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

[Mason <slash.tmp@free.fr>]

[ Nothing new added below.
  Vinod, was the description of my HW's quirks clear enough?
  Is there a way to write a driver within the existing framework?
  How can I get that HW block supported upstream?
  Regards. ]

On 25/11/2016 13:46, Mason wrote:

> On 25/11/2016 05:55, Vinod Koul wrote:
> 
>> On Wed, Nov 23, 2016 at 11:25:44AM +0100, Mason wrote:
>>
>>> 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.
> 
> It seems there is a disconnect between what Linux expects - an IRQ
> when the transfer is complete - and the quirks of this HW :-(
> 
> On this system, there are MBUS "agents" connected via a "switch box".
> An agent fires an IRQ when it has dealt with its *half* of the transfer.
> 
> SOURCE_AGENT <---> SBOX <---> DESTINATION_AGENT
> 
> Here are the steps for a transfer, in the general case:
> 
> 1) setup the sbox to connect SOURCE TO DEST
> 2) configure source to send N bytes
> 3) configure dest to receive N bytes
> 
> When SOURCE_AGENT has sent N bytes, it fires an IRQ
> When DEST_AGENT has received N bytes, it fires an IRQ
> The sbox connection can be torn down only when the destination
> agent has received all bytes.
> (And the twist is that some agents do not have an IRQ line.)
> 
> The system provides 3 RAM-to-sbox agents (read channels)
> and 3 sbox-to-RAM agents (write channels).
> 
> The NAND Flash controller read and write agents do not have
> IRQ lines.
> 
> So for a NAND-to-memory transfer (read from device)
> - nothing happens when the NFC has finished sending N bytes to the sbox
> - the write channel fires an IRQ when it has received N bytes
> 
> In that case, one IRQ fires when the transfer is complete,
> like Linux expects.
> 
> For a memory-to-NAND transfer (write to device)
> - the read channel fires an IRQ when it has sent N bytes
> - the NFC driver is supposed to poll the NFC to determine
> when the controller has finished writing N bytes
> 
> In that case, the IRQ does not indicate that the transfer
> is complete, merely that the sending half has finished
> its part.
> 
> For a memory-to-memory transfer (memcpy)
> - the read channel fires an IRQ when it has sent N bytes
> - the write channel fires an IRQ when it has received N bytes
> 
> So you actually get two IRQs in that case, which I don't
> think Linux (or the current DMA driver) expects.
> 
> I'm not sure how we're supposed to handle this kind of HW
> in Linux? (That's why I started this thread.)
> 
> 
>> If that is not doable, then since you claim this is custom part which
>> other vendors won't use (hope we are wrong down the line),
> 
> I'm not sure how to interpret "you claim this is custom part".
> Do you mean I may be wrong, that it is not custom?
> I don't know if other vendors may have HW with the same
> quirky behavior. What do you mean about being 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.
> 
> I don't think that would work. The fundamental issue is
> that Linux expects a single IRQ to indicate "transfer
> complete". And the driver (as written) starts a new
> transfer as soon as the IRQ fires.
> 
> But the HW may generate 0, 1, or even 2 IRQs for a single
> transfer. And when there is a single IRQ, it may not
> indicate "transfer complete" (as seen above).
> 
>> Only problem with that would be it wont be a generic solution
>> and you seem to be fine with that.
> 
> I think it is possible to have a generic solution:
> Right now, the callback is called from tasklet context.
> If we can have a new flag to have the callback invoked
> directly from the ISR, then the driver for the client
> device can do what is required.
> 
> For example, the NFC driver waits for the IRQ from the
> memory agent, and then polls the controller itself.
> 
> I can whip up a proof-of-concept if it's better to
> illustrate with a patch?