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[PATCH 1/3] dmaengine: virt-dma: don't always free descriptor upon completion

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  [PATCH 1/3] dmaengine: virt-dma: don't always free descriptor upon completion Robert Jarzmik <robert.jarzmik@free.fr> - 2015-09-06 13:50 +0200
    Re: [PATCH 1/3] dmaengine: virt-dma: don't always free descriptor upon completion Robert Jarzmik <robert.jarzmik@free.fr> - 2015-09-12 15:20 +0200
      Re: [PATCH 1/3] dmaengine: virt-dma: don't always free descriptor upon completion Robert Jarzmik <robert.jarzmik@free.fr> - 2015-09-16 21:30 +0200

#1219804 — [PATCH 1/3] dmaengine: virt-dma: don't always free descriptor upon completion

This patch attempts to enhance the case of a transfer submitted multiple
times, and where the cost of creating the descriptors chain is not
negligible.

This happens with big video buffers (several megabytes, ie. several
thousands of linked descriptors in one scatter-gather list). In these
cases, a video driver would want to do :
 - tx = dmaengine_prep_slave_sg()
 - dma_engine_submit(tx);
 - dma_async_issue_pending()
 - wait for video completion
 - read video data (or not, skipping a frame is also possible)
 - dma_engine_submit(tx)
   => here, the descriptors chain recalculation will take time
   => the dma coherent allocation over and over might create holes in
      the dma pool, which is counter-productive.
 - dma_async_issue_pending()
 - etc ...

In order to cope with this case, virt-dma is modified to prevent freeing
the descriptors upon completion if DMA_CTRL_REUSE flag is set in the
transfer.

This patch is a respin of the former DMA_CTRL_ACK approach, which was
reverted due to a regression in audio drivers.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
---
 drivers/dma/virt-dma.c | 36 ++++++++++++++++++++++++++++++------
 drivers/dma/virt-dma.h | 15 ++++++++++++++-
 2 files changed, 44 insertions(+), 7 deletions(-)

diff --git a/drivers/dma/virt-dma.c b/drivers/dma/virt-dma.c
index 6f80432a3f0a..e33caa87cbbb 100644
--- a/drivers/dma/virt-dma.c
+++ b/drivers/dma/virt-dma.c
@@ -29,7 +29,7 @@ dma_cookie_t vchan_tx_submit(struct dma_async_tx_descriptor *tx)
 	spin_lock_irqsave(&vc->lock, flags);
 	cookie = dma_cookie_assign(tx);
 
-	list_add_tail(&vd->node, &vc->desc_submitted);
+	list_move_tail(&vd->node, &vc->desc_submitted);
 	spin_unlock_irqrestore(&vc->lock, flags);
 
 	dev_dbg(vc->chan.device->dev, "vchan %p: txd %p[%x]: submitted\n",
@@ -39,6 +39,23 @@ dma_cookie_t vchan_tx_submit(struct dma_async_tx_descriptor *tx)
 }
 EXPORT_SYMBOL_GPL(vchan_tx_submit);
 
+int vchan_tx_desc_free(struct dma_async_tx_descriptor *tx)
+{
+	struct virt_dma_chan *vc = to_virt_chan(tx->chan);
+	struct virt_dma_desc *vd = to_virt_desc(tx);
+	unsigned long flags;
+
+	spin_lock_irqsave(&vc->lock, flags);
+	list_del(&vd->node);
+	spin_unlock_irqrestore(&vc->lock, flags);
+
+	dev_dbg(vc->chan.device->dev, "vchan %p: txd %p[%x]: freeing\n",
+		vc, vd, vd->tx.cookie);
+	vc->desc_free(vd);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(vchan_tx_desc_free);
+
 struct virt_dma_desc *vchan_find_desc(struct virt_dma_chan *vc,
 	dma_cookie_t cookie)
 {
@@ -83,8 +100,10 @@ static void vchan_complete(unsigned long arg)
 		cb_data = vd->tx.callback_param;
 
 		list_del(&vd->node);
-
-		vc->desc_free(vd);
+		if (dmaengine_desc_test_reuse(&vd->tx))
+			list_add(&vd->node, &vc->desc_allocated);
+		else
+			vc->desc_free(vd);
 
 		if (cb)
 			cb(cb_data);
@@ -96,9 +115,13 @@ void vchan_dma_desc_free_list(struct virt_dma_chan *vc, struct list_head *head)
 	while (!list_empty(head)) {
 		struct virt_dma_desc *vd = list_first_entry(head,
 			struct virt_dma_desc, node);
-		list_del(&vd->node);
-		dev_dbg(vc->chan.device->dev, "txd %p: freeing\n", vd);
-		vc->desc_free(vd);
+		if (dmaengine_desc_test_reuse(&vd->tx)) {
+			list_move_tail(&vd->node, &vc->desc_allocated);
+		} else {
+			dev_dbg(vc->chan.device->dev, "txd %p: freeing\n", vd);
+			list_del(&vd->node);
+			vc->desc_free(vd);
+		}
 	}
 }
 EXPORT_SYMBOL_GPL(vchan_dma_desc_free_list);
@@ -108,6 +131,7 @@ void vchan_init(struct virt_dma_chan *vc, struct dma_device *dmadev)
 	dma_cookie_init(&vc->chan);
 
 	spin_lock_init(&vc->lock);
+	INIT_LIST_HEAD(&vc->desc_allocated);
 	INIT_LIST_HEAD(&vc->desc_submitted);
 	INIT_LIST_HEAD(&vc->desc_issued);
 	INIT_LIST_HEAD(&vc->desc_completed);
diff --git a/drivers/dma/virt-dma.h b/drivers/dma/virt-dma.h
index 181b95267866..a4156af93d88 100644
--- a/drivers/dma/virt-dma.h
+++ b/drivers/dma/virt-dma.h
@@ -29,6 +29,7 @@ struct virt_dma_chan {
 	spinlock_t lock;
 
 	/* protected by vc.lock */
+	struct list_head desc_allocated;
 	struct list_head desc_submitted;
 	struct list_head desc_issued;
 	struct list_head desc_completed;
@@ -55,10 +56,17 @@ static inline struct dma_async_tx_descriptor *vchan_tx_prep(struct virt_dma_chan
 	struct virt_dma_desc *vd, unsigned long tx_flags)
 {
 	extern dma_cookie_t vchan_tx_submit(struct dma_async_tx_descriptor *);
+	extern int vchan_tx_desc_free(struct dma_async_tx_descriptor *);
+	unsigned long flags;
 
 	dma_async_tx_descriptor_init(&vd->tx, &vc->chan);
 	vd->tx.flags = tx_flags;
 	vd->tx.tx_submit = vchan_tx_submit;
+	vd->tx.desc_free = vchan_tx_desc_free;
+
+	spin_lock_irqsave(&vc->lock, flags);
+	list_add_tail(&vd->node, &vc->desc_allocated);
+	spin_unlock_irqrestore(&vc->lock, flags);
 
 	return &vd->tx;
 }
@@ -122,7 +130,8 @@ static inline struct virt_dma_desc *vchan_next_desc(struct virt_dma_chan *vc)
 }
 
 /**
- * vchan_get_all_descriptors - obtain all submitted and issued descriptors
+ * vchan_get_all_descriptors - obtain all allocated, submitted and issued
+ *                             descriptors
  * vc: virtual channel to get descriptors from
  * head: list of descriptors found
  *
@@ -134,6 +143,7 @@ static inline struct virt_dma_desc *vchan_next_desc(struct virt_dma_chan *vc)
 static inline void vchan_get_all_descriptors(struct virt_dma_chan *vc,
 	struct list_head *head)
 {
+	list_splice_tail_init(&vc->desc_allocated, head);
 	list_splice_tail_init(&vc->desc_submitted, head);
 	list_splice_tail_init(&vc->desc_issued, head);
 	list_splice_tail_init(&vc->desc_completed, head);
@@ -141,11 +151,14 @@ static inline void vchan_get_all_descriptors(struct virt_dma_chan *vc,
 
 static inline void vchan_free_chan_resources(struct virt_dma_chan *vc)
 {
+	struct virt_dma_desc *vd;
 	unsigned long flags;
 	LIST_HEAD(head);
 
 	spin_lock_irqsave(&vc->lock, flags);
 	vchan_get_all_descriptors(vc, &head);
+	list_for_each_entry(vd, &head, node)
+		dmaengine_desc_clear_reuse(&vd->tx);
 	spin_unlock_irqrestore(&vc->lock, flags);
 
 	vchan_dma_desc_free_list(vc, &head);
-- 
2.1.4

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#1223413

Robert Jarzmik <robert.jarzmik@free.fr> writes:

> This patch attempts to enhance the case of a transfer submitted multiple
> times, and where the cost of creating the descriptors chain is not
> negligible.
>
> This happens with big video buffers (several megabytes, ie. several
> thousands of linked descriptors in one scatter-gather list). In these
> cases, a video driver would want to do :
>  - tx = dmaengine_prep_slave_sg()
>  - dma_engine_submit(tx);
>  - dma_async_issue_pending()
>  - wait for video completion
>  - read video data (or not, skipping a frame is also possible)
>  - dma_engine_submit(tx)
>    => here, the descriptors chain recalculation will take time
>    => the dma coherent allocation over and over might create holes in
>       the dma pool, which is counter-productive.
>  - dma_async_issue_pending()
>  - etc ...
>
> In order to cope with this case, virt-dma is modified to prevent freeing
> the descriptors upon completion if DMA_CTRL_REUSE flag is set in the
> transfer.
>
> This patch is a respin of the former DMA_CTRL_ACK approach, which was
> reverted due to a regression in audio drivers.
>
> Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>

Hi Jun, Lars-Peter and Vinod,

The revert of the former patch of this type, 8c8fe97b2b8a ("Revert "dmaengine:
virt-dma: don't always free descriptor upon completion"") broke pxa_dma driver.

The reason behind is that pxa_dma was designed, upon transfer submission (in
pxad_tx_submit()), to "move" the virtual descriptor to the submitted list,
rather than adding it at its tail (because it was previously on the allocated
list).

As a consequence, the list_head is not initialized, pxa_dma fails, and the
current status is that pxa_dma Oopses in linux-next, and therefore in Linus's
tree once the merge window is closed.

I'd really like to have this patch as the fix, ie. that at transfer preparation
in vchan_tx_prep(), the virtual descriptor is added on "allocated" list tail.

But to be sure there is no regression on other platforms, I need a test,
especially from someone who suffered from the former version of this patch
(where we had DMA_CTRL_ACK instead of DMA_CTRL_REUSE).

Could anybody give it a try, as I need a fix to go in the early -rc of v4.3
please ?

Cheers.

-- 
Robert
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#1226411

Robert Jarzmik <robert.jarzmik@free.fr> writes:

> Robert Jarzmik <robert.jarzmik@free.fr> writes:
> Hi Jun, Lars-Peter and Vinod,
>
> The revert of the former patch of this type, 8c8fe97b2b8a ("Revert "dmaengine:
> virt-dma: don't always free descriptor upon completion"") broke pxa_dma
> driver.

So ... what's the plan, Vinod ? I need to fix pxa-dma driver in the early -rc
stages, how does it work with your schedule ?

Cheers.

--
Robert
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