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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Mar 12, 2019 at 06:37:57PM +0100, Noralf Trønnes wrote:
> 
> 
> 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@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.

Some drivers/devices may have working (and hopefully even reliable)
gpu reset, some may not.

-- 
Ville Syrjälä
Intel
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