linux-kernel - Re: [RFC PATCH v2 0/5] drm/log: Introduce a new boot logger to draw the kmsg on the screen

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Message-ID: <3578804.V25eIC5XRa@nerdopolis2>
Date: Sat, 17 Aug 2024 09:45:22 -0400
From: nerdopolis <bluescreen_avenger@...izon.net>
To: 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>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Cc: Jocelyn Falempe <jfalempe@...hat.com>
Subject: Re: [RFC PATCH v2 0/5] drm/log: Introduce a new boot logger to draw the kmsg
 on the screen

On Friday, August 16, 2024 8:52:32 AM EDT Jocelyn Falempe 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 a circular buffer so the console->write() callback is very quick, and will never stall.
>   * Drawing is done asynchronously using a workqueue.
>   * drm_log can only be built-in (and drm must be built-in too).
>     The reason is that, if you build it as a module, then a userspace application will be more appropriate than this module.
>   * When nbcon will be ready, I will use it. It should simplify this a lot, but I prefer not to depend on it yet.
>  
> The first patch is not for review/merge, it's a squash of my pending drm_panic series:
> https://patchwork.freedesktop.org/series/135944/
> 
> The second patch, moves the drawing function from drm_panic.c, to drm_draw.c, so they can be re-used by drm_log.
> The next patches are the actual drm_log implementation.
> 
> v2:
>  * Use vmap_local() api, with that change, I've tested it successfully on simpledrm, virtio-gpu, amdgpu, and nouveau.
>  * Stop drawing when the drm_master is taken. This avoid wasting CPU cycle if the buffer is not visible.
>  * Use deferred probe. Only do the probe the first time there is a log to draw. With this, if you boot with quiet, drm_log won't do any modeset.
>  * Add color support for the timestamp prefix, like what dmesg does.
>  * Add build dependency on  disabling the fbdev emulation, as they are both drm_client, and there is no way to choose which one gets the focus.
> 
> Thanks and best regards,
> 
> 
I tested it, and it seems pretty cool. And fast, it seems to keep up when I echo 100,000 lines to /dev/kmsg with seq