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

[Sameer Pujar <spujar@nvidia.com>]

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
>> 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.
>> - 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,
>>    * 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.