Fbdev issue after the drm updates 'drm-next-2023-10-31-1'

[Geert Uytterhoeven]

Hi Christian,

On Sun, Nov 12, 2023 at 3:23 PM Christian Zigotzky
<chzigotzky at xenosoft.de> wrote:
> On 07 November 2023 at 09:36 am, Christian Zigotzky wrote:
> > I have found out that fbdev no longer works with virtio-gpu-pci and
> > virtio-vga. It is not a problem with the penguin logos.
> >
> > Could you please check fbdev in QEMU virtual machines with
> > virtio-gpu-pci and virtio-vga graphics?
>
> > On 02 November 2023 at 03:45 pm, Christian Zigotzky wrote:
> >> There is a fbdev issue with the virtio-gpu-pci and virtio-vga. (The
> >> penguins are not displayed at boot time)
> >>
> >> Error message:  [    0.889302] virtio-pci 0000:00:02.0: [drm] *ERROR*
> >> fbdev: Failed to setup generic emulation (ret=-2)
> >>
> >> The kernel 6.6 final doesn't have this issue.
> >>
> >> Please check the fbdev changes in the drm updates
> >> 'drm-next-2023-10-31-1'.

Thanks for your report!

I can confirm there is no graphics output with m68k/virt, and
bisected this to my own commit 6ae2ff23aa43a0c4 ("drm/client: Convert
drm_client_buffer_addfb() to drm_mode_addfb2()"), ouch...

It turns out the old call to drm_mode_addfb() caused a translation
from a fourcc to a bpp/depth pair to a _different_ fourcc, due to the
quirk processing in drm_driver_legacy_fb_format().
I.e. on m68k/virt, the original requested format was XR24, which was
translated to BX24. The former doesn't work, the latter works.

The following (gmail-whitespace-damaged) patch fixed the issue for me:

--- a/drivers/gpu/drm/drm_client.c
+++ b/drivers/gpu/drm/drm_client.c
@@ -400,6 +400,16 @@ static int drm_client_buffer_addfb(struct
drm_client_buffer *buffer,

        fb_req.width = width;
        fb_req.height = height;
+       if (client->dev->mode_config.quirk_addfb_prefer_host_byte_order) {
+               if (format == DRM_FORMAT_XRGB8888)
+                       format = DRM_FORMAT_HOST_XRGB8888;
+               if (format == DRM_FORMAT_ARGB8888)
+                       format = DRM_FORMAT_HOST_ARGB8888;
+               if (format == DRM_FORMAT_RGB565)
+                       format = DRM_FORMAT_HOST_RGB565;
+               if (format == DRM_FORMAT_XRGB1555)
+                       format = DRM_FORMAT_HOST_XRGB1555;
+       }
        fb_req.pixel_format = format;
        fb_req.handles[0] = handle;
        fb_req.pitches[0] = buffer->pitch;

However, I don't think we want to sprinkle more of these
translations around... So perhaps we should (re)add a call to
drm_driver_legacy_fb_format() to drm_client_buffer_addfb()?

Second, as I doubt you are using a big-endian system, you are probably
running into a slightly different issue.

Oh wait, you did CC linuxppc-dev, so perhaps you are running on a
big-endian machine?

If not, please add

    pr_info("%s: format = %p4cc\n", __func__, &format);

to drivers/gpu/drm/drm_client.c:drm_client_buffer_addfb(), and,
after reverting commit 6ae2ff23aa43a0c4, add

    pr_info("%s: bpp %u/depth %u => r.pixel_format = %p4cc\n",
__func__, or->bpp, or->depth, &r.pixel_format);

to drivers/gpu/drm/drm_framebuffer.c:drm_mode_addfb(), so we know the
translation in your case?

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