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

[Ville Syrjälä]

On Mon, Apr 24, 2017 at 03:57:02PM +0900, Michel Dänzer wrote:
> On 22/04/17 07:05 PM, Ville Syrjälä wrote:
> > On Fri, Apr 21, 2017 at 06:14:31PM +0200, Gerd Hoffmann wrote:
> >>   Hi,
> >>
> >>>> 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 ...
> >>
> >> Ok, added some printk's to the ADDFB and ADDFB2 code paths and tested a
> >> bit.  Apparently pretty much all userspace still uses the ADDFB ioctl.
> >> xorg (modesetting driver) does.  gnome-shell in wayland mode does.
> >> Seems the big transition to ADDFB2 didn't happen yet.
> >>
> >> I guess that makes changing drm_mode_legacy_fb_format + drivers a
> >> reasonable option ...
> > 
> > Yeah, I came to the same conclusion after chatting with some
> > folks on irc.
> > 
> > So my current idea is that we change any driver that wants to follow the
> > CPU endianness
> 
> This isn't really optional for various reasons, some of which have been
> covered in this discussion.
> 
> 
> > to declare support for big endian formats if the CPU is
> > big endian. Presumably these are mostly the virtual GPU drivers.
> > 
> > Additonally we'll make the mapping performed by drm_mode_legacy_fb_format()
> > driver controlled. That way drivers that got changed to follow CPU
> > endianness can return a framebuffer that matches CPU endianness. And
> > drivers that expect the GPU endianness to not depend on the CPU
> > endianness will keep working as they do now. The downside is that users
> > of the legacy addfb ioctl will need to magically know which endianness
> > they will get, but that is apparently already the case. And users of
> > addfb2 will keep on specifying the endianness explicitly with
> > DRM_FORMAT_BIG_ENDIAN vs. 0.
> 
> I'm afraid it's not that simple.
> 
> The display hardware of older (pre-R600 generation) Radeon GPUs does not
> support the "big endian" formats directly. In order to allow userspace
> to access pixel data in native endianness with the CPU, we instead use
> byte-swapping functionality which only affects CPU access.

OK, I'm getting confused. Based on our irc discussion I got the
impression you don't byte swap CPU accesses. But since you do, how
do you deal with mixing 8bpp vs. 16bpp vs. 32bpp?

> This means
> that the GPU and CPU effectively see different representations of the
> same video memory contents.
> 
> Userspace code dealing with GPU access to pixel data needs to know the
> format as seen by the GPU, whereas code dealing with CPU access needs to
> know the format as seen by the CPU. I don't see any way to express this
> with a single format definition.

Hmm. Well that certainly makes life even more interesting.

-- 
Ville Syrjälä
Intel OTC