[Intel-gfx] [PATCH v2 1/3] drm: Add support for panic message output

[Noralf Trønnes]

Den 12.03.2019 18.37, skrev Noralf Trønnes:
> 
> 
> Den 12.03.2019 18.25, skrev Ville Syrjälä:
>> On Tue, Mar 12, 2019 at 06:15:24PM +0100, Noralf Trønnes wrote:
>>>
>>>
>>> Den 12.03.2019 17.17, skrev Ville Syrjälä:
>>>> On Tue, Mar 12, 2019 at 11:47:04AM +0100, Michel Dänzer wrote:
>>>>> On 2019-03-11 6:42 p.m., Noralf Trønnes wrote:
>>>>>> This adds support for outputting kernel messages on panic().
>>>>>> A kernel message dumper is used to dump the log. The dumper iterates
>>>>>> over each DRM device and it's crtc's to find suitable framebuffers.
>>>>>>
>>>>>> All the other dumpers are run before this one except mtdoops.
>>>>>> Only atomic drivers are supported.
>>>>>>
>>>>>> Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
>>>>>> ---
>>>>>>  [...]
>>>>>>
>>>>>> diff --git a/include/drm/drm_framebuffer.h b/include/drm/drm_framebuffer.h
>>>>>> index f0b34c977ec5..f3274798ecfe 100644
>>>>>> --- a/include/drm/drm_framebuffer.h
>>>>>> +++ b/include/drm/drm_framebuffer.h
>>>>>> @@ -94,6 +94,44 @@ struct drm_framebuffer_funcs {
>>>>>>  		     struct drm_file *file_priv, unsigned flags,
>>>>>>  		     unsigned color, struct drm_clip_rect *clips,
>>>>>>  		     unsigned num_clips);
>>>>>> +
>>>>>> +	/**
>>>>>> +	 * @panic_vmap:
>>>>>> +	 *
>>>>>> +	 * Optional callback for panic handling.
>>>>>> +	 *
>>>>>> +	 * For vmapping the selected framebuffer in a panic context. Must
>>>>>> +	 * be super careful about locking (only trylocking allowed).
>>>>>> +	 *
>>>>>> +	 * RETURNS:
>>>>>> +	 *
>>>>>> +	 * NULL if it didn't work out, otherwise an opaque cookie which is
>>>>>> +	 * passed to @panic_draw_xy. It can be anything: vmap area, structure
>>>>>> +	 * with more details, just a few flags, ...
>>>>>> +	 */
>>>>>> +	void *(*panic_vmap)(struct drm_framebuffer *fb);
>>>>>
>>>>> FWIW, the panic_vmap hook cannot work in general with the amdgpu/radeon
>>>>> drivers:
>>>>>
>>>>> Framebuffers are normally tiled, writing to them with the CPU results in
>>>>> garbled output.
>>>>>
>>>
>>> In which case the driver needs to support the ->panic_draw_xy callback,
>>> or maybe it's possible to make a generic helper for tiled buffers.
>>>
>>>>> With a discrete GPU having a large amount of VRAM, the framebuffer may
>>>>> not be directly CPU accessible at all.
>>>>>
>>>
>>> I would have been nice to know how Windows works around this.
>>>
>>>>>
>>>>> There would need to be a mechanism for switching scanout to a linear,
>>>>> CPU accessible framebuffer.
>>>>
>>>> I suppose panic_vmap() could just provide a linear temp buffer
>>>> to the panic handler, and panic_unmap() could copy the contents
>>>> over to the real fb.
>>>>
>>>> That said, this approach of scribbling over the primary plane's
>>>> framebuffer has some clear limitations:
>>>> * something may overwrite the oops message before the user
>>>>   can even read it
>>>
>>> When the dumper drm_panic_kmsg_dump() runs, the other CPU's should have
>>> been stopped. See panic().
>>
>> GPUs etc. may still be executing away.
>>
> 
> Would it be safe to stop it in a panic situation? It would ofc be bad to
> crash the box even harder.
> 
>>>
>>>> * there may be other planes obscuring part or all of the
>>>>   primary plane
>>>>
>>>
>>> Yeah, this is a problem, again I wonder how Windows deals with this.
>>
>> Probably just disables all other planes. Not that it uses planes
>> all that heavily.
>>
>>>
>>>> Also scribbling over the user's framebuffer seems rather rude
>>>> to me, so I'm thinking this approach should be limited to kernel
>>>> panics only.
>>>>
>>>
>>> Yes this will only happen on kernel panics:
>>>
>>> panic() -> kmsg_dump() -> drm_panic_kmsg_dump()
>>>
>>> (Unless invoking through debugfs ofc)
>>
>> I thought you set the max_level or whatever to OOPS. Doesn't that mean
>> it gets involved for non-panics as well?
>>
> 
> I do that in the debugfs code, but I can't remember why I lower level,

Now I remember, it is so I can catch the null pointer debugfs test, but
that's not necessary since this is designed for panics, so I should
remove that test.

Noralf.

> I
> think can just change the level when invoking the dumper:
>  drm_panic_file_panic_write(...)
> -		kmsg_dump(KMSG_DUMP_OOPS);
> +		kmsg_dump(KMSG_DUMP_PANIC);
> 
> 
> This is the dumper config:
> 
> static struct kmsg_dumper drm_panic_kmsg_dumper = {
> 	.dump = drm_panic_kmsg_dump,
> 	.max_reason = KMSG_DUMP_PANIC,
> };
> 
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