Virtualization - Apr 2021 - [PATCH v4 0/9] drm: Support simple-framebuffer devices and firmware fbs

On Mon, Apr 19, 2021 at 10:00:56AM +0200, Geert Uytterhoeven
wrote:
> Hi Thomas,
> 
> On Fri, Apr 16, 2021 at 11:00 AM Thomas Zimmermann <tzimmermann at
suse.de> wrote:
> > This patchset adds support for simple-framebuffer platform devices and
> > a handover mechanism for native drivers to take-over control of the
> > hardware.
> >
> > The new driver, called simpledrm, binds to a simple-frambuffer
platform
> > device. The kernel's boot code creates such devices for
firmware-provided
> > framebuffers, such as EFI-GOP or VESA. Typically the BIOS, UEFI or
boot
> > loader sets up the framebuffers. Description via device tree is also
an
> > option.
> 
> I guess this can be used as a replacement for offb, too...
> 
> > Patches 4 to 8 add the simpledrm driver. It's build on simple DRM
helpers
> > and SHMEM. It supports 16-bit, 24-bit and 32-bit RGB framebuffers.
During
> 
> .... if support for 8-bit frame buffers would be added?
Is that 8-bit greyscale or 8-bit indexed with 256 entry palette? Former
shouldn't be a big thing, but the latter is only really supported by the
overall drm ecosystem in theory. Most userspace assumes that xrgb8888
works, and we keep that illusion up by emulating it in kernel for hw which
just doesn't support it. But reformatting xrgb8888 to c8 is tricky at
best. The uapis are all there for setting the palette, and C8 is a defined
format even with atomic kms interface, but really there's not much
userspace for it. In other words, it would work as well as current offb
would, but that's at least that.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Hi Daniel,

On Tue, Apr 20, 2021 at 10:46 AM Daniel Vetter <daniel at ffwll.ch>
wrote:
> On Mon, Apr 19, 2021 at 10:00:56AM +0200, Geert Uytterhoeven wrote:
> > On Fri, Apr 16, 2021 at 11:00 AM Thomas Zimmermann <tzimmermann at
suse.de> wrote:
> > > This patchset adds support for simple-framebuffer platform
devices and
> > > a handover mechanism for native drivers to take-over control of
the
> > > hardware.
> > >
> > > The new driver, called simpledrm, binds to a simple-frambuffer
platform
> > > device. The kernel's boot code creates such devices for
firmware-provided
> > > framebuffers, such as EFI-GOP or VESA. Typically the BIOS, UEFI
or boot
> > > loader sets up the framebuffers. Description via device tree is
also an
> > > option.
> >
> > I guess this can be used as a replacement for offb, too...
> >
> > > Patches 4 to 8 add the simpledrm driver. It's build on simple
DRM helpers
> > > and SHMEM. It supports 16-bit, 24-bit and 32-bit RGB
framebuffers. During
> >
> > .... if support for 8-bit frame buffers would be added?
>
> Is that 8-bit greyscale or 8-bit indexed with 256 entry palette? Former
8-bit indexed with 256 entry palette.
> shouldn't be a big thing, but the latter is only really supported by
the
> overall drm ecosystem in theory. Most userspace assumes that xrgb8888
> works, and we keep that illusion up by emulating it in kernel for hw which
> just doesn't support it. But reformatting xrgb8888 to c8 is tricky at
> best. The uapis are all there for setting the palette, and C8 is a defined
> format even with atomic kms interface, but really there's not much
> userspace for it. In other words, it would work as well as current offb
> would, but that's at least that.
Oh, that's good to know!
Does this mean fbdev is not deprecated for anything <= C8? ;-)

A while ago, I was looking into converting an fbdev driver to drm, and
one of the things I ran into is lack of C4, C2, C1, Y8, Y4, Y2, and
monochrome support.  On top of that, lots of internal code seems to
assume pixels are never smaller than a byte (thus ignoring
char_per_block[]/block_w).  The lack of support for planar modes isn't
that bad, combined with the need for copying, as c2p conversion can be
done while copying, thus even making life easier for userspace
applications that can just always work on chunky data.
Then real work kicked in, before I got anything working...

Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at
linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or
something like that.
                                -- Linus Torvalds

Hi,
> > > Patches 4 to 8 add the simpledrm driver. It's build on simple
DRM helpers
> > > and SHMEM. It supports 16-bit, 24-bit and 32-bit RGB
framebuffers. During
> > 
> > .... if support for 8-bit frame buffers would be added?
> 
> Is that 8-bit greyscale or 8-bit indexed with 256 entry palette? Former
> shouldn't be a big thing, but the latter is only really supported by
the
> overall drm ecosystem in theory. Most userspace assumes that xrgb8888
> works, and we keep that illusion up by emulating it in kernel for hw which
> just doesn't support it. But reformatting xrgb8888 to c8 is tricky at
> best.
Well.  cirrus converts xrgb8888 on the fly to rgb888 or rgb565
(depending on display resolution).  We could pull off the same trick
here and convert to rgb332 (assuming we can program the palette with the
color cube needed for that).  Wouldn't look pretty, but would probably
work better than expecting userspace know what color palettes are in
2021 ...

take care,
  Gerd

Hi

Am 20.04.21 um 10:46 schrieb Daniel Vetter:
> On Mon, Apr 19, 2021 at 10:00:56AM +0200, Geert Uytterhoeven wrote:
>> Hi Thomas,
>>
>> On Fri, Apr 16, 2021 at 11:00 AM Thomas Zimmermann <tzimmermann at
suse.de> wrote:
>>> This patchset adds support for simple-framebuffer platform devices
and
>>> a handover mechanism for native drivers to take-over control of the
>>> hardware.
>>>
>>> The new driver, called simpledrm, binds to a simple-frambuffer
platform
>>> device. The kernel's boot code creates such devices for
firmware-provided
>>> framebuffers, such as EFI-GOP or VESA. Typically the BIOS, UEFI or
boot
>>> loader sets up the framebuffers. Description via device tree is
also an
>>> option.
>>
>> I guess this can be used as a replacement for offb, too...
>>
>>> Patches 4 to 8 add the simpledrm driver. It's build on simple
DRM helpers
>>> and SHMEM. It supports 16-bit, 24-bit and 32-bit RGB framebuffers.
During
>>
>> .... if support for 8-bit frame buffers would be added?
Offb doesn't seem to be tied to the simple-framebuffer support. So 
adding a new driver or extending the simple-framebuffer code would be 
required. Not a big deal, though. Patch 3 of this patchset adds the 
ability to create generic drivers within DRM.
> 
> Is that 8-bit greyscale or 8-bit indexed with 256 entry palette? Former
> shouldn't be a big thing, but the latter is only really supported by
the
> overall drm ecosystem in theory. Most userspace assumes that xrgb8888
> works, and we keep that illusion up by emulating it in kernel for hw which
> just doesn't support it. But reformatting xrgb8888 to c8 is tricky at
> best. The uapis are all there for setting the palette, and C8 is a defined
> format even with atomic kms interface, but really there's not much
> userspace for it. In other words, it would work as well as current offb
> would, but that's at least that.
I think we can just use a shadow palette in the drm driver: If the drm 
framebuffer is in C8, use the userspace's palette. If the drm 
framebuffer is in XRGB, use a palette that represents RGB332. The driver 
would do on-the-fly conversion; just like cirrus does.

Best regards
Thomas
> -Daniel
> 
-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 N?rnberg, Germany
(HRB 36809, AG N?rnberg)
Gesch?ftsf?hrer: Felix Imend?rffer

-------------- next part --------------
A non-text attachment was scrubbed...
Name: OpenPGP_signature
Type: application/pgp-signature
Size: 840 bytes
Desc: OpenPGP digital signature
URL:
<http://lists.linuxfoundation.org/pipermail/virtualization/attachments/20210426/eb361a12/attachment.sig>