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

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Message-ID: <9e9ddab2-5bcd-f826-2343-d1db691a308e@daenzer.net>
Date:   Mon, 25 Oct 2021 15:24:33 +0200
From:   Michel Dänzer <michel@...nzer.net>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Peter Robinson <pbrobinson@...il.com>,
        Neal Gompa <ngompa13@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [RFC PATCH] drm/aperture: Add param to disable conflicting
 framebuffers removal

On 2021-10-25 14:28, Javier Martinez Canillas 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 ?

No, it was simply a brain fart on my part. :}

Thanks for confirming this works!


-- 
Earthling Michel Dänzer            |                  https://redhat.com
Libre software enthusiast          |         Mesa and Xwayland developer