[PATCH RESEND v4 v4 2/4] drm/vc4: Support nomodeset

[Javier Martinez Canillas]

Hello Maxime

On Mon, Dec 13, 2021 at 5:26 PM Maxime Ripard <maxime at cerno.tech> wrote:
>
> If we have nomodeset on the kernel command line we should have the
> firmware framebuffer driver kept as is and not try to load the
> full-blown KMS driver.
>

Patch looks good to me. I just have a question, but I'm OK with either way.

Reviewed-by: Javier Martinez Canillas <javierm at redhat.com>

> In this case, let's just register the v3d driver.
>

I wonder if the v3d driver should be registered if nomodeset is
present. Most (if not all?) drivers that currently check for this
parameter disable both KMS and DRM. So even when it seems to imply
that's about kernel mode settings, it is also used to disable DRM.

This semantic was never documented and I attempted to do that in
commit b22a15a5aca3 ("Documentation/admin-guide: Document nomodeset
kernel parameter"). After feedback from folks in the list, the text
ended as follows:

```
Disable kernel modesetting. DRM drivers will not perform
display-mode changes or accelerated rendering. Only the
system framebuffer will be available for use if this was
set-up by the firmware or boot loader.

Useful as fallback, or for testing and debugging.
```

So maybe vc4_drm_register() should just return -EINVAL if
(drm_firmware_drivers_only()) like the other drivers do?

Best regards,
Javier