Re: [PATCH 04/12] dma-buf: add optional invalidate_mappings callback v5

[Daniel Vetter]

On Thu, Apr 18, 2019 at 08:28:51AM +0000, Koenig, Christian wrote:
> Am 18.04.19 um 10:08 schrieb Daniel Vetter:
> > On Wed, Apr 17, 2019 at 09:13:22PM +0200, Christian König wrote:
> >> Am 17.04.19 um 21:07 schrieb Daniel Vetter:
> >>> On Tue, Apr 16, 2019 at 08:38:33PM +0200, Christian König wrote:
> >>>> Each importer can now provide an invalidate_mappings callback.
> >>>>
> >>>> This allows the exporter to provide the mappings without the need to pin
> >>>> the backing store.
> >>>>
> >>>> v2: don't try to invalidate mappings when the callback is NULL,
> >>>>       lock the reservation obj while using the attachments,
> >>>>       add helper to set the callback
> >>>> v3: move flag for invalidation support into the DMA-buf,
> >>>>       use new attach_info structure to set the callback
> >>>> v4: use importer_priv field instead of mangling exporter priv.
> >>>> v5: drop invalidation_supported flag
> >>>>
> >>>> Signed-off-by: Christian König <christian.koenig@xxxxxxx>
> >>>> ---
> >>>>    drivers/dma-buf/dma-buf.c | 37 +++++++++++++++++++++++++++++++++++++
> >>>>    include/linux/dma-buf.h   | 33 +++++++++++++++++++++++++++++++--
> >>>>    2 files changed, 68 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> >>>> index 83c92bfd964c..a3738fab3927 100644
> >>>> --- a/drivers/dma-buf/dma-buf.c
> >>>> +++ b/drivers/dma-buf/dma-buf.c
> >>>> @@ -563,6 +563,8 @@ struct dma_buf_attachment *dma_buf_attach(const struct dma_buf_attach_info *info
> >>>>    	attach->dev = info->dev;
> >>>>    	attach->dmabuf = dmabuf;
> >>>> +	attach->importer_priv = info->importer_priv;
> >>>> +	attach->invalidate = info->invalidate;
> >>>>    	mutex_lock(&dmabuf->lock);
> >>>> @@ -571,7 +573,9 @@ struct dma_buf_attachment *dma_buf_attach(const struct dma_buf_attach_info *info
> >>>>    		if (ret)
> >>>>    			goto err_attach;
> >>>>    	}
> >>>> +	reservation_object_lock(dmabuf->resv, NULL);
> >>>>    	list_add(&attach->node, &dmabuf->attachments);
> >>>> +	reservation_object_unlock(dmabuf->resv);
> >>>>    	mutex_unlock(&dmabuf->lock);
> >>>> @@ -615,7 +619,9 @@ void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
> >>>>    					   DMA_BIDIRECTIONAL);
> >>>>    	mutex_lock(&dmabuf->lock);
> >>>> +	reservation_object_lock(dmabuf->resv, NULL);
> >>>>    	list_del(&attach->node);
> >>>> +	reservation_object_unlock(dmabuf->resv);
> >>>>    	if (dmabuf->ops->detach)
> >>>>    		dmabuf->ops->detach(dmabuf, attach);
> >>>> @@ -653,7 +659,16 @@ dma_buf_map_attachment_locked(struct dma_buf_attachment *attach,
> >>>>    	if (attach->sgt)
> >>>>    		return attach->sgt;
> >>>> +	/*
> >>>> +	 * Mapping a DMA-buf can trigger its invalidation, prevent sending this
> >>>> +	 * event to the caller by temporary removing this attachment from the
> >>>> +	 * list.
> >>>> +	 */
> >>>> +	if (attach->invalidate)
> >>>> +		list_del(&attach->node);
> >>> Just noticed this: Why do we need this? invalidate needs the reservation
> >>> lock, as does map_attachment. It should be impssoble to have someone else
> >>> sneak in here.
> >> I was having problems with self triggered invalidations.
> >>
> >> E.g. client A tries to map an attachment, that in turn causes the buffer to
> >> move to a new place and client A is informed about that movement with an
> >> invalidation.
> > Uh, that sounds like a bug in ttm or somewhere else in the exporter. If
> > you evict the bo that you're trying to map, that's bad.
> >
> > Or maybe it's a framework bug, and we need to track whether an attachment
> > has a map or not. That would make more sense ...
> 
> Well neither, as far as I can see this is perfectly normal behavior.
> 
> We just don't want any invalidation send to a driver which is currently 
> making a mapping.
> 
> If you want I can do this in the driver as well, but at least of hand it 
> looks like a good idea to have that in common code.

Hm. This sounds like we'd want to invalidate a specific mapping.

> Tracking the mappings could work as well, but the problem here is that I 
> actually want the lifetime of old and new mappings to overlap for 
> pipelining.

Aside: Overlapping mappings being explicitly allowed should be in the
docs. The current kerneldoc for invalidate leaves that up for
interpretation. This answers one of the questions I had overnight, about
whether we expect ->invalidate to tear down the mapping or not.

Imo a better semantics would be that ->invalidate must tear down the
mapping, but the exporter must delay actual unmap until all fences have
cleared. Otherwise you could end up with fun stuff where the exporter
releases the memory (it wanted to invalidate after all), while the
importer still has a mapping around. That's not going to end well I think.

That would also solve the issue of getting an invalidate while you map, at
least if we filter per attachment.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch