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

From: Sameer Pujar
Date: Mon Jun 17 2019 - 03:12:37 EST

On 6/13/2019 10:13 AM, Vinod Koul wrote:
On 06-06-19, 09:19, Sameer Pujar wrote:

you are really going other way around about the whole picture. FWIW that
is how *other* folks do audio with dmaengine!
I discussed this internally with HW folks and below is the reason why DMA
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.
- 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
when is signal triggered? When there is space available or some
threshold of space is reached?
Signal is triggered when FIFO read/write happens on the peripheral side. In other words
this happens when data is pushed/popped out of ADMAIF from/to one of the AHUB modules (I2S
for example) This is on peripheral side and ADMAIF signals ADMA per WORD basis.
ADMA <---(1. DMA transfers)---> ADMAIF <------ (2. FIFO read/write) ------> I2S
To be more clear ADMAIF signals ADMA when [2] happens.

FIFO_THRESHOLD field in ADMAIF is just to indicate when can ADMAIF do operation [2].
Also please note FIFO_THRESHOLD field is present only for memory---->AHUB path (playback path)
and there is no such threshold concept for AHUB----> memory path (capture path)
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.
That doesn't sound too different from typical dmaengine. To give an
example of a platform (and general DMAengine principles as well) I worked
on the FIFO was 16 word deep. DMA didn't knew!

Peripheral driver would signal to DMA when a threshold is reached and
No, In our case ADMAIF does not do any threshold based signalling to ADMA.
DMA would send a burst controlled by src/dst_burst_size. For example if
you have a FIFO with 16 words depth, typical burst_size would be 8 words
and peripheral will configure signalling for FIFO having 8 words, so
signal from peripheral will make dma transfer 8 words.
The scenario is different in ADMA case, as ADMAIF cannot configure the
signalling based on FIFO_THRESHOLD settings.

Here the peripheral driver FIFO is important, but the driver
configures it and sets burst_size accordingly.

So can you explain me what is the difference here that the peripheral
cannot configure and use burst size with passing fifo depth?
Say for example FIFO_THRESHOLD is programmed as 16 WORDS, BURST_SIZE as 8 WORDS.
ADMAIF does not push data to AHUB(operation [2]) till threshold of 16 WORDS is
reached in ADMAIF FIFO. Hence 2 burst transfers are needed to reach the threshold.
As mentioned earlier, threshold here is to just indicate when data transfer can happen
to AHUB modules.

Once the data is popped from ADMAIF FIFO, ADMAIF signals ADMA. ADMA is the master
and it keeps track of the buffer occupancy by knowing the FIFO_DEPTH and the signalling.
Then finally it decides when to do next burst transfer depending on the free space
available in ADMAIF.
- Now consider two cases based on above logic,
ÂÂÂ In this case, ADMA thinks that there is enough space available for
transfer, when actually the FIFO data
ÂÂÂ on slave is not consumed yet. It would result in OVERRUN.
ÂÂÂ This is case where ADMA wonât 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.