Message-Id: <A5D4ED10-F3EB-4DA8-A846-C28842BDAA9F@konsulko.com>
Date:	Wed, 18 Feb 2015 18:39:01 +0200
From:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>
To:	Ludovic Desroches <ludovic.desroches@...el.com>
Cc:	Mark Rutland <mark.rutland@....com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Matt Porter <matt.porter@...aro.org>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Guenter Roeck <linux@...ck-us.net>,
	Rob Herring <robherring2@...il.com>,
	Tony Lindgren <tony@...mide.com>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/4] of: DT quirks infrastructure

Hi Ludovic,

> On Feb 18, 2015, at 18:32 , Ludovic Desroches <ludovic.desroches@...el.com> wrote:
> 
> Hi,
> 
> Great something we are waiting for a long time!
> 
> On Wed, Feb 18, 2015 at 05:53:50PM +0200, Pantelis Antoniou wrote:
>> Hi Mark,
>> 
>>> On Feb 18, 2015, at 17:41 , Mark Rutland <mark.rutland@....com> wrote:
>>> 
>>> Hi Pantelis,
>>> 
>>> On Wed, Feb 18, 2015 at 02:59:34PM +0000, Pantelis Antoniou wrote:
>>>> Implement a method of applying DT quirks early in the boot sequence.
>>>> 
>>>> A DT quirk is a subtree of the boot DT that can be applied to
>>>> a target in the base DT resulting in a modification of the live
>>>> tree. The format of the quirk nodes is that of a device tree overlay.
>>>> 
>>>> For details please refer to Documentation/devicetree/quirks.txt
>>>> 
>>>> Signed-off-by: Pantelis Antoniou <pantelis.antoniou@...sulko.com>
>>>> ---
>>>> Documentation/devicetree/quirks.txt | 101 ++++++++++
>>>> drivers/of/dynamic.c                | 358 ++++++++++++++++++++++++++++++++++++
>>>> include/linux/of.h                  |  16 ++
>>>> 3 files changed, 475 insertions(+)
>>>> create mode 100644 Documentation/devicetree/quirks.txt
>>>> 
>>>> diff --git a/Documentation/devicetree/quirks.txt b/Documentation/devicetree/quirks.txt
>>>> new file mode 100644
>>>> index 0000000..789319a
>>>> --- /dev/null
>>>> +++ b/Documentation/devicetree/quirks.txt
>>>> @@ -0,0 +1,101 @@
>>>> +A Device Tree quirk is the way which allows modification of the
>>>> +boot device tree under the control of a per-platform specific method.
>>>> +
>>>> +Take for instance the case of a board family that comprises of a
>>>> +number of different board revisions, all being incremental changes
>>>> +after an initial release.
>>>> +
>>>> +Since all board revisions must be supported via a single software image
>>>> +the only way to support this scheme is by having a different DTB for each
>>>> +revision with the bootloader selecting which one to use at boot time.
>>> 
>>> Not necessarily at boot time. The boards don't have to run the exact
>>> same FW/bootloader binary, so the relevant DTB could be programmed onto
>>> each board.
>>> 
>> 
>> That has not been the case in any kind of board I’ve worked with.
>> 
>> A special firmware image that requires a different programming step at
>> the factory to select the correct DTB for each is always one more thing
>> that can go wrong.
>> 
> 
> I agree. We have boards with several display modules, even if it seems
> quite easy to know which dtb has to be loaded since we use a prefix
> describing the display module (_pda4, _pda7, etc.) it is a pain for
> customers. Moreover you can add the revision of the board, we have a
> mother board and a cpu module so it can quickly lead to something like
> this:
> at91-sama5d31ek_mb-revc_cm-revd_pda7.
> 
> It is only an example, at the moment it is a bit less complicated but I
> am not so far from the reality: sama5d31ek_revc_pda7.dts,
> sama5d33ek_revc_pda4.dts, etc. For a SoC family, we have 27 DTS files…
> 

I bet it’s easy to screw up no? Any horror stories from the factory floor? :)

> As for the single zImage, we should find a way to have a single DTB.
> 
>>>> +While this may in theory work, in practice it is very cumbersome
>>>> +for the following reasons:
>>>> +
>>>> +1. The act of selecting a different boot device tree blob requires
>>>> +a reasonably advanced bootloader with some kind of configuration or
>>>> +scripting capabilities. Sadly this is not the case many times, the
>>>> +bootloader is extremely dumb and can only use a single dt blob.
>>> 
>>> You can have several bootloader builds, or even a single build with
>>> something like appended DTB to get an appropriate DTB if the same binary
>>> will otherwise work across all variants of a board.
>>> 
>> 
>> No, the same DTB will not work across all the variants of a board.
>> 
>>> So it's not necessarily true that you need a complex bootloader.
>>> 
>> 
>>>> +2. On many instances boot time is extremely critical; in some cases
>>>> +there are hard requirements like having working video feeds in under
>>>> +2 seconds from power-up. This leaves an extremely small time budget for
>>>> +boot-up, as low as 500ms to kernel entry. The sanest way to get there
>>>> +is by removing the standard bootloader from the normal boot sequence
>>>> +altogether by having a very small boot shim that loads the kernel and
>>>> +immediately jumps to kernel, like falcon-boot mode in u-boot does.
>>> 
>>> Given my previous comments above I don't see why this is relevant.
>>> You're already passing _some_ DTB here, so if you can organise for the
>>> board to statically provide a sane DTB that's fine, or you can resort to
>>> appended DTB if it's not possible to update the board configuration.
>>> 
>> 
>> You’re missing the point. I can’t use the same DTB for each revision of the
>> board. Each board is similar but it’s not identical.
>> 
>>>> +3. Having different DTBs/DTSs for different board revisions easily leads to
>>>> +drift between versions. Since no developer is expected to have every single
>>>> +board revision at hand, things are easy to get out of sync, with board versions
>>>> +failing to boot even though the kernel is up to date.
>>> 
>>> I'm not sure I follow. Surely if you don't have every board revision at
>>> hand you can't test quirks exhaustively either?
>>> 
>> 
>> It’s one less thing to worry about. For example in the current mainline kernel
>> already there is a drift between the beaglebone white and the beaglebone black.
>> 
>> Having the same DTS is just easier to keep things in sync.
>> 
>>> Additionally you face the problem that two boards of the same variant
>>> could have different base DTBs that you would need to test that each
>>> board's quirks worked for a range of base DTBs.
>>> 
>> 
>> This is not a valid case. This patch is about boards that have the same base DTB.
>> 
>>>> +4. One final problem is the static way that device tree works.
>>>> +For example it might be desirable for various boards to have a way to
>>>> +selectively configure the boot device tree, possibly by the use of command
>>>> +line options.  For instance a device might be disabled if a given command line
>>>> +option is present, different configuration to various devices for debugging
>>>> +purposes can be selected and so on. Currently the only way to do so is by
>>>> +recompiling the DTS and installing, which is an chore for developers and
>>>> +a completely unreasonable expectation from end-users.
>>> 
>>> I'm not sure I follow here.
>>> 
>>> Which devices do you envisage this being the case for?
>>> 
>>> Outside of debug scenarios when would you envisage we do this?
>>> 
>> 
>> We already have to do this on the beaglebone black. The onboard EMMC and HDMI
>> devices conflict with any capes that use the same pins. So you have to
>> have a way to disable them so that the attached cape will work.
>> 
>>> For the debug case it seems reasonable to have command line parameters
>>> to get the kernel to do what we want. Just because there's a device in
>>> the DTB that's useful in a debug scenario doesn't mean we have to use it
>>> by default.
>> 
>> I don’t follow. Users need this functionality to work. I.e. pass a command
>> line option to use different OPPs etc. Real world usage is messy.
>> 
>>> 
>>>> +Device Tree quirks solve all those problems by having an in-kernel interface
>>>> +which per-board/platform method can use to selectively modify the device tree
>>>> +right after unflattening.
>>>> +
>>>> +A DT quirk is a subtree of the boot DT that can be applied to
>>>> +a target in the base DT resulting in a modification of the live
>>>> +tree. The format of the quirk nodes is that of a device tree overlay.
>>>> +
>>>> +As an example the following DTS contains a quirk.
>>>> +
>>>> +/ {
>>>> +       foo: foo-node {
>>>> +               bar = <10>;
>>>> +       };
>>>> +
>>>> +       select-quirk = <&quirk>;
>>>> +
>>>> +       quirk: quirk {
>>>> +               fragment@0 {
>>>> +                       target = <&foo>;
>>>> +                       __overlay {
>>>> +                               bar = <0xf00>;
>>>> +                               baz = <11>;
>>>> +                       };
>>>> +               };
>>>> +       };
>>>> +};
>>>> +
>>>> +The quirk when applied would result at the following tree:
>>>> +
>>>> +/ {
>>>> +       foo: foo-node {
>>>> +               bar = <0xf00>;
>>>> +               baz = <11>;
>>>> +       };
>>>> +
>>>> +       select-quirk = <&quirk>;
>>>> +
>>>> +       quirk: quirk {
>>>> +               fragment@0 {
>>>> +                       target = <&foo>;
>>>> +                       __overlay {
>>>> +                               bar = <0xf00>;
>>>> +                               baz = <11>;
>>>> +                       };
>>>> +               };
>>>> +       };
>>>> +
>>>> +};
>>>> +
>>>> +The two public method used to accomplish this are of_quirk_apply_by_node()
>>>> +and of_quirk_apply_by_phandle();
>>>> +
>>>> +To apply the quirk, a per-platform method can retrieve the phandle from the
>>>> +select-quirk property and pass it to the of_quirk_apply_by_phandle() node.
>>>> +
>>>> +The method which the per-platform method is using to select the quirk to apply
>>>> +is out of the scope of the DT quirk definition, but possible methods include
>>>> +and are not limited to: revision encoding in a GPIO input range, board id
>>>> +located in external or internal EEPROM or flash, DMI board ids, etc.
>>> 
>>> It seems rather unfortunate that to get a useful device tree we have to
>>> resort to board-specific mechanisms. That means yet more platform code,
>>> which is rather unfortunate. This would also require any other DT users
>>> to understand and potentially have to sanitize any quirks (e.g. in the
>>> case of Xen).
>> 
>> The original internal version of the patches used early platform devices and
>> a generic firmware quirk mechanism, but I was directed to the per-platform
>> method instead. It is perfectly doable to go back at the original implementation
>> but I need to get the ball rolling with a discussion about the internals.
> 
> I also think we should used early platform devices to not add platform
> specific code. What were the cons to swith to per-platform method?
> 

Supposedly easier to accept. /me shrugs

>> 
>>> 
>>> Mark.
>> 
>> Regards
>> 
>> — Pantelis
>> 
> 
> Regards
> 
> Ludovic

Regards

— Pantelis

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