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 08:43:51 +0000
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Mark Brown <broonie@...nel.org>
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 01/04/2019 09.34, Mark Brown wrote:
> On Wed, Mar 27, 2019 at 02:30:48PM +0000, Rasmus Villemoes wrote:
>> I doubt patches 3 and 4 are acceptable, but I'd still like to get
>> comments and/or alternative suggestions for making large transfers
>> faster.
> 
> I see no problem with this from a framework point of view FWIW, it's
> going to be a question of if there's any glitches like you say.  I'm not
> sure how we can get wider testing/review unless the patches actually get
> merged though...  I'll leave them for a bit longer but unless someone
> sees a problem I'll probably go ahead and apply them.
> 

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.

Rasmus