Re: [PATCH] dmaengine: omap-dma: Do not call omap_dma_callback() from tx_status()

[Peter Ujfalusi <peter.ujfalusi@ti.com>]

On 2016-02-26 13:25, Russell King - ARM Linux wrote:
>>> Your original commit adding the original hack that you're now removing
>>> above says that this is to support polled operation: I'm not aware of
>>> DMA engine supporting such a mode.  DMA_PREP_INTERRUPT is a mechanism
>>> where requests can be queued without an interrupt to allow batching.
>>
>> Also it is used to suppress DMA interrupts during audio playback for example.
>> In this case we will run w/o interrupts and the position is polled.
>>
>>> See the raid5/async_tx code, which queues a set of operations without
>>> DMA_PREP_INTERRUPT, with the final operation with DMA_PREP_INTERRUPT
>>> set.
>>
>> We only allow the interrupts to be disabled in cyclic or memcpy mode. With
>> slave_sg we have interrupts as it is needed to move to the next SG.
>>
>>> As the driver is reliant on interrupts to move to the next transfer,
>>> the patch which causes DMA_PREP_INTERRUPT to influence whether
>>> interrupts are sent is actually buggy, and will prevent several
>>> queued DMA operations to fail.
>>
>> Yes, the omap-dma only allows the interrupts to be actually disabled when it
>> is save to do so. slave_sg can not work w/o interrupts so there we don't
>> disable them.
> 
> I get the impression that you haven't taken in what I've said, because
> each fragment of your reply is just repeating what the previous fragment
> said.

Sorry about that. I got it.

> Let me state that I don't believe you need any hacks here, and this patch
> is not necessary: the final operation in a set of chained memcpy()s should
> have DMA_PREP_INTERRUPT set.

calling the omap_dma_callback() from the tx_status() was a bad idea, this is
why I'm fixing it.
As for the DMA_PREP_INTERRUPT not enabling interrupt was introduced by:
4ce98c0a20bef (dmaengine: omap-dma: Add support for memcpy)

While we do not have users submitting multiple descriptors I agree that this
is a possibility and the driver should not fail in such a case.
I can send a followup patch to fix the omap_dma_prep_dma_memcpy()

-- 
Péter