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

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Message-ID: <CAKb7UviirFVnut-6k6nO2-dspyzdu0AdZQ1CV8sAKAJkqAc3oA@mail.gmail.com>
Date:   Sat, 22 Apr 2017 09:40:47 -0400
From:   Ilia Mirkin <imirkin@...m.mit.edu>
To:     Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:     Gerd Hoffmann <kraxel@...hat.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Pekka Paalanen <ppaalanen@...il.com>,
        Michel Dänzer <michel@...nzer.net>,
        Alex Deucher <alexdeucher@...il.com>,
        amd-gfx@...ts.freedesktop.org,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm: fourcc byteorder: brings header file comments in
 line with reality.

On Sat, Apr 22, 2017 at 5:50 AM, Ville Syrjälä
<ville.syrjala@...ux.intel.com> wrote:
> On Sat, Apr 22, 2017 at 01:07:57AM -0400, Ilia Mirkin wrote:
>> On Fri, Apr 21, 2017 at 12:59 PM, Ville Syrjälä
>> <ville.syrjala@...ux.intel.com> wrote:
>> > On Fri, Apr 21, 2017 at 10:49:49AM -0400, Ilia Mirkin wrote:
>> >> On Fri, Apr 21, 2017 at 3:58 AM, Gerd Hoffmann <kraxel@...hat.com> wrote:
>> >> > While working on graphics support for virtual machines on ppc64 (which
>> >> > exists in both little and big endian variants) I've figured the comments
>> >> > for various drm fourcc formats in the header file don't match reality.
>> >> >
>> >> > Comments says the RGB formats are little endian, but in practice they
>> >> > are native endian.  Look at the drm_mode_legacy_fb_format() helper.  It
>> >> > maps -- for example -- bpp/depth 32/24 to DRM_FORMAT_XRGB8888, no matter
>> >> > whenever the machine is little endian or big endian.  The users of this
>> >> > function (fbdev emulation, DRM_IOCTL_MODE_ADDFB) expect the framebuffer
>> >> > is native endian, not little endian.  Most userspace also operates on
>> >> > native endian only.
>> >> >
>> >> > So, go update the comments for all 16+24+32 bpp RGB formats.
>> >> >
>> >> > Leaving the yuv formats as-is.  I have no idea if and how those are used
>> >> > on bigendian machines.
>> >>
>> >> I think this is premature. The current situation is that I can't get
>> >> modetest to work *at all* on my NV34 / BE setup (I mean, it runs, just
>> >> the colors displayed are wrong). I believe that currently it packs
>> >> things in "cpu native endian". I've tried futzing with that without
>> >> much success, although I didn't spend too much time on it. I have a
>> >> NV34 plugged into my LE setup as well although I haven't tested to
>> >> double-check that it all works there. However I'm quite sure it used
>> >> to, as I used modetest to help develop the YUV overlay support for
>> >> those GPUs.
>> >
>> > I just took a quick stab at fixing modetest to respect the current
>> > wording in drm_fourcc.h:
>> >
>> > git://github.com/vsyrjala/libdrm.git modetest_endian
>>
>> Looks like there was some careless testing on my part :( So ... it
>> looks like the current modetest without those changes does, in fact,
>> work on NV34/BE. With the changes, it breaks (and the handling of the
>> b* modes is a little broken in those patches -- they're not selectable
>> from the cmdline.) Which means that, as Michel & co predicted, it
>> appears to be taking BE input not LE input. This is very surprising to
>> me, but it is what it is. As I mentioned before, the details of how
>> the "BE" mode works on the GPUs is largely unknown to us beyond a few
>> basics. Note that only XR24 works, AR24 ends up with all black
>> displayed. This also happens on LE.
>
> Did you try 8bpp or 16bpp formats? I expect that if you've just blindly
> enabled some magic byte swapper in the hardware it will only for
> a specific pixel size.

Thankfully dispnv04 exposes no such madness - just XR24 (and AR24,
although that doesn't appear functional). Yes, it's likely that
there's a byteswap happening somewhere. In fact the copy engines have
parameters somewhere to tell how the swap should be done (basically
what the element size is). I don't quite know how to set that though
on this generation. I should poke at VRAM via the mmio peephole and
see what's actually being stored. Although of course MMIO accesses are
also auto-byteswapped. It's all just one big massive headache.

  -ilia