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

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Date:   Sun, 24 Oct 2021 22:32:35 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     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

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.

Currently what most distros do is to pass "nomodeset" in this mode. But now
that we have the simpledrm driver, would be nice to just use the frame buffer
set by the system firmware in those cases.

Best regards,
-- 
Javier Martinez Canillas
Linux Engineering
Red Hat