Re: [PATCH 1/7] dmaengine: edma: fix two faults which happen with the 8250_dma user

Sebastian Andrzej Siewior
2014-08-28T09:06:43Z

On 08/28/2014 09:06 AM, Vinod Koul wrote:
> On Thu, Aug 21, 2014 at 03:09:12PM +0200, Sebastian Andrzej Siewior wrote:
>> On 08/19/2014 05:12 PM, Vinod Koul wrote:
>>>>
>>>> desc = dmaengine_prep_slave_single(rxchan, …);
>>>> rx_cookie = dmaengine_submit(desc);
>>>> dma_async_issue_pending(rxchan);
>>>>
>>>> ssleep(2);
>>>> /* Now assume that the transfer did not start */
>>>> st = dmaengine_tx_status(rxchan, rx_cookie, NULL);
>>>> /* st is now DMA_IN_PROGRESS as expected */
>>>>
>>>> dmaengine_terminate_all(rxchan);
>>>> st = dmaengine_tx_status(rxchan, rx_cookie, NULL);
>>> and here is the culprit. You have terminated the channel. This means that
>>> dmaengine driver is free to clean up all the allocated descriptors on the
>>> channels and do whatever it decides to do with them.
>>
>> descriptors, yes.
> and by that logic when you query the driver would have freed up!

I don't understand. What "would have freed up"?

>>> You have already terminated the channel so what is the point in querying the
>>> status of the cookie, which you shouldn't use anyway after invoking
>>> terminate_all() as its result is not correct.
>>
>> The point is to check (later, after terminate_all()) if there is an
>> outstanding DMA transfer or not _and_ how much data was really
>> transfered. Looking at edma it does not really support the latter if
>> the transfer is already completed. On the plus side the HW does not
>> support partly transfers :)
> well that can be achieved properly and differently!

I am all yours.

> Why don't we pause the channel, get the residue, status and then
> terminate.

This is done that way. For the protocol: pause is not supported by
driver (edma and omap-dma) unless the channel is used in "continuous"
mode. But this does not matter here.
The root problems remains: You get DMA_IN_PROGRESS returned as long as
the cookie has not been completed _even_ after the invocation of
terminate_all(). So that part that knows it terminates the thing for
the first time does everything correct.
The other part that only enqueues a transfer if there is not another
one pending does never do anything.

>> But where is it written that the life time of the cookie is limited?
>> Looking at the "cooking check" code there is no such thing. It is
>> simply compare of completed vs passed number but okay, this is an
>> implementation detail.
>> From [0] it says under "4. Submit the transaction":
>>
>> | This returns a cookie can be used to check the progress of DMA engine
>> | activity via other DMA engine calls not covered in this document.
>>
>> no life time limit mentioned here. Which brings to the question: Why is
>> it okay to use the cookie after the transaction "terminated" normally
>> but not if it has been canceled?
> Due to the special nature of terminate. The point here is that you don't
> terminate a transaction but channel operation

So the channel is terminated and the transaction is still in progress.
Is this what you are telling me? And why should a transaction remain in
progress after the channel has been terminated? I mean once you
terminate a bridge crossing point there are no cars going over that
bridge. It is closed. Their trip has been terminated as well.

The question that remains is which of those cars passed the bridge
before it was terminated. I understand that you want first to pause
that channel looking at the bytes and then terminate. Totally
understand. Otherwise the number of bytes transfered (reside) can no
longer be queried because the driver cleaned up. Fine.

But it should be reported that this transaction is no longer in
movement. It won't happen. Never. And this is what bugs me.

>> And from [0] the API explanation "4. … dma_async_is_tx_complete()":
>
>
>>
>> |Note:
>> | Not all DMA engine drivers can return reliable information for
>> | a running DMA channel. It is recommended that DMA engine users
>> | pause or stop (via dmaengine_terminate_all) the channel before
>> | using this API.
>>
>> So the documentation says to use the cookie with
>> dma_async_is_tx_complete() and before doing so it is recommended that
>> the transfer should be paused or stopped. _Exactly_ what is done here.
>>
>>>> /* st is still DMA_IN_PROGRESS but _I_ expect DMA_COMPLETE because
>>>> * it has been terminated / canceled
>>>> */
>>>>
>>>> Both dma driver clean up all / terminate all descriptors as required but
>>>> _none_ of them completes the cookie. As a result dma_cookie_status()
>>>> still thinks that the transfer is in progress.
>>>
>>> Btw how does it matter in the driver here if the transaction completed or
>>> not after terminate_all() was invoked. What is the purpose of querying
>>> status.
>>
>> In the RX interrupt (of the 8250 unit), the code checks if the transfer
>> has been already started or not via querying the status. So if it
>> returns DMA_COMPLETE then a new transfer will be started. If it returns
>> DMA_IN_PROGRESS then the code returns doing nothing because the DMA
>> engine should start moving data anytime now so the RX interrupt goes
>> away.
>>
>> That means: If the transfer is canceled then it won't be started again.
>>
>> Btw: the current (probably only) dma driver that is used by 8250-dma is
>> dw/core.c. That one does cookie complete on terminate:
>> dwc_control(DMA_TERMINATE_ALL) -> dwc_descriptor_complete() ->
>> dma_cookie_complete().
> Yes but would above flow work for you :)

No, it wont. I ask you to accept the two patches since now the behavior
of dma_async_is_tx_complete() is different depending on whether a
transaction completed on its own or got terminated. And reporting
in_progress does not reflect the reality. I can even add a patch on top
updating the documentation where it state that you must not trust the
dma_tx_state struct once the request is completed (or aborted).

Or we can trade :) I send you a patch which removes the "cookie
complete" part from the dwc-dma driver with a CC stable tag.
And you explain it to the intel folks that merged that part of the 8250
dma code that rely on the fact (that a aborted transaction reports
complete) that what they do is wrong.

but I think it will end up with this: I will change the 8250-dma code to
keep track whether or not it enqueued a DMA transfer and rely on this
information instead of dma_async_is_tx_complete(). I will update the
documentation stating that dma_async_is_tx_complete() may report bogus
information on completed and terminated transaction.

Sebastian