Message-ID: <20170103202418.GE16032@dtor-ws>
Date:   Tue, 3 Jan 2017 12:24:18 -0800
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Pali Rohár <pali.rohar@...il.com>
Cc:     Benjamin Tissoires <benjamin.tissoires@...hat.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 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? In general, if
you want the same hardware be handled by 2 different drivers you are
going to have bad time.

It seems to be that /dev/freefall in dell-smo8800 and lis3lv02d are the
same, right? 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)?

Thanks.

-- 
Dmitry

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