linux-kernel - Re: [PATCH 3/5] drm: fix drm_mode

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Date:   Mon, 3 Sep 2018 13:07:52 -0400
From:   Ilia Mirkin <imirkin@...m.mit.edu>
To:     Gerd Hoffmann <kraxel@...hat.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        Sean Paul <sean@...rly.run>
Subject: Re: [PATCH 3/5] drm: fix drm_mode_addfb() on big endian machines.

On Mon, Sep 3, 2018 at 12:45 PM, Daniel Vetter <daniel@...ll.ch> wrote:
> On Mon, Sep 03, 2018 at 12:57:54PM +0200, Gerd Hoffmann wrote:
>> Userspace on big endian machhines typically expects the ADDFB ioctl
>> returns a big endian framebuffer.  drm_mode_addfb() will call
>> drm_mode_addfb2() unconditionally with little endian DRM_FORMAT_*
>> values though, which is wrong.  This patch fixes that.
>>
>> Drivers (both kernel and xorg) have quirks in place to deal with the
>> broken drm_mode_addfb() behavior.  Because of this we can't just change
>> drm_mode_addfb() behavior for everybody without breaking things.  So add
>> a new driver feature flag DRIVER_PREFER_HOST_BYTE_ORDER, so drivers can
>> opt-in.
>>
>> Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>
>> ---
>>  include/drm/drm_drv.h             |  1 +
>>  drivers/gpu/drm/drm_framebuffer.c | 11 +++++++++++
>>  2 files changed, 12 insertions(+)
>>
>> diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
>> index 46a8009784..9cf12596cd 100644
>> --- a/include/drm/drm_drv.h
>> +++ b/include/drm/drm_drv.h
>> @@ -57,6 +57,7 @@ struct drm_printer;
>>  #define DRIVER_KMS_LEGACY_CONTEXT    0x20000
>>  #define DRIVER_SYNCOBJ                  0x40000
>>  #define DRIVER_PREFER_XBGR_30BPP        0x80000
>> +#define DRIVER_PREFER_HOST_BYTE_ORDER   0x100000
>
> Hm, not a huge fan of using driver_flags for random little quirks. I think
> a boolean in sturct drm_mode_config would be much better. Bonus if you
> also move the 30bpp hack over to that. Something like
> mode_config.quirk_addfb_host_byte_order and
> mode_config.quirk_addfb_prefer_xbgr_30bpp or whatever. That has the upside
> of giving us a really nice place to put a huge comment about what this is
> supposed to do.
>
> I think otherwise this looks overall rather reasonable. I think the only
> other driver that ever cared about big endian was radeon/amdgpu. Would be
> good to get at least an ack from amd folks, or a "meh, stopped caring".

and nouveau.

I do believe that ADDFB should be made to always prefer host byte
order -- this is how all of the existing implementations work in
practice. However e.g. nouveau wants DRM_FORMAT_XRGB8888. But it still
treats it as host byte order. This will become more important in a
world where ADDFB2 is more common.

So, I think that this change should be applied, drivers (I suspect
just nouveau and radeon) fixed up to consume the "new" formats, and
then made to be the one-and-only way for ADDFB to function. That way
we'll no longer be lying about the DRM_FORMAT being used.

Cheers,

  -ilia