VT console blank ignored by DRM drivers on QEMU

[Takashi Iwai]

Hi,

we've casually found a weird behavior of DRM drivers on QEMU (cirrus,
bochs, virtio) via openQA: namely, VT console blank is ignored on such
drivers.  The whole screen is kept shown while the cursor blinking
stops.

I took a closer look, and it seems that drm_fb_helper_blank() just
calls drm_fb_helper_dpms(), by a naive assumption that every driver
properly implements DPMS handling.  Meanwhile bochs driver has a
code to ignore the whole DPMS hilariously.  Ditto for virtio.

(The cirrus is a bit different story; it has a DPMS implementation,
 but QEMU side seems ignoring it.)

In the fbcon side, there is a fallback to the explicit clear when the
fb_blank() returns an error, so we should be able to handle it if we
return an error properly.  But the dpms callback is a void function,
so the driver doesn't tell it for now.


I guess we have several options to address it.  An easy one would be
to provide an own fb_blank function for returning an error and use it
for the drivers for VM.  The driver can't use any longer
DRM_FB_HELPER_DEFAULT_OPS, thus it'll a bit ugly, though.

Another is to change dpms callback allowing to return an error.  But
it'd affect so many codes.

Yet another option would be to define some flag and let
drm_fb_helper_blank() returns an error.  But I also hesitate to do it
just for such a workaround.


Any suggestions?


thanks,

Takashi