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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>,
        Gerd Hoffmann <kraxel@redhat.com>,
        David Airlie <airlied@linux.ie>, Sean Paul <sean@poorly.run>
References: <20180903105756.24912-1-kraxel@redhat.com>
 <20180903105756.24912-4-kraxel@redhat.com>
 <20180903164558.GL21634@phenom.ffwll.local>
 <CAKb7Uvh-UB6pR1r-D5sX0HwWwTExVzjxcdeAKNAmRWdqh=hKGw@mail.gmail.com>
Cc:     dri-devel <dri-devel@lists.freedesktop.org>,
        open list <linux-kernel@vger.kernel.org>
From:   =?UTF-8?Q?Michel_D=c3=a4nzer?= <michel@daenzer.net>
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Date:   Tue, 4 Sep 2018 10:00:27 +0200
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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.


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