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Subject: Re: [PATCH] [RFC] dmaengine: add fifo_size member
From:   Dmitry Osipenko <digetx@gmail.com>
To:     Jon Hunter <jonathanh@nvidia.com>,
        Peter Ujfalusi <peter.ujfalusi@ti.com>,
        Sameer Pujar <spujar@nvidia.com>, Vinod Koul <vkoul@kernel.org>
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        dmaengine@vger.kernel.org, linux-kernel@vger.kernel.org,
        sharadg@nvidia.com, rlokhande@nvidia.com, dramesh@nvidia.com,
        mkumard@nvidia.com, linux-tegra <linux-tegra@vger.kernel.org>
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Date:   Thu, 6 Jun 2019 16:55:16 +0300
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06.06.2019 16:45, Dmitry Osipenko пишет:
> 06.06.2019 15:37, Jon Hunter пишет:
>>
>> 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
>>
> 
> Hi,
> 
> If I understood everything correctly, the FIFO buffer is shared among
> all of the ADMA clients and hence it should be up to the ADMA driver to
> manage the quotas of the clients. So if there is only one client that
> uses ADMA at a time, then this client will get a whole FIFO buffer, but
> once another client starts to use ADMA, then the ADMA driver will have
> to reconfigure hardware to split the quotas.
> 

You could also simply hardcode the quotas per client in the ADMA driver
if the quotas are going to be static anyway.

-- 
Dmitry