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

[Christian Zigotzky]

On 13 November 2023 at 01:48 pm, Geert Uytterhoeven wrote:

Hi Christian,

On Sun, Nov 12, 2023 at 3:23 PM Christian Zigotzky
<chzigot...@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 -- ge...@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 Geert,

Thanks for your answer! I use a big-endian system.

Cheers,
Christian