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Subject: Re: [PATCH 3/5] drm: fix drm_mode_addfb() on big endian machines.
To:     Ilia Mirkin <imirkin@alum.mit.edu>
Cc:     David Airlie <airlied@linux.ie>, Sean Paul <sean@poorly.run>,
        Gerd Hoffmann <kraxel@redhat.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        open list <linux-kernel@vger.kernel.org>
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From:   =?UTF-8?Q?Michel_D=c3=a4nzer?= <michel@daenzer.net>
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Date:   Tue, 4 Sep 2018 17:02:12 +0200
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On 2018-09-04 3:05 p.m., Ilia Mirkin wrote:
> On Tue, Sep 4, 2018 at 4:00 AM, Michel Dänzer <michel@daenzer.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@ffwll.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@redhat.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?

ADDFB, not ADDFB2. The latter probably didn't even exist yet when this
was made to work. :)


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer