On Wed, 4 Jul 2018, Noralf Trønnes wrote:
AFAIU calling unregister_framebuffer() with open fd's is just fine as long as fb_info with buffers stay intact. All it does is to remove the fbX from userspace. Cleanup can be done in fb_ops->fb_destroy.
I have been working on generic fbdev emulation for DRM [1] and I did a test now to see what would happen if I did unbind the driver from the device. It worked as expected if I didn't have another fbdev present, but if there is an fb0 and I remove fb1 with a console on it, I would sometimes get crashes, often with a call to cursor_timer_handler() in the traceback.
I think there's index mixup in fbcon_fb_unbind(), at least this change seems to solve the immediate problem:
diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c index 5fb156bdcf4e..271b9b988b73 100644 --- a/drivers/video/fbdev/core/fbcon.c +++ b/drivers/video/fbdev/core/fbcon.c @@ -3066,7 +3072,7 @@ static int fbcon_fb_unbind(int idx) for (i = first_fb_vc; i <= last_fb_vc; i++) { if (con2fb_map[i] != idx && con2fb_map[i] != -1) { - new_idx = i; + new_idx = con2fb_map[i]; break; } }
Reviewed-by: Mikulas Patocka mpatocka@redhat.com
Yes - that's a good point. i is virtual console index and it is assigned to new_idx and new_idx is interpreted as a framebuffer device index.
I haven't got time to follow up on this now, but making sure DRM generic fbdev emulation device unplug works is on my TODO.
BTW, I believe the udl drm driver should be able to use the generic fbdev emulation if it had a drm_driver->gem_prime_vmap hook. It uses a shadow buffer which would also make fbdev mmap work for udl. (shmem buffers and fbdev deferred I/O doesn't work together since they both use page->lru/mapping)
I have sent patch for this: https://patchwork.kernel.org/patch/10445393/
Mikulas