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

[Petr Mladek <pmladek@suse.com>]

On Wed 2024-12-18 12:41:39, Jocelyn Falempe wrote:
> On 18/12/2024 12:00, Geert Uytterhoeven wrote:
> > Hi Jocelyn,
> > 
> > On Wed, Dec 18, 2024 at 11:14 AM Jocelyn Falempe <jfalempe@redhat.com> wrote:
> > > On 17/12/2024 15:54, Geert Uytterhoeven wrote:
> > > > On Tue, Dec 17, 2024 at 3:46 PM Jocelyn Falempe <jfalempe@redhat.com> wrote:
> > > > > On 17/12/2024 15:19, Geert Uytterhoeven wrote:
> > > > > > On Wed, Dec 4, 2024 at 6:41 PM Jocelyn Falempe <jfalempe@redhat.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?


> 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.

Best Regards,
Petr