Re: [PATCH] drm: fourcc byteorder: brings header file comments in line with reality.

["Christian König" <christian.koenig@amd.com>]

Adding Harry to this mail thread as well, cause is one of the people 
really affected by this.

Christian.

Am 21.04.2017 um 15:21 schrieb Christian König:
> Am 21.04.2017 um 15:12 schrieb Gerd Hoffmann:
>>    Hi,
>>
>>>> "native" to me feels more like "native to the GPU" since these things
>>>> really are tied to the GPU not the CPU. That's also why I went with 
>>>> the
>>>> explicit endianness originally so that the driver could properly 
>>>> declare
>>>> what the GPU supports.
>>> And to be honest I would really prefer to stick with that approach for
>>> exactly that reason.
>>>
>>> The proposed change would require that drivers have different code path
>>> for different CPU byte order. Those code path tend to be not tested 
>>> very
>>> well and are additional complexity we probably don't want inside the 
>>> driver.
>> We can add fixed-endian #defines without too much effort, at least for
>> the 8 bits per color formats.  In qemu we have the same problem, only
>> with pixman.  Those formats are native endian too, but often we have to
>> handle a fixed format, so we did this:
>>
>> /*
>>   * pixman image formats are defined to be native endian,
>>   * that means host byte order on qemu.  So we go define
>>   * fixed formats here for cases where it is needed, like
>>   * feeding libjpeg / libpng and writing screenshots.
>>   */
>>
>> #ifdef HOST_WORDS_BIGENDIAN
>> # define PIXMAN_BE_r8g8b8     PIXMAN_r8g8b8
>> # define PIXMAN_BE_x8r8g8b8   PIXMAN_x8r8g8b8
>> # define PIXMAN_BE_a8r8g8b8   PIXMAN_a8r8g8b8
>> # define PIXMAN_BE_b8g8r8x8   PIXMAN_b8g8r8x8
>> # define PIXMAN_BE_b8g8r8a8   PIXMAN_b8g8r8a8
>> # define PIXMAN_BE_r8g8b8x8   PIXMAN_r8g8b8x8
>> # define PIXMAN_BE_r8g8b8a8   PIXMAN_r8g8b8a8
>> # define PIXMAN_BE_x8b8g8r8   PIXMAN_x8b8g8r8
>> # define PIXMAN_BE_a8b8g8r8   PIXMAN_a8b8g8r8
>> # define PIXMAN_LE_x8r8g8b8   PIXMAN_b8g8r8x8
>> #else
>> # define PIXMAN_BE_r8g8b8     PIXMAN_b8g8r8
>> # define PIXMAN_BE_x8r8g8b8   PIXMAN_b8g8r8x8
>> # define PIXMAN_BE_a8r8g8b8   PIXMAN_b8g8r8a8
>> # define PIXMAN_BE_b8g8r8x8   PIXMAN_x8r8g8b8
>> # define PIXMAN_BE_b8g8r8a8   PIXMAN_a8r8g8b8
>> # define PIXMAN_BE_r8g8b8x8   PIXMAN_x8b8g8r8
>> # define PIXMAN_BE_r8g8b8a8   PIXMAN_a8b8g8r8
>> # define PIXMAN_BE_x8b8g8r8   PIXMAN_r8g8b8x8
>> # define PIXMAN_BE_a8b8g8r8   PIXMAN_r8g8b8a8
>> # define PIXMAN_LE_x8r8g8b8   PIXMAN_x8r8g8b8
>> #endif
>
> Exactly what Mesa did as well.
>
>>> My personal opinion is that formats in drm_fourcc.h should be
>>> independent of the CPU byte order and the function
>>> drm_mode_legacy_fb_format() and drivers depending on that incorrect
>>> assumption be fixed instead.
>> The problem is this isn't a kernel-internal thing any more. With the
>> addition of the ADDFB2 ioctl the fourcc codes became part of the
>> kernel/userspace abi ...
>
> I know and that's exactly the reason I'm going to object those changes.
>
> The kernel/userspace abi is fixed and changing it like this could 
> potentially break drivers I'm the co-maintainer of. So that whole 
> approach is a clear NAK from my side.
>
> If you find a driver or userspace which doesn't use the formats as 
> defined in the comments of drm_fourcc.h the fix the driver instead of 
> trying to adjust the common header to broken behavior. Cause the later 
> will clearly cause problems with drivers who correctly implemented the 
> interface.
>
> Regards,
> Christian.
>
>>
>> cheers,
>>    Gerd
>>
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>
>
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