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

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Message-ID: <1492780323.25675.45.camel@redhat.com>
Date:   Fri, 21 Apr 2017 15:12:03 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Christian König <christian.koenig@....com>
Cc:     Ville Syrjälä 
        <ville.syrjala@...ux.intel.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        Michel Dänzer <michel@...nzer.net>,
        open list <linux-kernel@...r.kernel.org>,
        dri-devel@...ts.freedesktop.org,
        Pekka Paalanen <ppaalanen@...il.com>,
        Sean Paul <seanpaul@...omium.org>,
        amd-gfx@...ts.freedesktop.org,
        Alex Deucher <alexdeucher@...il.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Ilia Mirkin <imirkin@...m.mit.edu>
Subject: Re: [PATCH] drm: fourcc byteorder: brings header file comments in
 line with reality.

  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

> 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 ...

cheers,
  Gerd