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

[Jocelyn Falempe]

On 17/12/2024 15:54, Geert Uytterhoeven wrote:
> Hi Jocelyn.
> 
> On Tue, Dec 17, 2024 at 3:46 PM Jocelyn Falempe <jfalempe at redhat.com> wrote:
>> On 17/12/2024 15:19, Geert Uytterhoeven wrote:
>>> On Wed, Dec 4, 2024 at 6:41 PM Jocelyn Falempe <jfalempe at 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.

> 
> Gr{oetje,eeting}s,
> 
>                          Geert
>