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

On Thu, Aug 04, 2016 at 11:27:46AM -0400, Sinan Kaya wrote:

On 8/4/2016 10:40 AM, Russell King - ARM Linux wrote:

quoted

On Thu, Aug 04, 2016 at 10:17:24AM -0400, Sinan Kaya wrote:

quoted

On 8/4/2016 8:55 AM, Vinod Koul wrote:

quoted

Dmaengine tells transaction is complete. It does not say if the txn is
success or failure. It can transfer data and not say if data was
correct. A successful transaction implies data integrity as well, which
dmaengine can't provide.

Thanks for describing this. I was confused about DMA_SUCCESS and DMA_COMPLETE.
I now understand that tx_success API just returns information that the request
was executed whether the result is error or not. This makes sense now.

However, if the txn is failure; then we should never call the client callback
since DMA engine cannot provide such feedback to the client without Dave's patch.
You are saying that the calling the callback is optional.

Then, the callback cannot be optional in the error case for old behavior.

How does the client know if memcpy executed or not? The client got its callback
and tx_status is also DMA_COMPLETE.

If an error occurred, then dma_async_is_tx_complete() is supposed to
return DMA_ERROR.  It's up to the DMA engine driver to ensure that
this happens if it has error detection abilities.

Well, that's not what I'm getting from Vinod and also from the current usage
in most of the drivers that I looked.

Sorry but, you are not interpreting it correctly. Me and Russell are saying the
same thing!

The current drivers implement tx_status as follows.

static enum dma_status xyz_tx_status(struct dma_chan *chan,
	dma_cookie_t cookie, struct dma_tx_state *state) 
{
...
	ret = dma_cookie_status(&c->vc.chan, cookie, state);
	if (ret == DMA_COMPLETE)
		return ret; 
...
}

What Vinod is telling me that I need to set the cookie to complete 
whether the transaction is successful or not if the request was accepted
by HW. xyz_tx_status is just an indication that the transaction was accepted
by HW. An error can happen as a result of transaction execution.

Nope, if the txn is completed you mark it complete. If you can detect error
(can you??) then you can report DMA_ERROR.

In that latter case do not use dma_async_is_complete() to check. You would
need to store and report that cookie 'x' failed which you report status in
.tx_statis()

If I call dma_cookie_complete for all transactions regardless of transaction
success or not, then the xyz_tx_status returns DMA_COMPLETE. 

Again that is based on your implementation.

This also matches what Vinod is saying. The transaction is complete but
it may not be success. I'm saying that if we follow this scheme, then
we should not call the callback.

That is not in driver's control. If the callback is set, you have to call
it. Client may choose to not set it.

I also made the argument that the driver should not call dma_cookie_complete
for failure cases and somehow return an error for transactions that failed. 
This is your suggestion.

I don't think it matches Vinod's expectation.

It does!

quoted

Most of the helpers in drivers/dma/dmaengine.h are there to _assist_
the driver writer - they can't do magic.  dma_cookie_status() will
return from the point of view of the generic DMA code what the status
of a particular cookie is, and the cookie state.  It doesn't take
care of whether a particular transaction associated with a cookie
failed or not - that's up to the driver.

So, if dma_cookie_status() says that a cookie has DMA_COMPLETED
status, and the DMA engine is able to detect errors on individual
transfers, then the driver needs to do further status lookup to
determine whether the particular transaction referred to by the
cookie did fail, and modify the returned status appropriately.

If dma_cookie_status() says that the cookie is DMA_IN_PROGRESS,
then the driver is expected to calculate and report the residue
(the remaining number of bytes) of the referred to transaction.

This part is fine. I'm worried about transactions that are failing.

And you issue is complete orthogonal to this debate. I am not saying we
should not discuss this, but you fix would be entirely different here (going
by data you have provided till now)

-- 
~Vinod