linux-kernel - Re: [PATCH v2 08/11] spi: cadence-qspi: add early busywait to cqspi_wait_for

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Date: Mon, 08 Apr 2024 16:42:43 +0200
From: Théo Lebrun <theo.lebrun@...tlin.com>
To: "Mark Brown" <broonie@...nel.org>
Cc: "Rob Herring" <robh+dt@...nel.org>, "Krzysztof Kozlowski"
 <krzysztof.kozlowski+dt@...aro.org>, "Conor Dooley" <conor+dt@...nel.org>,
 "Vaishnav Achath" <vaishnav.a@...com>, "Thomas Bogendoerfer"
 <tsbogend@...ha.franken.de>, "Rob Herring" <robh@...nel.org>,
 <linux-spi@...r.kernel.org>, <devicetree@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, <linux-mips@...r.kernel.org>, "Vladimir
 Kondratiev" <vladimir.kondratiev@...ileye.com>, "Gregory CLEMENT"
 <gregory.clement@...tlin.com>, "Thomas Petazzoni"
 <thomas.petazzoni@...tlin.com>, "Tawfik Bayouk"
 <tawfik.bayouk@...ileye.com>
Subject: Re: [PATCH v2 08/11] spi: cadence-qspi: add early busywait to
 cqspi_wait_for_bit()

Hello,

On Mon Apr 8, 2024 at 4:16 PM CEST, Mark Brown wrote:
> On Fri, Apr 05, 2024 at 05:02:18PM +0200, Théo Lebrun wrote:
>
> > If the CQSPI_BUSYWAIT_EARLY quirk flag is on, call
> > readl_relaxed_poll_timeout() with no sleep at the start of
> > cqspi_wait_for_bit(). If its short timeout expires, a sleeping
> > readl_relaxed_poll_timeout() call takes the relay.
> > 
> > Behavior is hidden behind a quirk flag to keep the previous behavior the
> > same on all platforms.
> > 
> > The reason is to avoid hrtimer interrupts on the system. All read
> > operations take less than 100µs.
>
> Why would this be platform specific, this seems like a very standard
> optimisation technique?

It does not make sense if you know that all read operations take more
than 100µs. I preferred being conservative. If you confirm it makes
sense I'll remove the quirk.

Regards,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com