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

[Måns Rullgård]

Vinod Koul <vinod.koul@xxxxxxxxx> writes:

> On Wed, Nov 23, 2016 at 11:25:44AM +0100, Mason wrote:
>> Hello,
>> 
>> On my platform, setting up a DMA transfer is a two-step process:
>> 
>> 1) configure the "switch box" to connect a device to a memory channel
>> 2) configure the transfer details (address, size, command)
>> 
>> When the transfer is done, the sbox setup can be torn down,
>> and the DMA driver can start another transfer.
>> 
>> The current software architecture for my NFC (NAND Flash controller)
>> driver is as follows (for one DMA transfer).
>> 
>>   sg_init_one
>>   dma_map_sg
>>   dmaengine_prep_slave_sg
>>   dmaengine_submit
>>   dma_async_issue_pending
>>   configure_NFC_transfer
>>   wait_for_IRQ_from_DMA_engine // via DMA_PREP_INTERRUPT
>>   wait_for_NFC_idle
>>   dma_unmap_sg
>
> Looking at thread and discussion now, first thinking would be to ensure
> the transaction is completed properly and then isr fired. You may need
> to talk to your HW designers to find a way for that. It is quite common
> that DMA controllers will fire and complete whereas the transaction is
> still in flight.

The hardware is what it is, and it has been deployed in some form or
other for years.

> If that is not doable, then since you claim this is custom part which
> other vendors wont use (hope we are wrong down the line), then we can
> have a custom api,
>
> foo_sbox_configure(bool enable, ...);
>
> This can be invoked from NFC driver when required for configuration and
> teardown. For very specific cases where people need some specific
> configuration we do allow custom APIs.
>
> Only problem with that would be it wont be a generic solution and you
> seem to be fine with that.

The same DMA unit is also used for SATA, which is an off the shelf
Designware controller with an in-kernel driver.  This interrupt timing
glitch can actually explain some intermittent errors I've observed with
it.

One possible solution is to add a new function for device drivers to
call when their end is complete.  Existing DMA drivers would simply do
nothing, and device drivers could have this call added whenever the need
arises.

-- 
Måns Rullgård