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

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Date:   Tue, 18 Apr 2017 14:58:41 +0900
From:   Michel Dänzer <michel@...nzer.net>
To:     Ilia Mirkin <imirkin@...m.mit.edu>
Cc:     amd-gfx@...ts.freedesktop.org,
        open list <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [RfC PATCH] drm: fourcc byteorder: brings header file comments in
 line with reality.

On 18/04/17 02:04 PM, Ilia Mirkin wrote:
> On Mon, Apr 17, 2017 at 10:53 PM, Michel Dänzer <michel@...nzer.net> wrote:
>> On 17/04/17 03:43 PM, Ilia Mirkin wrote:
>>> On Tue, Apr 11, 2017 at 10:18 AM, Ilia Mirkin <imirkin@...m.mit.edu> wrote:
>>>>> However, I totally agree with Alex that someone with a BE machine
>>>>> should review the whole stack before we could be confident with anything.
>>>>
>>>> Here's what I'm confident about: xf86-video-nouveau worked just fine
>>>> on top of kernel 4.3 on an AGP GeForce FX 5200 (with AGPGART turned
>>>> off because ... well ... uninorth). fbcon/fbdev accel worked,
>>>> xf86-video-nouveau's 2d accel worked, and simple demos (ala glxgears)
>>>> worked after I fixed up mesa and nv30 driver items in version ... 11.1
>>>> it seems. As I recall it had gotten all broken in 10.0 or so by Adam
>>>> Jackson in the name of making llvmpipe work on BE, declaring all other
>>>> drivers broken, with various fixes by Michel Dänzer to get it back to
>>>> working over the years.
>>>>
>>>> Anyone "fixing" the stack has to maintain that level of functioning
>>>> through their various fixing.
>>>>
>>>> I will double-check that the above still works with the latest
>>>> kernel/xorg/xf86-video-nouveau/mesa and report back (hopefully by this
>>>> weekend). If there are any patches you'd like me to test, now's the
>>>> time to ask -- getting the box up and running is the hard part,
>>>> booting up an extra kernel -- easy.
>>>
>>> OK, so I revived my PowerMac7,3 G5 setup (PPC64 BE, NV34 GPU). Booted
>>> it with an upstream 4.11-rc7 kernel, loaded up the nouveau kernel
>>> module (which is included in that kernel), updated X to 1.19.2 and
>>> mesa to 17.0.3. Everything works fine. Specifically:
>>>
>>> fbcon on top of fbdev provided by nouveau -- colors are fine
>>> glxgears hw-accelerated by mesa on top of xf86-video-nouveau using
>>> DRI2 -- colors are fine
>>> glxgears softpipe-accelerated by mesa on top of xf86-video-nouveau --
>>> colors are fine
>>> glxgears softpipe-accelerated by mesa on top of xf86-video-modesetting
>>> -- colors are fine
>>> xterm on top of xf86-video-nouveau -- colors are fine
>>> xterm on top of xf86-video-modesetting -- colors are fine
>>>
>>> I couldn't test anything with GLAMOR since GLAMOR requires GL 2.1 or
>>> higher, whereas nouveau's NV3x acceleration only provides GL 1.5 (due
>>> to lacking NPOT and a handful of other things).
>>>
>>> The modetest utility did have trouble with AR24 and I'm pretty sure
>>> the XR24 pattern was off too. However I wouldn't be surprised if the
>>> modetest utility itself had endian issues in the pattern generation
>>> logic. (Seems to be the case, based on a quick glance at the
>>> tests/util/format.c logic and how it's used in pattern.c.)
>>>
>>> So in short, I think the current definitions of format are fine.
>>
>> I agree with Pekka that it's not that simple. What you've established is
>> that things look fine after going through several layers of abstraction.
>> It's possible that multiple bugs in those layers cancel each other out;
>> in particular, it's quite likely that the code dealing with DRM formats
>> is treating them as using native endianness (one possible giveaway for
>> that is using shifts for (un)packing colour components).
> 
> Quite true that this proves nothing. However one should note that
> fbcon -> fbdev works,

BTW, this supports Gerd's patch, since the KMS fbdev emulation code uses
e.g. DRM_FORMAT_XRGB8888 for depth/bpp 24/32, and the fbdev API uses
native endian packed colour values.


> and both mesa hw driver and softpipe driver work, in addition to regular
> DDX accel.

Similarly, the X11 protocol uses native endian packed colour values.


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