Hello Thomas,

Thanks for the feedback. It was very useful.

On 4/25/22 11:15, Thomas Zimmermann wrote:

Hi

Am 25.04.22 um 10:54 schrieb Thomas Zimmermann:

quoted

Hi

Am 20.04.22 um 10:52 schrieb Javier Martinez Canillas:

quoted

Hello,

The patches in this series are mostly changes suggested by Daniel Vetter
to fix some race conditions that exists between the fbdev core (fbmem)
and sysfb with regard to device registration and removal.

For example, it is currently possible for sysfb to register a platform
device after a real DRM driver was registered and requested to remove the
conflicting framebuffers.

A symptom of this issue, was worked around with by commit fb561bf9abde
("fbdev: Prevent probing generic drivers if a FB is already registered")
but that's really a hack and should be reverted.

As I mentioned on IRC, I think this series should be merged for the 
reasons I give in the other comments.

You meant that should *not* get merged, as we discussed over IRC.

quoted

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This series attempt to fix it more properly and revert the mentioned 
hack.
That will also unblock a pending patch to not make the num_registered_fb
variable visible to drivers anymore, since that's internal to fbdev core.

Here's as far as I understand the problem:

  1) build DRM/fbdev and sysfb code into the kernel
  2) during boot, load the DRM/fbdev modules and have them acquire I/O 
ranges
  3) afterwards load sysfb and have it register platform devices for the 
generic framebuffers
  4) these devices now conflict with the already-registered DRM/fbdev 
devices

That's correct, yes.

quoted

If that is the problem here, let's simply set a sysfb_disable flag in 
sysfb code when the first DRM/fbdev driver first loads. With the flag 
set, sysfb won't create any platform devices. We assume that there are 
now DRM/fbdev drivers for the framebuffers and sysfb won't be needed.

We can set the flag internally from drm_aperture_detach_drivers() [1] 
and do_remove_conflicting_framebuffers() [2].

And further thinking about it, it would be better to set such a flag 
after successfully registering a DRM/fbdev device.  So we know that 
there's at least one working display in the system. We don't have to 
rely on generic framebuffers after that.

Exactly, should be done when the device is registered rather than when
the driver is registered or a call is made to remove the conflicting FB.

I'll rework this series with only the bits for sysfb_disable() and drop
the rest. We can go back to the discussion of the remaining parts later
if that makes sense (I still think that patch 3/5 is a better approach,
but let's defer that for a different series).

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat