Re: [PATCH 2/4] of: DT quirks infrastructure

From: Ludovic Desroches
Date: Wed Feb 18 2015 - 11:33:03 EST

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@xxxxxxx> 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@xxxxxxxxxxxx>
> >> ---
> >> 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 Iâve 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...

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.
> >
>
> Youâre missing the point. I canât use the same DTB for each revision of the
> board. Each board is similar but itâs 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?
> >
>
> Itâs 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 donât 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?

>
> >
> > Mark.
>
> Regards
>
> â Pantelis
>

Regards

Ludovic
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