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

From: Måns Rullgård
Date: Thu Nov 24 2016 - 09:17:27 EST

Mason <slash.tmp@xxxxxxx> writes:

>>> I'm confused. Are you saying there is no solution to my problem
>>> within the existing DMA framework?
>>> In its current form, the tangox-dma.c driver will fail randomly
>>> for writes to a device (SATA, NFC).
>>> Maybe an extra hook can be added to the DMA framework?
>>> I'd like to hear from the framework's maintainers. Perhaps they
>>> can provide some guidance.
>> You could have the dma descriptor callback wait for the receiving device
>> to finish. Bear in mind this runs from a tasklet, so it's not allowed
>> to sleep.
> Thanks for the suggestion, but I don't think it works :-(
> This is my DMA desc callback:
> static void tango_dma_callback(void *arg)
> {
> printk("%s from %pf\n", __func__, __builtin_return_address(0));
> mdelay(10000);
> printk("DONE FAKE SPINNING\n");
> complete(arg);
> }
> I also added
> printk("%s from %pf\n", __func__, __builtin_return_address(0));
> after tangox_dma_pchan_detach(pchan);
> And I get this output:
> [ 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.

> 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.

Måns Rullgård