Re: Tearing down DMA transfer setup after DMA client has finished

From: Måns Rullgård
Date: Thu Nov 24 2016 - 11:37:24 EST

Mason <slash.tmp@xxxxxxx> writes:

> On 24/11/2016 15:17, Måns Rullgård wrote:
>> Mason wrote:
>>> [ 35.085854] SETUP DMA
>>> [ 35.088272] START NAND TRANSFER
>>> [ 35.091670] tangox_dma_pchan_start from tangox_dma_irq
>>> [ 35.096882] tango_dma_callback from vchan_complete
>>> [ 45.102513] DONE FAKE SPINNING
>>> So the IRQ rolls in, the ISR calls tangox_dma_pchan_start,
>>> which calls tangox_dma_pchan_detach to tear down the sbox
>>> setup; and only sometime later does the DMA framework call
>>> my callback function.
>> Yes, I realised this soon after I said it. The dma driver could be
>> rearranged to make it work though.
> There is a way to make the tasklet run and invoke the callback
> before the interrupt service routine proceeds?

No, but it would be possible to defer the teardown to the tasklet.
Having said that, I'm not sure it's such a great idea since the tasklet
could be held up for an arbitrary length of time waiting for the target
to finish.

>>> So far, the work-arounds I've tested are:
>>> 1) delay sbox tear-down by 10 µs in tangox_dma_pchan_detach.
>>> 2) statically setup sbox in probe, and never touch it henceforth.
>>> WA1 is fragile, it might break for devices other than NFC.
>>> WA2 is what I used when I wrote the NFC driver.
>>> Can tangox_dma_irq() be changed to have the framework call
>>> the client's callback *before* tangox_dma_pchan_start?
>>> (Thinking out loud) The DMA_PREP_INTERRUPT requests that the
>>> DMA framework invoke the callback from tasklet context,
>>> maybe a different flag DMA_PREP_INTERRUPT_EX can request
>>> calling the call-back directly from within the ISR?
>>> (Looking at existing flags) Could I use DMA_CTRL_ACK?
>>> Description sounds like some kind hand-shake between
>>> client and dmaengine.
>>> Grepping for DMA_PREP_INTERRUPT, I don't see where the framework
>>> checks that flag to spawn the tasklet? Or is that up to each
>>> driver individually?
>> Those flags all have defined meanings and abusing them for other things
>> is a bad idea. As far as possible, device drivers should work with any
>> dma driver.
> I was asking about introducing a new flag, not abusing existing
> flags. (I don't understand the semantics of DMA_CTRL_ACK.)

This needs more than a new flag anyhow.

> (FWIW, both the NFC and the MBUS agent are custom designs,
> not third-party IP blocks.)

Sure, but who knows what will be in the next chip?

Måns Rullgård