Re: [PATCH 1/3] drm/modeset: Prime modeset lock vs dma_resv

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Nov 20, 2019 at 09:34:03AM +0100, Christian König wrote:
> Am 19.11.19 um 22:08 schrieb Daniel Vetter:
> > It's kinda really hard to get this wrong on a driver with both display
> > and dma_resv locking. But who ever knows, so better to make sure that
> > really all drivers nest these the same way.
> > 
> > For actual lock semantics the acquire context nesting doesn't matter.
> > But to teach lockdep what's going on with ww_mutex the acquire ctx is
> > a fake lockdep lock, hence from a lockdep pov it does matter. That's
> > why I figured better to include it.
> > 
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Christian König <christian.koenig@amd.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> 
> Why not add another __init function like we did for dma_resv? That looked
> rather clean to me.

Hm, I've only done that because callers of dma_resv_init where holding
lots of locks already (ttm ghost objects). At least in i915 we try to do
all lockdep priming as close as possible to the mutex_init calls, so it's
all together. Since often there's no separate obj_init function, and you
need to use the same mutex_init to make sure you have the same static
lockdep key.

No strong opinion here from me, just wanted to explain why I'm biased to
this way of doing things.

> Either why feel free to add an Acked-by: Christian König
> <christian.koenig@amd.com> to the patch.

Thanks. Can you pls also look at patch 2, at least from a ttm/amdgpu pov?

Cheers, Daniel

> 
> Regards,
> Christian.
> 
> > ---
> >   drivers/gpu/drm/drm_mode_config.c | 28 ++++++++++++++++++++++++++++
> >   1 file changed, 28 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/drm_mode_config.c b/drivers/gpu/drm/drm_mode_config.c
> > index 3b570a404933..08e6eff6a179 100644
> > --- a/drivers/gpu/drm/drm_mode_config.c
> > +++ b/drivers/gpu/drm/drm_mode_config.c
> > @@ -27,6 +27,7 @@
> >   #include <drm/drm_file.h>
> >   #include <drm/drm_mode_config.h>
> >   #include <drm/drm_print.h>
> > +#include <linux/dma-resv.h>
> >   #include "drm_crtc_internal.h"
> >   #include "drm_internal.h"
> > @@ -415,6 +416,33 @@ void drm_mode_config_init(struct drm_device *dev)
> >   	dev->mode_config.num_crtc = 0;
> >   	dev->mode_config.num_encoder = 0;
> >   	dev->mode_config.num_total_plane = 0;
> > +
> > +	if (IS_ENABLED(CONFIG_LOCKDEP)) {
> > +		struct drm_modeset_acquire_ctx modeset_ctx;
> > +		struct ww_acquire_ctx resv_ctx;
> > +		struct dma_resv resv;
> > +		int ret;
> > +
> > +		dma_resv_init(&resv);
> > +
> > +		drm_modeset_acquire_init(&modeset_ctx, 0);
> > +		ret = drm_modeset_lock(&dev->mode_config.connection_mutex,
> > +				       &modeset_ctx);
> > +		if (ret == -EDEADLK)
> > +			ret = drm_modeset_backoff(&modeset_ctx);
> > +
> > +		ww_acquire_init(&resv_ctx, &reservation_ww_class);
> > +		ret = dma_resv_lock(&resv, &resv_ctx);
> > +		if (ret == -EDEADLK)
> > +			dma_resv_lock_slow(&resv, &resv_ctx);
> > +
> > +		dma_resv_unlock(&resv);
> > +		ww_acquire_fini(&resv_ctx);
> > +
> > +		drm_modeset_drop_locks(&modeset_ctx);
> > +		drm_modeset_acquire_fini(&modeset_ctx);
> > +		dma_resv_fini(&resv);
> > +	}
> >   }
> >   EXPORT_SYMBOL(drm_mode_config_init);
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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