linux-kernel - Re: [PATCH] [RFC] dmaengine: add fifo

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Date:   Fri, 7 Jun 2019 08:50:08 +0300
From:   Peter Ujfalusi <peter.ujfalusi@...com>
To:     Jon Hunter <jonathanh@...dia.com>,
        Sameer Pujar <spujar@...dia.com>, Vinod Koul <vkoul@...nel.org>
CC:     <dan.j.williams@...el.com>, <tiwai@...e.com>,
        <dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <sharadg@...dia.com>, <rlokhande@...dia.com>, <dramesh@...dia.com>,
        <mkumard@...dia.com>, linux-tegra <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH] [RFC] dmaengine: add fifo_size member

Jon,

On 06/06/2019 15.37, Jon Hunter wrote:
>> 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.

Hrm, it is still not clear how all of these fits together.

What happens if you configure ADMA side:
BURST = 10
TX/RXSIZE = 100 (100 * 64 bytes?) /* FIFO_SIZE? */
*THRES = 5

And if you change the *THRES to 10?
And if you change the TX/RXSIZE to 50 (50 * 64 bytes?)
And if you change the BURST to 5?

In other words what is the relation between all of these?

There must be a rule and constraints around these and if we do really
need a new parameter for ADMA's FIFO_SIZE I'd like it to be defined in a
generic way so others could benefit without 'misusing' a fifo_size
parameter for similar, but not quite fifo_size information.

- Péter

Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
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