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

[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.
> 
> With a discrete GPU having a large amount of VRAM, the framebuffer may
> not be directly CPU accessible at all.
> 
> 
> 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
* there may be other planes obscuring part or all of the
  primary plane

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.

-- 
Ville Syrjälä
Intel