linux-kernel - Re: [RFC PATCH 1/2] spi: Add QuadSPI driver for Atmel SAMA5D2

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Date:   Thu, 28 Jun 2018 14:02:19 +0200 (CEST)
From:   Piotr Bugalski <bugalski.piotr@...il.com>
To:     Tudor Ambarus <tudor.ambarus@...rochip.com>
cc:     Piotr Bugalski <bugalski.piotr@...il.com>,
        Boris Brezillon <boris.brezillon@...tlin.com>,
        Mark Brown <broonie@...nel.org>, linux-spi@...r.kernel.org,
        David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Marek Vasut <marek.vasut@...il.com>,
        Richard Weinberger <richard@....at>,
        linux-mtd@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Cyrille Pitchen <cyrille.pitchen@...rochip.com>,
        Piotr Bugalski <pbu@...ptera.com>
Subject: Re: [RFC PATCH 1/2] spi: Add QuadSPI driver for Atmel SAMA5D2


Hi Tudor,

On Thu, 28 Jun 2018, Tudor Ambarus wrote:

> Hi, Piotr,
>
> On 06/27/2018 10:52 AM, Piotr Bugalski wrote:
>>
>>> General things to consider for the limitation in performance:
>>> - is the serial flash memory operating in Quad SPI?
>>
>> Yes, I've checked signal using logic analyzer, data is transferred using
>> all four lines.
>>
>>> - QSCLK should be as high as possible
>>
>> Sure, but when we are using lower frequency CPU impact should be
>> negligible while efficiency is crap on every speed.
>>
>>> - transfer delays - I checked them, they have default values, we should be good.
>>> - use DMA, as you suggested
>>>
>>
>> I don't understand one thing. While CPU is not busy and during my tests
>> 100% of CPU can be used for communication, efficiency is still very low.
>> Why DMA has such impact?
>>
>> It is very interesting to observe signals using logic analyzer.
>> When CPU is used for communication, there are long delays after
>> every byte transferred. These delays are  much longer than it should be only because of writing next value by CPU.
>
> Are those consecutive transfers (same peripheral without removing chip select)?
> The delays between consecutive transfers can be set just in SPI mode. It would
> be strange to see this kind of delays in serial memory mode.
>

Yes, it's just single block transfer so no CS changes occurs.
I find this delays strange also, but I have no idea how to avoid them.
The same behaviour exists even when DMA is used in APB mode (write
to registers). Only using SMM with DMA helps.

>> I tried to change SPI frequency. If delay were CPU related,
>> delay time should stay the same. Unfortunately results were different -
>> lowering SPI freqency extends delay time.
>
> If QSCK is less than f-perif-clock/2, then setting DLYBS to 1 will shorten the
> DLYBS delay, but this is peanuts.
>

I have DLYBS, DLYCS and DLYBCT set to zeros. I can try DLYBS=1 if
you wish.

> Thanks,
> ta
>
>> Using DMA makes these delays to disappear, but how to acheive CPU
>> communication without delays?
>

Thank you for comments,
Piotr