Date:   Wed, 4 Jan 2017 17:06:43 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Michał Kępień <kernel@...pniu.pl>,
        Jean Delvare <jdelvare@...e.com>,
        Wolfram Sang <wsa@...-dreams.de>,
        Steven Honeyman <stevenhoneyman@...il.com>,
        Valdis.Kletnieks@...edu,
        Jochen Eisinger <jochen@...guin-breeder.org>,
        Gabriele Mazzotta <gabriele.mzt@...il.com>,
        Andy Lutomirski <luto@...nel.org>, Mario_Limonciello@...l.com,
        Alex Hung <alex.hung@...onical.com>,
        Takashi Iwai <tiwai@...e.de>, linux-i2c@...r.kernel.org,
        linux-kernel@...r.kernel.org, platform-driver-x86@...r.kernel.org
Subject: Re: [PATCH] i2c: i801: Register optional lis3lv02d i2c device on
 Dell machines

On Wednesday 04 January 2017 14:02:52 Benjamin Tissoires wrote:
> On Jan 04 2017 or thereabouts, Pali Rohár wrote:
> > On Wednesday 04 January 2017 12:22:23 Pali Rohár wrote:
> > > On Wednesday 04 January 2017 11:32:33 Benjamin Tissoires wrote:
> > > > On Jan 04 2017 or thereabouts, Pali Rohár wrote:
> > > > > On Wednesday 04 January 2017 11:13:06 Benjamin Tissoires wrote:
> > > > > > On Jan 04 2017 or thereabouts, Pali Rohár wrote:
> > > > > > > On Wednesday 04 January 2017 10:05:22 Benjamin Tissoires wrote:
> > > > > > > > On Jan 04 2017 or thereabouts, Pali Rohár wrote:
> > > > > > > > > On Tuesday 03 January 2017 12:59:37 Dmitry Torokhov wrote:
> > > > > > > > > > On Tue, Jan 03, 2017 at 09:39:13PM +0100, Pali Rohár wrote:
> > > > > > > > > > > On Tuesday 03 January 2017 21:24:18 Dmitry Torokhov wrote:
> > > > > > > > > > > > On Tue, Jan 03, 2017 at 09:05:51PM +0100, Pali Rohár wrote:
> > > > > > > > > > > > > On Tuesday 03 January 2017 20:48:12 Dmitry Torokhov wrote:
> > > > > > > > > > > > > > On Tue, Jan 03, 2017 at 07:50:17PM +0100, Pali Rohár wrote:
> > > > > > > > > > > > > > > On Tuesday 03 January 2017 19:38:43 Dmitry Torokhov wrote:
> > > > > > > > > > > > > > > > On Tue, Jan 03, 2017 at 10:06:41AM +0100, Benjamin Tissoires
> > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > wrote:
> > > > > > > > > > > > > > > > > On Dec 29 2016 or thereabouts, Pali Rohár wrote:
> > > > > > > > > > > > > > > > > > On Thursday 29 December 2016 22:09:32 Michał Kępień wrote:
> > > > > > > > > > > > > > > > > > > > On Thursday 29 December 2016 14:47:19 Michał Kępień
> > > > > > > > > > > > > > > > > > > > wrote:
> > > > > > > > > > > > > > > > > > > > > > On Thursday 29 December 2016 09:29:36 Michał
> > > > > > > > > > > > > > > > > > > > > > Kępień
> > > > > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > > > > wrote:
> > > > > > > > > > > > > > > > > > > > > > > > Dell platform team told us that some (DMI
> > > > > > > > > > > > > > > > > > > > > > > > whitelisted) Dell Latitude machines have ST
> > > > > > > > > > > > > > > > > > > > > > > > microelectronics accelerometer at i2c address
> > > > > > > > > > > > > > > > > > > > > > > > 0x29. That i2c address is not specified in
> > > > > > > > > > > > > > > > > > > > > > > > DMI or ACPI, so runtime detection without
> > > > > > > > > > > > > > > > > > > > > > > > whitelist which is below is not possible.
> > > > > > > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > > > > > > Presence of that ST microelectronics
> > > > > > > > > > > > > > > > > > > > > > > > accelerometer is verified by existence of
> > > > > > > > > > > > > > > > > > > > > > > > SMO88xx ACPI device which represent that
> > > > > > > > > > > > > > > > > > > > > > > > accelerometer. Unfortunately without i2c
> > > > > > > > > > > > > > > > > > > > > > > > address.
> > > > > > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > > > > > This part of the commit message sounded a bit
> > > > > > > > > > > > > > > > > > > > > > > confusing to me at first because there is
> > > > > > > > > > > > > > > > > > > > > > > already an ACPI driver which handles SMO88xx
> > > > > > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > > > > > devices (dell-smo8800).  My understanding is
> > > > > > > > > > > > > > > > > > > > > > > that:
> > > > > > > > > > > > > > > > > > > > > > >   * the purpose of this patch is to expose a
> > > > > > > > > > > > > > > > > > > > > > >   richer interface (as
> > > > > > > > > > > > > > > > > > > > > > >   
> > > > > > > > > > > > > > > > > > > > > > >     provided by lis3lv02d) to these devices on
> > > > > > > > > > > > > > > > > > > > > > >     some machines,
> > > > > > > > > > > > > > > > > > > > > > >   
> > > > > > > > > > > > > > > > > > > > > > >   * on whitelisted machines, dell-smo8800 and
> > > > > > > > > > > > > > > > > > > > > > >   lis3lv02d can work
> > > > > > > > > > > > > > > > > > > > > > >   
> > > > > > > > > > > > > > > > > > > > > > >     simultaneously (even though dell-smo8800
> > > > > > > > > > > > > > > > > > > > > > >     effectively duplicates the work that
> > > > > > > > > > > > > > > > > > > > > > >     lis3lv02d does).
> > > > > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > > > > No. dell-smo8800 reads from ACPI irq number and
> > > > > > > > > > > > > > > > > > > > > > exports /dev/freefall device which notify
> > > > > > > > > > > > > > > > > > > > > > userspace about falls. lis3lv02d is i2c driver
> > > > > > > > > > > > > > > > > > > > > > which exports axes of accelerometer. Additionaly
> > > > > > > > > > > > > > > > > > > > > > lis3lv02d can export also /dev/freefall if
> > > > > > > > > > > > > > > > > > > > > > registerer of i2c device provides irq number --
> > > > > > > > > > > > > > > > > > > > > > which is not case of this patch.
> > > > > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > > > > So both drivers are doing different things and
> > > > > > > > > > > > > > > > > > > > > > both are useful.
> > > > > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > > > > IIRC both dell-smo8800 and lis3lv02d represent
> > > > > > > > > > > > > > > > > > > > > > one HW device (that ST microelectronics
> > > > > > > > > > > > > > > > > > > > > > accelerometer) but due to complicated HW
> > > > > > > > > > > > > > > > > > > > > > abstraction and layers on Dell laptops it is
> > > > > > > > > > > > > > > > > > > > > > handled by two drivers, one ACPI and one i2c.
> > > > > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > > > > Yes, in ideal world irq number should be passed
> > > > > > > > > > > > > > > > > > > > > > to lis3lv02d driver and that would export whole
> > > > > > > > > > > > > > > > > > > > > > device (with /dev/freefall too), but due to HW
> > > > > > > > > > > > > > > > > > > > > > abstraction it is too much complicated...
> > > > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > > > Why?  AFAICT, all that is required to pass that IRQ
> > > > > > > > > > > > > > > > > > > > > number all the way down to lis3lv02d is to set the
> > > > > > > > > > > > > > > > > > > > > irq field of the struct i2c_board_info you are
> > > > > > > > > > > > > > > > > > > > > passing to i2c_new_device().  And you can extract
> > > > > > > > > > > > > > > > > > > > > that IRQ number e.g. in
> > > > > > > > > > > > > > > > > > > > > check_acpi_smo88xx_device(). However, you would
> > > > > > > > > > > > > > > > > > > > > then need to make sure dell-smo8800 does not
> > > > > > > > > > > > > > > > > > > > > attempt to request the same IRQ on whitelisted
> > > > > > > > > > > > > > > > > > > > > machines.  This got me thinking about a way to
> > > > > > > > > > > > > > > > > > > > > somehow incorporate your changes into dell-smo8800
> > > > > > > > > > > > > > > > > > > > > using Wolfram's bus_notifier suggestion, but I do
> > > > > > > > > > > > > > > > > > > > > not have a working solution for now.  What is
> > > > > > > > > > > > > > > > > > > > > tempting about this approach is that you would not
> > > > > > > > > > > > > > > > > > > > > have to scan the ACPI namespace in search of
> > > > > > > > > > > > > > > > > > > > > SMO88xx devices, because smo8800_add() is
> > > > > > > > > > > > > > > > > > > > > automatically called for them. However, I fear that
> > > > > > > > > > > > > > > > > > > > > the resulting solution may be more complicated than
> > > > > > > > > > > > > > > > > > > > > the one you submitted.
> > > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > > Then we need to deal with lot of problems. Order of
> > > > > > > > > > > > > > > > > > > > loading .ko modules is undefined. Binding devices to
> > > > > > > > > > > > > > > > > > > > drivers registered by .ko module is also in "random"
> > > > > > > > > > > > > > > > > > > > order. At any time any of those .ko module can be
> > > > > > > > > > > > > > > > > > > > unloaded or at least device unbind (via sysfs) from
> > > > > > > > > > > > > > > > > > > > driver... And there can be some pathological
> > > > > > > > > > > > > > > > > > > > situation (thanks to adding ACPI layer as Andy
> > > > > > > > > > > > > > > > > > > > pointed) that there will be more SMO88xx devices in
> > > > > > > > > > > > > > > > > > > > ACPI. Plus you can compile kernel with and without
> > > > > > > > > > > > > > > > > > > > those modules and also you can blacklist loading
> > > > > > > > > > > > > > > > > > > > them (so compile time check is not enough). And
> > > > > > > > > > > > > > > > > > > > still some correct message notifier must be used.
> > > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > > I think such solution is much much more complicated,
> > > > > > > > > > > > > > > > > > > > there are lot of combinations of kernel configuration
> > > > > > > > > > > > > > > > > > > > and available dell devices...
> > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > I tried a few more things, but ultimately failed to
> > > > > > > > > > > > > > > > > > > find a nice way to implement this.
> > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > Another issue popped up, though.  Linus' master branch
> > > > > > > > > > > > > > > > > > > contains a recent commit by Benjamin Tissoires (CC'ed),
> > > > > > > > > > > > > > > > > > > 4d5538f5882a ("i2c: use an IRQ to report Host Notify
> > > > > > > > > > > > > > > > > > > events, not alert") which breaks your patch.  The
> > > > > > > > > > > > > > > > > > > reason for that is that lis3lv02d relies on the i2c
> > > > > > > > > > > > > > > > > > > client's IRQ being 0 to detect that it should not
> > > > > > > > > > > > > > > > > > > create /dev/freefall.
> > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > >  Benjamin's patch causes the Host Notify IRQ to be
> > > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > > assigned to the i2c client your patch creates, thus
> > > > > > > > > > > > > > > > > > > causing lis3lv02d to create /dev/freefall, which in
> > > > > > > > > > > > > > > > > > > turn conflicts with dell-smo8800 which is trying to
> > > > > > > > > > > > > > > > > > > create /dev/freefall itself.
> > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > > So 4d5538f5882a is breaking lis3lv02d driver...
> > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > Apologies for that.
> > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > I could easily fix this by adding a kernel API to know
> > > > > > > > > > > > > > > > > whether the provided irq is from Host Notify or if it was
> > > > > > > > > > > > > > > > > coming from an actual declaration. However, I have no idea
> > > > > > > > > > > > > > > > > how many other drivers would require this (hopefully only
> > > > > > > > > > > > > > > > > this one).
> > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > One other solution would be to reserve the Host Notify IRQ
> > > > > > > > > > > > > > > > > and let the actual drivers that need it to set it, but
> > > > > > > > > > > > > > > > > this was not the best solution according to Dmitri. On my
> > > > > > > > > > > > > > > > > side, I am not entirely against this given that it's a
> > > > > > > > > > > > > > > > > chip feature, so the driver should be able to know that
> > > > > > > > > > > > > > > > > it's available.
> > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > Dmitri, Wolfram, Jean, any preferences?
> > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > I read this:
> > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > "IIRC both dell-smo8800 and lis3lv02d represent one HW device
> > > > > > > > > > > > > > > > (that ST microelectronics accelerometer) but due to
> > > > > > > > > > > > > > > > complicated HW abstraction and layers on Dell laptops it is
> > > > > > > > > > > > > > > > handled by two drivers, one ACPI and one i2c."
> > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > and that is the core of the issue. You have 2 drivers
> > > > > > > > > > > > > > > > fighting over the same device. Fix this and it will all
> > > > > > > > > > > > > > > > work.
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > With my current implementation (which I sent in this patch),
> > > > > > > > > > > > > > > they are not fighting.
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > dell-smo8800 exports /dev/freefall (and nothing more) and
> > > > > > > > > > > > > > > lis3lv02d only accelerometer device as lis3lv02d driver does
> > > > > > > > > > > > > > > not get IRQ number in platform data.
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > As far as I can see hp_accel instantiates lis3lv02d and
> > > > > > > > > > > > > > > > accesses it via ACPI methods, can the same be done for Dell?
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > No, Dell does not have any ACPI methods. And as I wrote in ACPI
> > > > > > > > > > > > > > > or DMI is even not i2c address of device, so it needs to be
> > > > > > > > > > > > > > > specified in code itself.
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > Really there is no other way... :-(
> > > > > > > > > > > > > > 
> > > > > > > > > > > > > > Sure there is:
> > > > > > > > > > > > > > 
> > > > > > > > > > > > > > 1. dell-smo8800 instantiates I2C device as "dell-smo8800-accel".
> > > > > > > > > > > > > > 2. dell-smo8800 provides read/write functions for lis3lv02d that
> > > > > > > > > > > > > > simply forward requests to dell-smo8800-accel i2c client.
> > > > > > > > > > > > > > 3. dell-smo8800 instantiates lis3lv02d instance like hp_accel
> > > > > > > > > > > > > > does.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > Sorry, but I do not understand how you mean it... Why to provides
> > > > > > > > > > > > > new read/write i2c functions which are already implemented by
> > > > > > > > > > > > > i2c-i801 bus and lis3lv02d i2c driver?
> > > > > > > > > > > > 
> > > > > > > > > > > > Because that would allow you to avoid clashes with i2c creating
> > > > > > > > > > > > interrupt mapping for client residing on host-notify-capable
> > > > > > > > > > > > controller.
> > > > > > > > > > > > 
> > > > > > > > > > > > > > Alternatively, can lis3lv02d be tasked to create /dev/freefall?
> > > > > > > > > > > > > 
> > > > > > > > > > > > > If i2c_board_info contains IRQ then lis3lv02d create /dev/freefall
> > > > > > > > > > > > > device.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > But... what is problem with current implementation? Accelerometer
> > > > > > > > > > > > > HW provides two functions:
> > > > > > > > > > > > > 
> > > > > > > > > > > > > 1) 3 axes reports
> > > > > > > > > > > > > 2) Disk freefall detection
> > > > > > > > > > > > > 
> > > > > > > > > > > > > And 1) is handled by i2c driver lis3lv02d and 2) is by
> > > > > > > > > > > > > dell-smo8800. Both functions are independent here.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > I think you just trying to complicate this situation even more to
> > > > > > > > > > > > > be more complicated as currently is.
> > > > > > > > > > > > 
> > > > > > > > > > > > Because this apparently does not work for you, does it?
> > > > > > > > > > > 
> > > > > > > > > > > It is working fine. I do not see any problem.
> > > > > > > > > > > 
> > > > > > > > > > > > In general,
> > > > > > > > > > > > if you want the same hardware be handled by 2 different drivers you
> > > > > > > > > > > > are going to have bad time.
> > > > > > > > > > > 
> > > > > > > > > > > Yes, but in this case half of device is ACPI based and other half i2c 
> > > > > > > > > > > based. This is problem of ACPI and Dell design.
> > > > > > > > > > > 
> > > > > > > > > > > > It seems to be that /dev/freefall in dell-smo8800 and lis3lv02d are
> > > > > > > > > > > > the same, right?
> > > > > > > > > > > 
> > > > > > > > > > > Yes. I understand that clean solution is to have one driver which 
> > > > > > > > > > > provides everything.
> > > > > > > > > > > 
> > > > > > > > > > > But because half of data are ACPI and half i2c, you still needs to 
> > > > > > > > > > > create two drivers (one ACPI and one i2c). You can put both drivers into 
> > > > > > > > > > > one .ko module, but still these will be two drivers due to how ACPI and 
> > > > > > > > > > > i2c linux abstractions are different.
> > > > > > > > > > > 
> > > > > > > > > > > > So, instead of having 2 drivers split the
> > > > > > > > > > > > functionality, can you forego registering smo8800 ACPI driver on
> > > > > > > > > > > > your whitelisted boxes and instead instantiate full i2c client
> > > > > > > > > > > > device with properly assigned both address and IRQ and let lis3lv02d
> > > > > > > > > > > > handle it (providing both accelerometer data and /dev/freefall)?
> > > > > > > > > > > 
> > > > > > > > > > > With Michał we already discussed about it, see emails. Basically you can 
> > > > > > > > > > > enable/disable kernel modules at compile time or blacklist at runtime 
> > > > > > > > > > > (or even chose what will be compiled into vmlinux and what as external 
> > > > > > > > > > > .ko module).
> > > > > > > > > > 
> > > > > > > > > > This can be solved with a bit of Kconfig/IS_ENABLED() code.
> > > > > > > > > > 
> > > > > > > > > > > Some distributions blacklist i2c-i801.ko module... And 
> > > > > > > > > > 
> > > > > > > > > > Any particular reason for that?
> > > > > > > > > > 
> > > > > > > > > > > there can be also problem with initialization of i2c-i801 driver (fix is 
> > > > > > > > > > > in commit a7ae81952cda, but does not have to work at every time!). So 
> > > > > > > > > > > that move on whitelisted machines can potentially cause disappearance of 
> > > > > > > > > > > /dev/freefall and users will not have hdd protection which is currently 
> > > > > > > > > > > working.
> > > > > > > > > > 
> > > > > > > > > > Well, I gave you 2 possible solutions (roll your own i2c read/write,
> > > > > > > > > > forward them to i2c client) or have faith in your implementation and let
> > > > > > > > > > lis3lv02d handle it.
> > > > > > > > > > 
> > > > > > > > > > The 3rd one is to possibly add a flag to disable host notify to IRQ
> > > > > > > > > > mapping for given client (if Wolfram/Jean OK with it).
> > > > > > > > > > 
> > > > > > > > > > Oh, the 4th one: change the irq in lis3lv02d.h to be "int" and change
> > > > > > > > > > the check in lis3lv02d.c to be "lis->irq <= 0" and instantiate your
> > > > > > > > > > i2c_client with board_info->irq = -1.
> > > > > > > > > > 
> > > > > > > > > > Pick whichever you prefer.
> > > > > > > > > > 
> > > > > > > > > > By the way, what do you need accelerometer for on these devices? They
> > > > > > > > > > don't appear to be tablets that could use one... 
> > > > > > > > > 
> > > > > > > > > Ah, you are talking about problem that after 4d5538f5882a lis3lv02d will
> > > > > > > > > not work... I thought that discussion is about different mechanism how
> > > > > > > > > to implement bus registration notification to smo8800 driver (or
> > > > > > > > > different solution to not have registration in i801).
> > > > > > > > > 
> > > > > > > > 
> > > > > > > > Just because I am not sure I got everything right, could you confirm
> > > > > > > > that:
> > > > > > > > - in the current upstream tree, the dell-smo8800 driver is now broken
> > > > > > > >   after 4d5538f5882a (i2c: use an IRQ to report Host Notify events, not
> > > > > > > >   alert)
> > > > > > > 
> > > > > > > No, dell-smo8800 it is working fine. It is fully independent from i2c
> > > > > > > and lis3lv02d. It is pure ACPI driver which does not share anything with
> > > > > > > i2c.
> > > > > > > 
> > > > > > > > - this series adds an extra lis3lv02d on some machines and you have
> > > > > > > >   problem fighting for the irq (but this is not upstream yet).
> > > > > > > 
> > > > > > > Yes, this series (not merged yet) adds extra lis3lv02d device but is not
> > > > > > > working because of 4d5538f5882a.
> > > > > > > 
> > > > > > > > The extra lis3lv02d node is added from dell-smo8800
> > > > > > > 
> > > > > > > No, dell-smo8800 does not add new node in this patch.
> > > > > > > 
> > > > > > > > If the first point is not correct (by default, dell-smo8800 will not be
> > > > > > > > loaded at the same time than lis3lv02d), then it's a design issue with
> > > > > > > > the interactions between those 2 drivers.
> > > > > > > 
> > > > > > > No, there is no interactions between these two drivers (dell-smo8800 and
> > > > > > > lis3lv02d). dell-smo8800 is pure ACPI driver and exports just
> > > > > > > /dev/freefall device based on IRQ (and nothing more).
> > > > > > > 
> > > > > > > And lis3lv02d in *current* configuration in this patch exports only
> > > > > > > accelerometer input device, not /dev/freefall. It does not use IRQ.
> > > > > > > (Just there is problem with 4d5538f5882a which tells lis3lv02d IRQ
> > > > > > > number which is not freefall report, therefore lis3lv02d does not work).
> > > > > > > 
> > > > > > > To make it clear, ST Accelerometer provides two operations:
> > > > > > > * report free fall
> > > > > > > * report 3 axes
> > > > > > > 
> > > > > > > Free fall is reported by IRQ, state of 3 axes via i2c bus. Free fall IRQ
> > > > > > > is handled by dell-smo8800, state of 3 axes via i2c lis3lv02d driver.
> > > > > > > 
> > > > > > > lis3lv02d can handle also free fall IRQ is platform i2c data provides
> > > > > > > IRQ number for it -- but this is not case in our Dell configuration. But
> > > > > > > commit 4d5538f5882a inject some IRQ number to lis3lv02d driver which is
> > > > > > > not free fall detection and so is breaking lis3lv02 driver. In our Dell
> > > > > > > configuration (by this patch) there should be no IRQ number.
> > > > > > > 
> > > > > > > It is clear now?
> > > > > > 
> > > > > > I think I am starting to understand the problem.
> > > > > > 
> > > > > > Currently (upstream tree), 4d5538f5882a doesn't break anything.
> > > > > 
> > > > > I cannot prove it... lis3lv02d i2c driver itself uses this IRQ logic
> > > > > (missing = no freefall) and there could be already hardware/boards which
> > > > > register lis3lv02d i2c device without IRQ number. And definitions could
> > > > > be also in device tree and so outside of kernel sources...
> > > > > 
> > > > > So there can be potentially break. But at least not case of Dell.
> > > > > 
> > > > > > On the
> > > > > > mentioned Dell platforms, the dell-smo8800 gets loaded and provides
> > > > > > /dev/freefall. lis3lv02d is not loaded so everything just works.
> > > > > 
> > > > > Yes.
> > > > > 
> > > > > > The problem comes from this patch which doesn't set the irq in
> > > > > > i2c_board_info and so i2c_core sets the IRQ to Host Notify. I think
> > > > > > Dmitri already gave you the solution: set the irq to -1 (or -ENOENT)
> > > > > > in i2c_board_info in your patch and only forward it to
> > > > > > lis3lv02d_init_device() only if it is positive (valid).
> > > > > 
> > > > > Now I understood that Dmitri means. For this Dell platforms it is OK,
> > > > > but we have a problem for device tree platforms. Normally in device tree
> > > > > if device does not support something you just do not specify it. But
> > > > > with this approach you need to explicitly specify IRQ to -1 in device
> > > > > tree. And I think this is really not a clean solution.
> > > > > 
> > > 
> > > Here I was talking about other platforms/devices (not this Dell one).
> > > 
> > > > No. DT platforms won't have an issue: they don't change anything, and
> > > > there will be a new /dev/freefall misc device for the platforms that
> > > 
> > > And this is wrong! There should not be any /dev/freefall device
> > > connected with signal host notify. /dev/freefall is for hardware which
> > > supports free fall hdd detection.
> 
> On the principle, I agree. But let's be realistic: this device will be
> created but no events will be generated from it.

And I think this is a problem. This provides false information to kernel
and also to userspace applications that there is working free fall
sensor (even there is not).

> It's a capability for the chip to provide freefall,

That it is not truth. Not every chip has it. Platform data are there to
provides this information for driver, if HW has freefall capability or
not.

> so I don't see why it's such an issue to
> have one created which says nothing. (talking about non Dell platforms
> here too)
> 
> > > 
> > > > don't have the irq set *and* if the device is on a Host Notify capable
> > > > adapter, currently only i2c-i801.
> > > 
> > > I understood, but in future more bus drivers could support host notify.
> > 
> > This is really fragile. lis3lv02d will empty IRQ (0) will work on some
> > i2c buses and on some not. I would really expect that i2c driver
> > (lis3lv02d) will work in same way with any i2c bus driver. And not that
> > on i801 will acts differently as on e.g. omap3 i2c bus.
> 
> I really don't follow you here. On a bus with no Host Notify, the IRQ
> will be 0 and it will work in the same way.

That it OK.

> On a bus with Host Notify,
> the IRQ should be set to something negative in the i2c_board_info

Not only in i2c_board_info but also in DTS. Which means every one DTS
needs to be updates -- also those outside of linux kernel. Which is for
me wrong.

> to be
> sure to not have an Host Notify irq assigned. In both case the
> lis3lv02d_i2c driver will just do:
> 	if (client->irq > 0)
> 		lis3_dev.irq = client->irq;
> 
> So there is no bus adapter specifics in the driver.
> 
> > 
> > > This is basically incorrect if we use one property for two different
> > > things (host notify and hdd free fall).
> 
> Host Notify is a way for a I2C device to report an interrupt without
> having a dedicated line assigned (and so an IRQ).

Yes. But lis3lv02d i2c devices which I have uses dedicated IRQ. They
does not work in way like you describe.

> So the chip could use
> Host Notify to report hdd free fall if it supports the feature.

Old existing HW could not. As those were not designed for such purpose.

Yes, new HW can be designed in this way, but linux kernel should not
drop (or modify) correct support for old HW devices just because there
is new HW design.

> > > 
> > > For me it looks like that we should separate these two things into two
> > > IRQ properties. It is really wrong design if one property is used for
> > > two different things.
> 
> I won't say the design is wrong because Host Notify really is just an
> IRQ.

Yes it is IRQ, but different. And lis3lv02d does not issue host notify
IRQ, so it should not be assigned to lis3lv02d.

> Where the issue lies now is that lis3lv02d makes the assumption
> that an irq attached to an i2c_client means that it will report
> freefall.

Yes.

> It was correct before, but now it's not true anymore.

Agree.

> So maybe this series should also add a way to provide the source of
> the IRQ (Host Notify or ACPI or DT). Then, lis3lv02d could ignore or
> not the incoming IRQ.

Yes, this is an option how to fix this problem.

I would rather introduce new IRQ property in both DTS and lis3lv02d
platform data (e.g. irq-freefall) which will unambiguously identify
freefall IRQ.

But I do not think it should be part of my Dell patch. It is
independent fix for lis3lv02d driver for bug which was introduced by
commit 4d5538f5882a.

> 
> Cheers,
> Benjamin
> 
> > > 
> > > > But given that they are DT and not
> > > > ACPI, this won't conflict with dell-smo8800, so there won't be any
> > > > errors, just a dangling unused device node.
> > > 
> > > Yes, for upcoming Dell in this patch (with IRQ -1) it is not a problem.
> > > 
> > > > This approach is IMO the best if you want to have this in the kernel.
> > > > 
> > > > Cheers,
> > > > Benjamin
> > > 
> > 
> > -- 
> > Pali Rohár
> > pali.rohar@...il.com

-- 
Pali Rohár
pali.rohar@...il.com

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