On 10/10/23 11:25, Maxime Ripard wrote: > > > On Tue, Oct 10, 2023 at 10:55:09AM +0200, Thomas Zimmermann wrote: >>>> So if I understand correctly, drm_panic would pre-allocate a plane/commit, >>>> and use that when a panic occurs ? >>> >>> And have it checked already, yes. We would only wait for a panic to >>> happen to pull the trigger on the commit. >>> >>>> I have two concern about this approach: >>>> - How much memory would be allocated for this ? a whole framebuffer can be >>>> big for just this use case. >> >> As I outlined in my email at [1], there are a number of different scenarios. >> The question of atomic state and commits is entirely separate from the DRM >> panic handler. We should not throw them together. Whatever is necessary is >> get a scanout buffer, should happen on the driver side of >> get_scanout_buffer, not on the drm_panic side. >> >> [1] https://lore.kernel.org/dri-devel/39bd4c35-8a61-42ee-8675-ccea4f5d4ac6@xxxxxxx/T/#m22f116e9438e00a5f0a9dc43987d4153424f8be1 >> >>> >>> I'dd expect a whole framebuffer for the current >>> configuration/resolution. It would be typically 4MB for a full-HD system >>> which isn't a lot really and I guess we can always add an option to >>> disable the mechanism if needed. >>> >>>> - I find it risky to completely reconfigure the hardware in a panic handler. >>> >>> I would expect to only change the format and base address of the >>> framebuffer. I guess it can fail, but it doesn't seem that different to >>> the async plane update we already have and works well. >> >> The one thing I don't understand is: Why should we use atomic commits in the >> first place? It doesn't make sense for firmware-based drivers. > > Because this is generic infrastructure that is valuable for any drivers > and not only firmware-based drivers? > >> In some drivers, even the simple ast, we hold locks during the regular >> commit. Trying to run the panic commit concurrently will likely give a >> deadlock. > > You're in the middle of a panic. Don't take any locks and you won't deadlock. > >> In the end it's a per-driver decision, but in most cases, the driver can >> easily switch to a default mode with some ad-hoc code. > > When was the last time a per-driver decision has been a good thing? I'm > sorry, but the get_scanout_buffer approach buffer won't work for any > driver out there. > > I'm fine with discussing alternatives if you don't like the ones I > suggested, but they must allow the panic handler infrastructure to work > with any driver we have, not just 4. > Why can't we use the model[1] suggested by Daniel using a draw_pixel callback giving drivers full control on how they can put a pixel on the display? This will even work for the AMD debug interface. In the linear CPU accessible buffer case, we can provide a helper for that, maybe we can do helpers for other common cases as well. Adding to that we would need a panic_setup/enter and panic_teardown/exit callback. Noralf. [1] https://lore.kernel.org/dri-devel/20160810091529.GQ6232@phenom.ffwll.local/
