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

[Vinod Koul <vkoul@kernel.org>]

On 17-06-19, 12:37, Sameer Pujar wrote:
> 
> 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.

That is on every word read/write?

> 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)

That is sane and common. For memory you dont have a constraint so you
transfer at full throttle.

> > > 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.

So we have ADMA and AHUB and peripheral. You are talking to AHUB and that
is _not_ peripheral and if I have guess right the fifo depth is for AHUB
right?

> 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.

-- 
~Vinod