On Mon, Oct 25, 2021 at 8:28 AM Javier Martinez Canillas <javierm@xxxxxxxxxx> wrote: > > 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 ? > I would think that the 600 permissions would still permit it, since the root user can still access and manipulate it. -- 真実はいつも一つ！/ Always, there's only one truth!
