linux-kernel - Re: [PATCH 1/4] spi: lantiq: Synchronize interrupt handlers and transfers

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Message-ID: <20200429121310.GH4201@sirena.org.uk>
Date:   Wed, 29 Apr 2020 13:13:10 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Dilip Kota <eswara.kota@...ux.intel.com>
Cc:     Daniel Schwierzeck <daniel.schwierzeck@...il.com>, robh@...nel.org,
        linux-spi@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, hauke@...ke-m.de,
        andriy.shevchenko@...el.com, cheol.yong.kim@...el.com,
        chuanhua.lei@...ux.intel.com, qi-ming.wu@...el.com
Subject: Re: [PATCH 1/4] spi: lantiq: Synchronize interrupt handlers and
 transfers

On Wed, Apr 29, 2020 at 04:20:53PM +0800, Dilip Kota wrote:
> On 4/28/2020 7:10 PM, Daniel Schwierzeck wrote:

> > actually there is no real bottom half. Reading or writing the FIFOs is
> > fast and is therefore be done in hard IRQ context. But as the comment

> Doing FIFO r/w in threaded irqs shouldn't cause any impact on maximum
> transfer rate i think.

Have you actually tested this?  Generally adding extra latency is going
to lead to some opportunity for the hardware to idle and the longer the
hardware is idle the lower the throughput.

> Also the ISR should be quick enough, doing FIFO r/w in ISR adds up more
> latency to ISR.
> Handling the FIFOs r/w in threaded irq will be a better way.

Consider what happens on a heavily loaded system - the threaded
interrupt will have to be scheduled along with other tasks.

> > for lantiq_ssc_bussy_work() state, the driver needs some busy-waiting
> > after the last interrupt. I don't think it's worth to replace this with
> > threaded interrupts which add more runtime overhead and likely decrease
> > the maximum transfer speed.

> Workqueue has a higher chances of causing SPI transfers timedout.

because...?

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