Re: [RFC PATCH] drm/aperture: Add param to disable conflicting framebuffers removal

[Neal Gompa <ngompa13@gmail.com>]

On Mon, Oct 25, 2021 at 8:28 AM Javier Martinez Canillas
<javierm@redhat.com> 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.


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