linux-kernel - Re: [PATCH 3/3] spi: s3c64xx: support interrupt based pio mode

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Date:   Tue, 4 Apr 2023 14:33:06 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Jaewon Kim <jaewon02.kim@...sung.com>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Andi Shyti <andi@...zian.org>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Rob Herring <robh+dt@...nel.org>, linux-spi@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Chanho Park <chanho61.park@...sung.com>
Subject: Re: [PATCH 3/3] spi: s3c64xx: support interrupt based pio mode

On Tue, Apr 04, 2023 at 10:15:05PM +0900, Jaewon Kim wrote:
> On 23. 4. 4. 21:58, Mark Brown wrote:

> > Is there some lower limit where it's still worth using polling, for
> > example for just one or two bytes like a register address?  Taking an
> > interrupt isn't free...

> I did not considers lower limit.
> According to your review, interrupt seems to be called too often.
> However, It can't prevent the CPU utilization going to 100% during spi 
> transmission.

It's not so much that the interrupt could be called too often as that
the time taken to take the interrupt (including all the overhead the CPU
has) might be large compared to what busy waiting would take if the
transfer is very small.  If the FIFO is deep enough and the transfer is
long enough to use that then you start to see a win from interrupts.

> We will give more consideration and deliver a better solution to the 
> next patch version.

Great.

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