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: <f407194f-ae8e-4e0d-96af-9984e8f2123c@redhat.com>
Date: Wed, 18 Dec 2024 12:41:39 +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 18/12/2024 12:00, Geert Uytterhoeven wrote:
> Hi Jocelyn,
> 
> On Wed, Dec 18, 2024 at 11:14 AM Jocelyn Falempe <jfalempe@...hat.com> wrote:
>> On 17/12/2024 15:54, Geert Uytterhoeven wrote:
>>> On Tue, Dec 17, 2024 at 3:46 PM Jocelyn Falempe <jfalempe@...hat.com> wrote:
>>>> On 17/12/2024 15:19, Geert Uytterhoeven wrote:
>>>>> 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?
>>>
>>> Nothing interesting to see. Compared to the fbdev client:
>>>
>>>        rcar-du feb00000.display: [drm] Registered 2 planes with drm panic
>>>        [drm] Initialized rcar-du 1.0.0 for feb00000.display on minor 0
>>>        rcar-du feb00000.display: [drm] Device feb00000.display probed
>>>       -Console: switching to colour frame buffer device 240x67
>>>       -rcar-du feb00000.display: [drm] fb0: rcar-dudrmfb frame buffer device
>>>
>>> I did verify (by adding my own debug prints) that the code does
>>> get to the success case in drm_log_register().
>>> Thanks!
>>
>> Maybe you need to add console=drm_log to your kernel command line, so
>> the kernel will actually use this console.
> 
> Thanks, that does the trick!
> 
> Note that I do not need to specify any console= kernel command line
> parameter for the fbdev console.

Yes, the fbcon console is tty0, which is hardcoded for historical reason.
Some architectures use add_preferred_console() to enable specific 
consoles, I'm not sure it's allowed to use that from the 
drm_log_register() code.

I will still send a patch to add update the Kconfig help for drm_log, as 
this command line argument is required to have it working.

Best regards,

-- 

Jocelyn

> 
> With
> 
>      CONFIG_VT_CONSOLE=y
>      CONFIG_DRM_CLIENT_DEFAULT_FBDEV=y
> 
> I see all console messages on both the emulated fbdev console and on
> the serial console by default.
> 
> Gr{oetje,eeting}s,
> 
>                          Geert
>