RE: [PATCH 2/3] udmabuf: Sync buffer mappings for attached devices

[Kasireddy, Vivek]

> >> Currently this driver creates a SGT table using the CPU as the
> >> target device, then performs the dma_sync operations against
> >> that SGT. This is backwards to how DMA-BUFs are supposed to behave.
> >> This may have worked for the case where these buffers were given
> >> only back to the same CPU that produced them as in the QEMU case.
> >> And only then because the original author had the dma_sync
> >> operations also backwards, syncing for the "device" on begin_cpu.
> >> This was noticed and "fixed" in this patch[0].
> >>
> >> That then meant we were sync'ing from the CPU to the CPU using
> >> a pseudo-device "miscdevice". Which then caused another issue
> >> due to the miscdevice not having a proper DMA mask (and why should
> >> it, the CPU is not a DMA device). The fix for that was an even
> >> more egregious hack[1] that declares the CPU is coherent with
> >> itself and can access its own memory space..
> >>
> >> Unwind all this and perform the correct action by doing the dma_sync
> >> operations for each device currently attached to the backing buffer.
> > Makes sense.
> >
> >>
> >> [0] commit 1ffe09590121 ("udmabuf: fix dma-buf cpu access")
> >> [1] commit 9e9fa6a9198b ("udmabuf: Set the DMA mask for the udmabuf
> >> device (v2)")
> >>
> >> Signed-off-by: Andrew Davis <afd@xxxxxx>
> >> ---
> >>   drivers/dma-buf/udmabuf.c | 41 +++++++++++++++------------------------
> >>   1 file changed, 16 insertions(+), 25 deletions(-)
> >>
> >> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> >> index 3a23f0a7d112a..ab6764322523c 100644
> >> --- a/drivers/dma-buf/udmabuf.c
> >> +++ b/drivers/dma-buf/udmabuf.c
> >> @@ -26,8 +26,6 @@ MODULE_PARM_DESC(size_limit_mb, "Max size of a
> >> dmabuf, in megabytes. Default is
> >>   struct udmabuf {
> >>   	pgoff_t pagecount;
> >>   	struct page **pages;
> >> -	struct sg_table *sg;
> >> -	struct miscdevice *device;
> >>   	struct list_head attachments;
> >>   	struct mutex lock;
> >>   };
> >> @@ -169,12 +167,8 @@ static void unmap_udmabuf(struct
> >> dma_buf_attachment *at,
> >>   static void release_udmabuf(struct dma_buf *buf)
> >>   {
> >>   	struct udmabuf *ubuf = buf->priv;
> >> -	struct device *dev = ubuf->device->this_device;
> >>   	pgoff_t pg;
> >>
> >> -	if (ubuf->sg)
> >> -		put_sg_table(dev, ubuf->sg, DMA_BIDIRECTIONAL);
> > What happens if the last importer maps the dmabuf but erroneously
> > closes it immediately? Would unmap somehow get called in this case?
> >
> 
> Good question, had to scan the framework code a bit here. I thought
> closing a DMABUF handle would automatically unwind any current
> attachments/mappings, but it seems nothing in the framework does that.
> 
> Looks like that is up to the importing drivers[0]:
> 
> > Once a driver is done with a shared buffer it needs to call
> > dma_buf_detach() (after cleaning up any mappings) and then
> > release the reference acquired with dma_buf_get() by
> > calling dma_buf_put().
> 
> So closing a DMABUF after mapping without first unmapping it would
> be a bug in the importer, it is not the exporters problem to check
It may be a bug in the importer but wouldn't the memory associated
with the sg table and attachment get leaked if unmap doesn't get called
in this scenario?

Thanks,
Vivek

> for (although some more warnings in the framework checking for that
> might not be a bad idea..).
> 
> Andrew
> 
> [0] https://www.kernel.org/doc/html/v6.7/driver-api/dma-buf.html
> 
> > Thanks,
> > Vivek
> >
> >> -
> >>   	for (pg = 0; pg < ubuf->pagecount; pg++)
> >>   		put_page(ubuf->pages[pg]);
> >>   	kfree(ubuf->pages);
> >> @@ -185,33 +179,31 @@ static int begin_cpu_udmabuf(struct dma_buf
> >> *buf,
> >>   			     enum dma_data_direction direction)
> >>   {
> >>   	struct udmabuf *ubuf = buf->priv;
> >> -	struct device *dev = ubuf->device->this_device;
> >> -	int ret = 0;
> >> -
> >> -	if (!ubuf->sg) {
> >> -		ubuf->sg = get_sg_table(dev, buf, direction);
> >> -		if (IS_ERR(ubuf->sg)) {
> >> -			ret = PTR_ERR(ubuf->sg);
> >> -			ubuf->sg = NULL;
> >> -		}
> >> -	} else {
> >> -		dma_sync_sg_for_cpu(dev, ubuf->sg->sgl, ubuf->sg->nents,
> >> -				    direction);
> >> -	}
> >> +	struct udmabuf_attachment *a;
> >>
> >> -	return ret;
> >> +	mutex_lock(&ubuf->lock);
> >> +
> >> +	list_for_each_entry(a, &ubuf->attachments, list)
> >> +		dma_sync_sgtable_for_cpu(a->dev, a->table, direction);
> >> +
> >> +	mutex_unlock(&ubuf->lock);
> >> +
> >> +	return 0;
> >>   }
> >>
> >>   static int end_cpu_udmabuf(struct dma_buf *buf,
> >>   			   enum dma_data_direction direction)
> >>   {
> >>   	struct udmabuf *ubuf = buf->priv;
> >> -	struct device *dev = ubuf->device->this_device;
> >> +	struct udmabuf_attachment *a;
> >>
> >> -	if (!ubuf->sg)
> >> -		return -EINVAL;
> >> +	mutex_lock(&ubuf->lock);
> >> +
> >> +	list_for_each_entry(a, &ubuf->attachments, list)
> >> +		dma_sync_sgtable_for_device(a->dev, a->table, direction);
> >> +
> >> +	mutex_unlock(&ubuf->lock);
> >>
> >> -	dma_sync_sg_for_device(dev, ubuf->sg->sgl, ubuf->sg->nents,
> >> direction);
> >>   	return 0;
> >>   }
> >>
> >> @@ -307,7 +299,6 @@ static long udmabuf_create(struct miscdevice
> >> *device,
> >>   	exp_info.priv = ubuf;
> >>   	exp_info.flags = O_RDWR;
> >>
> >> -	ubuf->device = device;
> >>   	buf = dma_buf_export(&exp_info);
> >>   	if (IS_ERR(buf)) {
> >>   		ret = PTR_ERR(buf);
> >> --
> >> 2.39.2
> >