linux-kernel - Re: [PATCH v9 0/6] drm/log: Introduce a new boot logger to draw the kmsg on the screen

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Message-ID: <301714d8-0723-4881-83e8-24523c121bfe@redhat.com>
Date: Tue, 17 Dec 2024 15:46:12 +0100
From: Jocelyn Falempe <jfalempe@...hat.com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
 John Ogness <john.ogness@...utronix.de>,
 Javier Martinez Canillas <javierm@...hat.com>,
 "Guilherme G . Piccoli" <gpiccoli@...lia.com>,
 bluescreen_avenger@...izon.net, Caleb Connolly <caleb.connolly@...aro.org>,
 Petr Mladek <pmladek@...e.com>, Jani Nikula <jani.nikula@...ux.intel.com>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH v9 0/6] drm/log: Introduce a new boot logger to draw the
 kmsg on the screen

On 17/12/2024 15:19, Geert Uytterhoeven wrote:
> Hi Jocelyn,
> 
> On Wed, Dec 4, 2024 at 6:41 PM Jocelyn Falempe <jfalempe@...hat.com> wrote:
>> drm_log is a simple logger that uses the drm_client API to print the kmsg boot log on the screen.
>> This is not a full replacement to fbcon, as it will only print the kmsg.
>> It will never handle user input, or a terminal because this is better done in userspace.
>>
>> If you're curious on how it looks like, I've put a small demo here:
>> https://people.redhat.com/jfalempe/drm_log/drm_log_draft_boot_v2.mp4
>>
>> Design decisions:
>>    * It uses the drm_client API, so it should work on all drm drivers from the start.
>>    * It doesn't scroll the message, that way it doesn't need to redraw the whole screen for each new message.
>>      It also means it doesn't have to keep drawn messages in memory, to redraw them when scrolling.
>>    * It uses the new non-blocking console API, so it should work well with PREEMPT_RT
> 
> I gave this a try on Koelsch (R-Car M2-W), using rcar-du.
> Unfortunately I don't see any kernel messages, and my monitor complains
> about no signal. Does this require special support from the driver?

It doesn't require a special support from the driver. But as it is the 
first drm client other than fbdev emulation, I'm not surprised it's 
broken on some driver.
I know it works on virtio-gpu, nouveau, amdgpu, and even on a OnePlus 6 
(Qualcomm SDM845/freedreno), without requiring driver changes.

Do you have a serial console on this device, to check if there is 
something in kmsg?

Best regards,

-- 

Jocelyn

> 
>      CONFIG_DRM_CLIENT=y
>      CONFIG_DRM_CLIENT_LIB=y
>      CONFIG_DRM_CLIENT_SELECTION=y
>      CONFIG_DRM_CLIENT_SETUP=y
>      CONFIG_DRM_CLIENT_LOG=y
>      # CONFIG_DRM_CLIENT_DEFAULT_FBDEV is not set
>      CONFIG_DRM_CLIENT_DEFAULT_LOG=y
>      CONFIG_DRM_CLIENT_DEFAULT="log"
> 
> Switching to fbdev gives a working display, as before:
> 
>      CONFIG_DRM_CLIENT_DEFAULT_FBDEV=y
>      # CONFIG_DRM_CLIENT_DEFAULT_LOG is not set
>      CONFIG_DRM_CLIENT_DEFAULT="fbdev"
> 
> Thanks!
> 
> Gr{oetje,eeting}s,
> 
>                          Geert
>