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

[Jon Hunter <jonathanh@nvidia.com>]

On 06/06/2019 12:54, Peter Ujfalusi wrote:
> 
> 
> On 06/06/2019 13.49, Jon Hunter wrote:
>>
>> On 06/06/2019 11:22, Peter Ujfalusi wrote:
>>
>> ...
>>
>>>>>> It does sounds like that FIFO_SIZE == src/dst_maxburst in your case as
>>>>>> well.
>>>>> Not exactly equal.
>>>>> ADMA burst_size can range from 1(WORD) to 16(WORDS)
>>>>> FIFO_SIZE can be adjusted from 16(WORDS) to 1024(WORDS) [can vary in
>>>>> multiples of 16]
>>>>
>>>> So I think that the key thing to highlight here, is that the as Sameer
>>>> highlighted above for the Tegra ADMA there are two values that need to
>>>> be programmed; the DMA client FIFO size and the max burst size. The ADMA
>>>> has register fields for both of these.
>>>
>>> How does the ADMA uses the 'client FIFO size' and 'max burst size'
>>> values and what is the relation of these values to the peripheral side
>>> (ADMAIF)?
>>
>> Per Sameer's previous comment, the FIFO size is used by the ADMA to
>> determine how much space is available in the FIFO. I assume the burst
>> size just limits how much data is transferred per transaction.
>>
>>>> As you can see from the above the FIFO size can be much greater than the
>>>> burst size and so ideally both of these values would be passed to the DMA.
>>>>
>>>> We could get by with just passing the FIFO size (as the max burst size)
>>>> and then have the DMA driver set the max burst size depending on this,
>>>> but this does feel quite correct for this DMA. Hence, ideally, we would
>>>> like to pass both.
>>>>
>>>> We are also open to other ideas.
>>>
>>> I can not find public documentation (I think they are walled off by
>>> registration), but correct me if I'm wrong:
>>
>> No unfortunately, you are not wrong here :-(
>>
>>> ADMAIF - peripheral side
>>>  - kind of a small DMA for audio preipheral(s)?
>>
>> Yes this is the interface to the APE (audio processing engine) and data
>> sent to the ADMAIF is then sent across a crossbar to one of many
>> devices/interfaces (I2S, DMIC, etc). Basically a large mux that is user
>> configurable depending on the use-case.
>>
>>>  - Variable FIFO size
>>
>> Yes.
>>
>>>  - sends DMA request to ADMA per words
>>
>> From Sameer's notes it says the ADMAIF send a signal to the ADMA per
>> word, yes.
>>
>>> ADMA - system DMA
>>>  - receives the DMA requests from ADMAIF
>>>  - counts the requests
>>>  - based on some threshold of the counter it will send/read from ADMAIF?
>>>   - maxburst number of words probably?
>>
>> Sounds about right to me.
>>
>>> ADMA needs to know the ADMAIF's FIFO size because, it is the one who is
>>> managing that FIFO from the outside, making sure that it does not over
>>> or underrun?
>>
>> Yes.
>>
>>> And it is the one who sets the pace (in effect the DMA burst size - how
>>> many bytes the DMA jumps between refills) of refills to the ADMAIF's FIFO?
>>
>> Yes.
>>
>> So currently, if you look at the ADMA driver
>> (drivers/dma/tegra210-adma.c) you will see we use the src/dst_maxburst
>> for the burst, but the FIFO size is hard-coded (see the
>> TEGRA210_FIFO_CTRL_DEFAULT and TEGRA186_FIFO_CTRL_DEFAULT definitions).
>> Ideally, we should not hard-code this but pass it.
> 
> Sure, hardcoding is never good ;)
> 
>> Given that there are no current users of the ADMA upstream, we could
>> change the usage of the src/dst_maxburst, but being able to set the FIFO
>> size as well would be ideal.
> 
> Looking at the drivers/dma/tegra210-adma.c for the
> TEGRA*_FIFO_CTRL_DEFAULT definition it is still not clear where the
> remote FIFO size would fit.
> There are fields for overflow and starvation(?) thresholds and TX/RX
> size (assuming word length, 3 == 32bits?).

The TX/RX size are the FIFO size. So 3 equates to a FIFO size of 3 * 64
bytes.

> Both threshold is set to one, so I assume currently ADMA is
> pushing/pulling data word by word.

That's different. That indicates thresholds when transfers start.

> Not sure what the burst size is used for, my guess would be that it is
> used on the memory (DDR) side for optimized, more efficient accesses?

That is the actual burst size.

> My guess is that the threshold values are the counter limits, if the DMA
> request counter reaches it then ADMA would do a threshold limit worth of
> push/pull to ADMAIF.
> Or there is another register where the remote FIFO size can be written
> and ADMA is counting back from there until it reaches the threshold (and
> pushes/pulling again threshold amount of data) so it keeps the FIFO
> filled with at least threshold amount of data?
> 
> I think in both cases the threshold would be the maxburst.
> 
> I suppose you have the patch for adma on how to use the fifo_size
> parameter? That would help understand what you are trying to achieve better.

Its quite simple, we would just use the FIFO size to set the fields
TEGRAXXX_ADMA_CH_FIFO_CTRL_TXSIZE/RXSIZE in the
TEGRAXXX_ADMA_CH_FIFO_CTRL register. That's all.

Jon

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