Message-ID: <CAMuHMdWoC6SyDCRvBfGUj6CabmofLEhzkxEnEuPJ3mKMCwvw9A@mail.gmail.com>
Date: Wed, 18 Dec 2024 15:25:05 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Petr Mladek <pmladek@...e.com>
Cc: Jocelyn Falempe <jfalempe@...hat.com>, 
	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>, 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

Hi Petr,

On Wed, Dec 18, 2024 at 3:18 PM Petr Mladek <pmladek@...e.com> wrote:
> On Wed 2024-12-18 12:41:39, Jocelyn Falempe wrote:
> > On 18/12/2024 12:00, Geert Uytterhoeven wrote:
> > > 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.
>
> add_preferred_console() is used when the console should get enabled
> intentionally. I would split the intentions into two categories:
>
>   + requested by the end-user on the command line, see
>        __add_preferred_console(..., true) in console_setup()
>
>   + enabled by default by a hardware definition (manufacture), see
>        add_preferred_console() in:
>
>         + of_console_check(): generic solution via device tree
>         + acpi_parse_spcr(): generic solution via SPCR table
>         + *: hardcoded in few more HW-specific drivers
>
> add_preferred_console() causes the console will always get enabled
> when it is successfully initialized.
>
> So, should the "drm_log" console get always enabled?

Good question!

> > 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.
>
> I guess that the drm_log consoles should get enabled only when
> requested by the user => documenting the command line parameter
> is the right solution here.

There are actually two ways to request it:
  1. Through CONFIG_DRM_CLIENT_DEFAULT, at kernel configuration
     time,
  2. Through drm_client_lib.active=log, at kernel boot or module load time.

Currently both options need console=drm_log on top of that, which
sounds like overkill?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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