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

[Jocelyn Falempe]

On 18/12/2024 15:18, Petr Mladek wrote:
> 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 at redhat.com> wrote:
>>>> On 17/12/2024 15:54, Geert Uytterhoeven wrote:
>>>>> 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.
>>>
>>> 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?

drm_log is a replacement for fbcon, which is always enabled, so I think 
it should also be always enabled. Otherwise you won't get any console as 
fbcon is no more available.
drm_log doesn't really fit in the architecture/hardware model, because 
drm drivers are available for a wide range of architecture, and a GPU 
can do either fbdev/fbcon or drm_log.

> 
> 
>> 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.

Most embedded linux specify the console on the command line, but for 
laptop/desktop distributions, it's not the case as fbcon is the default 
since the beginning.

I see a few options here:
1) Use add_preferred_console("drm_log") if DRM_CLIENT_LOG is enabled for 
x86_64 and maybe arm64, so at least the main users are covered.
2) Have a DRM_CLIENT_LOG_PREFERRED_CONSOLE config, so that it's easier 
to setup than changing the kernel command line.
3) Use the kernel command line.

Best Regards,

-- 

Jocelyn

> 
> Best Regards,
> Petr
>