[RFC][PATCH v2 0/2] drm/panic: Add a drm panic handler

[Jocelyn Falempe]

On 19/09/2023 01:19, Noralf Trønnes wrote:
> Hi,
> 
> On 9/15/23 10:28, Jocelyn Falempe wrote:
>> This introduces a new drm panic handler, which displays a message when a panic occurs.
>> So when fbcon is disabled, you can still see a kernel panic.
>>
>> This is one of the missing feature, when disabling VT/fbcon in the kernel:
>> https://www.reddit.com/r/linux/comments/10eccv9/config_vtn_in_2023/
>> Fbcon can be replaced by a userspace kms console, but the panic screen must be done in the kernel.
>>
>> This is a proof of concept, and works only with simpledrm, using a new get_scanout_buffer() api
>>
> 
> There's a panic handling entry in Documentation/gpu/todo.rst pointing to
> some work done in this area.

Thanks a lot for this pointer, I wasn't aware of this previous drm panic 
attempt. I hope this one will go a bit further ;)
Also I wasn't sure about what is allowed or not in the panic handler, 
and this doc makes things really clear.

Regarding the fbcon interactions, I prevent this at build time, I rely 
on userspace consoles, and hope they will progressively replace fbcon.

My real question is how did I missed that ? My google-fu failed me this 
time.

Best regards,

-- 

Jocelyn




> 
> Noralf.
> 
>> To test it, make sure you're using the simpledrm driver, and trigger a panic:
>> echo c > /proc/sysrq-trigger
>>
>> v2
>>   * Use get_scanout_buffer() instead of the drm client API. (Thomas Zimmermann)
>>   * Add the panic reason to the panic message (Nerdopolis)
>>   * Add an exclamation mark (Nerdopolis)
>>   
>> I didn't reuse the fbdev functions yet, that would need some fbdev refactoring, because they rely on struct fb_info, and struct vc_data (for font/console). But I still plan to at least try it for v3.
>>
>> A few more though:
>>   1) what about gpu with multiple monitor connected ?
>> maybe get_scanout_buffer() could return a list of scanout buffers ?
>>   2) I think for some GPU drivers, there might need a flush_scanout_buffer() function, that should be called after the scanout buffer has been filled ?
>>
>> Best regards,
>>
>> Jocelyn Falempe (2):
>>    drm/panic: Add a drm panic handler
>>    drm/simpledrm: Add drm_panic support
>>
>>   drivers/gpu/drm/Kconfig          |  11 ++
>>   drivers/gpu/drm/Makefile         |   1 +
>>   drivers/gpu/drm/drm_drv.c        |   3 +
>>   drivers/gpu/drm/drm_panic.c      | 270 +++++++++++++++++++++++++++++++
>>   drivers/gpu/drm/tiny/simpledrm.c |  17 ++
>>   include/drm/drm_drv.h            |  14 ++
>>   include/drm/drm_panic.h          |  41 +++++
>>   7 files changed, 357 insertions(+)
>>   create mode 100644 drivers/gpu/drm/drm_panic.c
>>   create mode 100644 include/drm/drm_panic.h
>>
>>
>> base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
>