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

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 13.03.19 um 16:38 schrieb Michel Dänzer:
> On 2019-03-13 2:37 p.m., Christian König wrote:
>> Am 13.03.19 um 14:31 schrieb Ville Syrjälä:
>>> On Wed, Mar 13, 2019 at 10:35:08AM +0100, Michel Dänzer wrote:
>>>> On 2019-03-12 6:15 p.m., 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.
>>>> I'm afraid that won't help, at least not without porting big chunks of
>>>> https://gitlab.freedesktop.org/mesa/mesa/tree/master/src/amd/addrlib
>>>> into the kernel, none of which will be used for anything else.
>>>>
>>>>
>>>>>>> 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.
>>>> Copy how? Using a GPU engine?
>>> CPU maybe? Though I suppose that won't work if the buffer isn't CPU
>>> accesible :/
>> Well we do have a debug path for accessing invisible memory with the CPU.
>>
>> E.g. three registers: DATA and auto increment OFFSET_LO/HI. So you can
>> just read/write DATA over and over again if you want to access some memory.
> Right. I assume that'll be very slow, but I guess it could do when the
> memory isn't directly CPU accessible.

Just made a quick test and reading 33423360 bytes (4096x2040x4) using 
that interfaces takes about 13 seconds.

IIRC we don't use the auto increment optimization yet, so that can 
probably be improved by a factor of 3 or more.

>> But turning of tilling etc is still extremely tricky when the system is
>> already unstable.
> Maybe we could add a little hook to the display code, which just
> disables tiling for scanout and maybe disables non-primary planes, but
> doesn't touch anything else. Harry / Nicholas, does that seem feasible?
>
>
> I'm coming around from "this is never going to work" to "it might
> actually work" with our hardware...

Yeah, agree. It's a bit tricky, but doable.

Takeaway for Noralf is that this whole vmap on panic won't even remotely 
work. We need to get the data byte by byte without a page mapping if 
that is ever going to fly.

Christian.

>
>

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