linux-kernel - Re: [RFC PATCH 0/4] spi: spi-fsl-spi: try to make cpu-mode transfers faster

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Date:   Tue, 2 Apr 2019 16:10:05 +0700
From:   Mark Brown <broonie@...nel.org>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc:     "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Fabio Estevam <festevam@...il.com>,
        Rasmus Villemoes <Rasmus.Villemoes@...vas.se>
Subject: Re: [RFC PATCH 0/4] spi: spi-fsl-spi: try to make cpu-mode transfers
 faster

On Tue, Apr 02, 2019 at 08:43:51AM +0000, Rasmus Villemoes wrote:

> Thanks! There's one other option I can think of: don't do the interrupts
> at all, but just busy-wait for the completion of each word transfer (in
> a cpu_relax() loop). That could be guarded by something like
> 1000000*bits_per_word < hz (roughly, the word transfer takes less than 1
> us). At least on -rt, having the interrupt thread scheduled in and out
> again easily takes more than 1us of cpu time, and AFAIU we'd still be
> preemptible throughout - and/or one can throw in a cond_resched() every
> nnn words. But this might be a bit -rt specific, and the 1us threshold
> is rather arbitrary.

Yeah, that's definitely worth exploring as a mitigation but obviously
with things like flash I/O that gets a bit rude.  Hopefully what's there
at the minute turns out to be robust enough.

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