Hello Michel, On 10/25/21 12:45, Michel Dänzer wrote: > On 2021-10-24 22:32, Javier Martinez Canillas wrote: >> Hello Ville, >> >> On 10/22/21 21:12, Ville Syrjälä wrote: >>> On Fri, Oct 22, 2021 at 04:40:40PM +0200, Javier Martinez Canillas wrote: >>>> The simpledrm driver allows to use the frame buffer that was set-up by the >>>> firmware. This gives early video output before the platform DRM driver is >>>> probed and takes over. >>>> >>>> But it would be useful to have a way to disable this take over by the real >>>> DRM drivers. For example, there may be bugs in the DRM drivers that could >>>> cause the display output to not work correctly. >>>> >>>> For those cases, it would be good to keep the simpledrm driver instead and >>>> at least get a working display as set-up by the firmware. >>>> >>>> Let's add a drm.remove_fb boolean kernel command line parameter, that when >>>> set to false will prevent the conflicting framebuffers to being removed. >>>> >>>> Since the drivers call drm_aperture_remove_conflicting_framebuffers() very >>>> early in their probe callback, this will cause the drivers' probe to fail. >>> >>> Why is that better than just modprobe.blacklisting those drivers? >> >> Because would allow to deny list all native (as Thomas called it) DRM drivers >> and only allow the simpledrm driver to be probed. This is useful for distros, >> since could add a "Basic graphics mode" to the boot menu entries, that could >> boot the kernel passing a "drm.disable_native_drivers=1" cmdline option. >> >> That way, if there's any problem with a given DRM driver, the distro may be >> installed and booted using the simpledrm driver and troubleshoot why a native >> DRM driver is not working. Or try updating the kernel package, etc. > > For troubleshooting, it'll be helpful if this new parameter can be enabled for the boot via the kernel command line, then disabled again after boot-up. One simple possibility for this would be allowing the parameter to be changed via /sys/module That's already the case with the current patch, i.e: $ grep -o drm.* /proc/cmdline drm.disable_native_drivers=1 $ cat /proc/fb 0 simpledrm $ modprobe virtio_gpu $ dmesg [ 125.731549] [drm] pci: virtio-vga detected at 0000:00:01.0 [ 125.732410] virtio_gpu: probe of virtio0 failed with error -16 $ echo 0 > /sys/module/drm/parameters/disable_native_drivers $ modprobe virtio_gpu $ dmesg [ 187.889136] [drm] pci: virtio-vga detected at 0000:00:01.0 [ 187.894578] Console: switching to colour dummy device 80x25 [ 187.897090] virtio-pci 0000:00:01.0: vgaarb: deactivate vga console [ 187.899983] [drm] features: -virgl +edid -resource_blob -host_visible [ 187.907176] [drm] number of scanouts: 1 [ 187.907714] [drm] number of cap sets: 0 [ 187.914108] [drm] Initialized virtio_gpu 0.1.0 0 for virtio0 on minor 1 [ 187.930807] Console: switching to colour frame buffer device 128x48 [ 187.938737] virtio_gpu virtio0: [drm] fb0: virtio_gpu frame buffer device $ cat /proc/fb 0 virtio_gpu /drm/parameters/<name>, which I suspect doesn't work with the patch as is (due to the 0600 permissions). > > I followed the convention used by other drm module parameters, hence the 0600. Do you mean that for this parameter we should be less restrictive ? Best regards, -- Javier Martinez Canillas Linux Engineering Red Hat
