Re: [PATCH 07/13] i2c: nomadik: support short xfer timeouts using waitqueue & hrtimer

From: "Théo Lebrun" <theo.lebrun@bootlin.com>
To: "Linus Walleij" <linus.walleij@linaro.org>
Cc: "Andi Shyti" <andi.shyti@kernel.org>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Krzysztof Kozlowski" <krzysztof.kozlowski+dt@linaro.org>,
	"Conor Dooley" <conor+dt@kernel.org>,
	"Thomas Bogendoerfer" <tsbogend@alpha.franken.de>,
	<linux-arm-kernel@lists.infradead.org>,
	<linux-i2c@vger.kernel.org>, <devicetree@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>, <linux-mips@vger.kernel.org>,
	"Gregory Clement" <gregory.clement@bootlin.com>,
	"Vladimir Kondratiev" <vladimir.kondratiev@mobileye.com>,
	"Thomas Petazzoni" <thomas.petazzoni@bootlin.com>,
	"Tawfik Bayouk" <tawfik.bayouk@mobileye.com>
Subject: Re: [PATCH 07/13] i2c: nomadik: support short xfer timeouts using waitqueue & hrtimer
Date: Mon, 19 Feb 2024 15:31:45 +0100	[thread overview]
Message-ID: <CZ94MU7BQIOE.2KYB4TWVOJISN@bootlin.com> (raw)
In-Reply-To: <CACRpkdYC=vVBA-s6GmsaED=NdXfsr0JDzzF+x8q8C3tqQ0F8YQ@mail.gmail.com>

Hello,

On Mon Feb 19, 2024 at 3:19 PM CET, Linus Walleij wrote:
> On Thu, Feb 15, 2024 at 5:52 PM Théo Lebrun <theo.lebrun@bootlin.com> wrote:
>
> > Replace the completion by a waitqueue for synchronization from IRQ
> > handler to task. For short timeouts, use hrtimers, else use timers.
> > Usecase: avoid blocking the I2C bus for too long when an issue occurs.
> >
> > The threshold picked is one jiffy: if timeout is below that, use
> > hrtimers. This threshold is NOT configurable.
> >
> > Implement behavior but do NOT change fetching of timeout. This means the
> > timeout is unchanged (200ms) and the hrtimer case will never trigger.
> >
> > A waitqueue is used because it supports both desired timeout approaches.
> > See wait_event_timeout() and wait_event_hrtimeout(). An atomic boolean
> > serves as synchronization condition.
> >
> > Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
>
> Part of me want to go and fix completions to handle hrtimer timeouts
> for submicrosecond timeouts, BUT I realized that this is a bit thick
> request for a simple driver, so just a suggestion for something we could
> do one day. This is fine with me.
> Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

Indeed having to switch to another abstraction because we desire another
timeout method is nonsensical. Completion supporting hrtimeouts would
make sense. As you said though, this is too much for a simple driver.

Thanks,

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

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