Date:   Thu, 22 Dec 2022 20:41:59 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Mario Limonciello <mario.limonciello@....com>,
        Alex Deucher <alexander.deucher@....com>,
        linux-efi@...r.kernel.org, Thomas Zimmermann <tzimmermann@...e.de>
Cc:     Carlos Soriano Sanchez <csoriano@...hat.com>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, christian.koenig@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] Recover from failure to probe GPU

[adding Thomas Zimmermann to CC list]

Hello Mario,

Interesting case.

On 12/22/22 19:30, Mario Limonciello wrote:
> One of the first thing that KMS drivers do during initialization is
> destroy the system firmware framebuffer by means of
> `drm_aperture_remove_conflicting_pci_framebuffers`
>

The reason why that's done at the very beginning is that there are no
guarantees that the firmware-provided framebuffer would keep working
after the real display controller driver re-initializes the IP block.

> This means that if for any reason the GPU failed to probe the user
> will be stuck with at best a screen frozen at the last thing that
> was shown before the KMS driver continued it's probe.
>
> The problem is most pronounced when new GPU support is introduced
> because users will need to have a recent linux-firmware snapshot
> on their system when they boot a kernel with matching support.
>

Right. That's a problem indeed but as mentioned there's a gap between
the firmware-provided framebuffer is removed and the real driver sets
up its framebuffer.
 
> However the problem is further exaggerated in the case of amdgpu because
> it has migrated to "IP discovery" where amdgpu will attempt to load
> on "ALL" AMD GPUs even if the driver is missing support for IP blocks
> contained in that GPU.
> 
> IP discovery requires some probing and isn't run until after the
> framebuffer has been destroyed.
>
> This means a situation can occur where a user purchases a new GPU not
> yet supported by a distribution and when booting the installer it will
> "freeze" even if the distribution doesn't have the matching kernel support
> for those IP blocks.
> 
> The perfect example of this is Ubuntu 21.10 and the new dGPUs just
> launched by AMD.  The installation media ships with kernel 5.19 (which
> has IP discovery) but the amdgpu support for those IP blocks landed in
> kernel 6.0. The matching linux-firmware was released after 21.10's launch.
> The screen will freeze without nomodeset. Even if a user manages to install
> and then upgrades to kernel 6.0 after install they'll still have the
> problem of missing firmware, and the same experience.
> 
> This is quite jarring for users, particularly if they don't know
> that they have to use "nomodeset" to install.
>

I'm not familiar with AMD GPUs, but could be possible that this discovery
and firmware loading step be done at the beginning before the firmware FB
is removed ? That way the FB removal will not happen unless that succeeds.
 
> To help the situation, allow drivers to re-run the init process for the
> firmware framebuffer during a failed probe. As this problem is most
> pronounced with amdgpu, this is the only driver changed.
> 
> But if this makes sense more generally for other KMS drivers, the call
> can be added to the cleanup routine for those too.
> 

The problem I see is that depending on how far the driver's probe function
went, there may not be possible to re-run the init process. Since firmware
provided framebuffer may already been destroyed or the IP block just be in
a half initialized state.

I'm not against this series if it solves the issue in practice for amdgpu,
but don't think is a general solution and would like to know Thomas' opinion
on this before as well.

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat

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