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

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Date:   Tue, 4 Sep 2018 09:05:09 -0400
From:   Ilia Mirkin <imirkin@...m.mit.edu>
To:     Michel Dänzer <michel@...nzer.net>
Cc:     Gerd Hoffmann <kraxel@...hat.com>, David Airlie <airlied@...ux.ie>,
        Sean Paul <sean@...rly.run>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] drm: fix drm_mode_addfb() on big endian machines.

On Tue, Sep 4, 2018 at 4:00 AM, Michel Dänzer <michel@...nzer.net> wrote:
> On 2018-09-03 7:07 p.m., Ilia Mirkin wrote:
>> 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, [...]
>
> As explained before, that would break radeon userspace on big endian hosts.

We last discussed this about a year ago, so I hope you'll forgive my
lapse in memory...

There's userspace that uses ADDFB2 with DRM_FORMAT_XRGB8888 but
expects it to be host-endian? If so, that'll be a problem for nouveau
as well...

  -ilia