linux-kernel - Re: [PATCH v2 31/37] drm: rcar-du: Add support for the nomodeset kernel parameter

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Message-ID: <9ae2c7ba-133e-7c6f-190b-97158f241c7f@redhat.com>
Date:   Fri, 28 Jan 2022 12:09:05 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 31/37] drm: rcar-du: Add support for the nomodeset
 kernel parameter

On 1/28/22 11:46, Thomas Zimmermann wrote:
> Am 28.01.22 um 11:34 schrieb Laurent Pinchart:

[snip]

>>>
>>> It cannot be 'mode-setted'. We get a pre-configured framebuffer from the
>>> firmware or bootloader. Whatever we draw there shows up on the screen.
>>
>> I doubt that's going to work as you expect, clocks and regulators will
>> get disabled at boot if not used by any driver.
> 
> Simpledrm and simplefb attach to these firmware framebuffers. Both 
> drivers look at the device tree nodes to acquire the relevant clocks and 
> regulators.
>

Laurent is correct that in some cases it might be an issue. For example, if
a SystemReady ES image is booted using the U-boot EFI stub instead of bootz.

In this case the system will be presented as EFI and the firmware framebuffer
just taken from the EFI GOP. There won't be any Device Tree node describing
the clocks, regulators, etc that are required for the framebuffer to work.

The user then will have to pass clk_ignore_unused, pd_ignore_unused and maybe
other cmdline params (or even hack a DT to set regulator-always-on for needed
regulators).

But that is also true if the user wants to disable the real DRM driver with
modprobe.blacklist=rcar-du-drm.

What this patch does is just to make all DRM drivers consistent and honour the
nomodeset param (whose name is really misleading but we can't change that).

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