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

[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, 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,
};