Re: [PATCH] [RFC] dmaengine: add fifo_size member

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

Hi Sameer,

On 06/06/2019 6.49, Sameer Pujar wrote:
> Sorry for late reply.
> [Resending the reply since delivery failed for few recipients]
> I discussed this internally with HW folks and below is the reason why
> DMA needs
> to know FIFO size.
> 
> - FIFOs reside in peripheral device(ADMAIF), which is the ADMA interface
> to the audio sub-system.
> - ADMAIF has multiple channels and share FIFO buffer for individual
> operations. There is a provision
>   to allocate specific fifo size for each individual ADMAIF channel from
> the shared buffer.
> - Tegra Audio DMA(ADMA) architecture is different from the usual DMA
> engines, which you described earlier.

It is not really different than what other DMAs are doing.

> - The flow control logic is placed inside ADMA. Slave peripheral
> device(ADMAIF) signals ADMA whenever a
>   read or write happens on the FIFO(per WORD basis). Please note that
> the signaling is per channel. There is
>   no other signaling present from ADMAIF to ADMA.
> - ADMA keeps a counter related to above signaling. Whenever a sufficient
> space is available, it initiates a transfer.
>   But the question is, how does it know when to transfer. This is the
> reason, why ADMA has to be aware of FIFO
>   depth of ADMAIF channel. Depending on the counters and FIFO depth, it
> knows exactly when a free space is available
>   in the context of a specific channel. On ADMA, FIFO_SIZE is just a
> value which should match to actual FIFO_DEPTH/SIZE
>   of ADMAIF channel.
> - Now consider two cases based on above logic,
>   * Case 1: when DMA_FIFO_SIZE > SLAVE_FIFO_SIZE
>     In this case, ADMA thinks that there is enough space available for
> transfer, when actually the FIFO data
>     on slave is not consumed yet. It would result in OVERRUN.
>   * Case 2: when DMA_FIFO_SIZE < SLAVE_FIFO_SIZE
>     This is case where ADMA won’t transfer, even though sufficient space
> is available, resulting in UNDERRUN.
> - The guideline is to program, DMA_FIFO_SIZE(on ADMA side) =
> SLAVE_FIFO_SIZE(on ADMAIF side) and hence we need a
>   way to communicate fifo size info to ADMA.

The src_maxburst / dst_maxburst is exactly for this reason. To
communicate how much data should be transferred per DMA request to/from
peripheral.

In TI land we have now 3 DMA engines servicing McASP. McASP has FIFO
which is dynamically configured (you can see the AFIFO of McASP as a
small DMA: on McASP side it services the peripheral, on the other side
it interacts with the given system DMA used by the SoC - EDMA, sDMA or
UDMAP). All DMAs needs a bit different configuration, but the AFIFO
depth on the McASP side is coming in via the src/dst_maxburst and the
drivers just need to interpret it correctly.

It does sounds like that FIFO_SIZE == src/dst_maxburst in your case as well.

- Péter

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