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

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Date:   Mon, 25 Oct 2021 12:45:16 +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 2021-10-24 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/drm/parameters/<name>, which I suspect doesn't work with the patch as is (due to the 0600 permissions).


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