[PATCH] dmaengine: qcom_hidma: release the descriptor before the callback

Hi,

On 7/24/2016 2:24 AM, Vinod Koul wrote:

On Fri, Jul 15, 2016 at 09:00:52PM -0400, Sinan Kaya wrote:

quoted

Hi Vinod,

On 7/13/2016 10:57 PM, Sinan Kaya wrote:

quoted

There is a race condition between data transfer callback and descriptor
free code. The callback routine may decide to clear the resources even
though the descriptor has not yet been freed.

Instead of calling the callback first and then releasing the memory,
this code is changing the order to return the descriptor back to the
free pool and then call the user provided callback.

Signed-off-by: Sinan Kaya <redacted>
---
 drivers/dma/qcom/hidma.c | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/drivers/dma/qcom/hidma.c b/drivers/dma/qcom/hidma.c
index 41b5c6d..c41696e 100644
--- a/drivers/dma/qcom/hidma.c
+++ b/drivers/dma/qcom/hidma.c

@@ -111,6 +111,7 @@ static void hidma_process_completed(struct hidma_chan *mchan)
 	struct dma_async_tx_descriptor *desc;
 	dma_cookie_t last_cookie;
 	struct hidma_desc *mdesc;
+	struct hidma_desc *next;
 	unsigned long irqflags;
 	struct list_head list;
 

@@ -122,28 +123,31 @@ static void hidma_process_completed(struct hidma_chan *mchan)
 	spin_unlock_irqrestore(&mchan->lock, irqflags);
 
 	/* Execute callbacks and run dependencies */
-	list_for_each_entry(mdesc, &list, node) {
-		enum dma_status llstat;
+	list_for_each_entry_safe(mdesc, next, &list, node) {
+		dma_async_tx_callback callback;
+		void *param;
 
 		desc = &mdesc->desc;
 
 		spin_lock_irqsave(&mchan->lock, irqflags);
-		dma_cookie_complete(desc);
+		if (hidma_ll_status(mdma->lldev, mdesc->tre_ch)
+			== DMA_COMPLETE)
+			dma_cookie_complete(desc);

It looks like I introduced a behavioral change while refactoring the code.
The previous one would call the callback only if the transfer was successful
but it would always call dma_cookie_complete.

The new behavior is to call dma_cookie_complete only if the transfer is successful
and it calls the callback even in the case of error cases. Then, the client has
to query if transfer was successful.

Which one is the correct behavior?

Hi Sinan,

Cookie is always completed. That simply means trasactions was completed and
has no indication if the transaction was successfull or not.

Uptill now we had no way of reporting error, Dave's series adds that up, so
you can use it.

Callback is optional for users. Again we didnt convey success of
transaction, but a callback for reporting that trasaction was completed. So
invoking callback makes sense everytime.

HTH

Let's put Dave's series aside for the moment and assume an error case where
something bad happened during the transfer. I can add the error code once Dave's
series get merged.

Here is the callback from dmatest.

static void dmatest_callback(void *arg)
{ 
	done->done = true;
}

Here is how the request is made.

dma_async_issue_pending(chan);

wait_event_freezable_timeout(done_wait, done.done,
			     msecs_to_jiffies(params->timeout));

status = dma_async_is_tx_complete(chan, cookie, NULL, NULL);
if (!done.done) {
	timeout
} else if (status != DMA_COMPLETE) { 
	error
}

success.

Based on what I see here, receiving callback all the time is OK. The client
checks if the callback is received or not first. 

Next, the client checks the status of the tx_status. 

In the error case mentioned, the callback will be executed. done.done will be true.

If I set dma_cookie_complete(desc) in error case, it would be wrong to tell the client
that the transfer is successful. 

In my opinion, the new behavior is correct. Calling dma_cookie_complete(desc) all the time
is not. Do you agree?

If yes, I can divide this patch into two. One to correct the ordering. Another one
for behavioral change.


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.