linux-kernel - Re: [SPAM] [PATCH v2 06/11] i2c: nomadik: support short xfer timeouts using waitqueue & hrtimer

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Message-Id: <CZL1F576XCJB.2DBGD5Z7UUXIH@bootlin.com>
Date: Mon, 04 Mar 2024 15:32:38 +0100
From: Théo Lebrun <theo.lebrun@...tlin.com>
To: "Andi Shyti" <andi.shyti@...nel.org>
Cc: "Linus Walleij" <linus.walleij@...aro.org>, "Rob Herring"
 <robh+dt@...nel.org>, "Krzysztof Kozlowski"
 <krzysztof.kozlowski+dt@...aro.org>, "Conor Dooley" <conor+dt@...nel.org>,
 "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>,
 <linux-arm-kernel@...ts.infradead.org>, <linux-i2c@...r.kernel.org>,
 <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <linux-mips@...r.kernel.org>, "Gregory Clement"
 <gregory.clement@...tlin.com>, "Vladimir Kondratiev"
 <vladimir.kondratiev@...ileye.com>, "Thomas Petazzoni"
 <thomas.petazzoni@...tlin.com>, "Tawfik Bayouk"
 <tawfik.bayouk@...ileye.com>
Subject: Re: [SPAM] [PATCH v2 06/11] i2c: nomadik: support short xfer
 timeouts using waitqueue & hrtimer

Hello,

On Mon Mar 4, 2024 at 2:54 PM CET, Andi Shyti wrote:
> Hi Theo,
>
> ...
>
> > +static bool nmk_i2c_wait_xfer_done(struct nmk_i2c_dev *priv)
> > +{
> > +	if (priv->timeout_usecs < jiffies_to_usecs(1)) {
> > +		unsigned long timeout_usecs = priv->timeout_usecs;
> > +		ktime_t timeout = ktime_set(0, timeout_usecs * NSEC_PER_USEC);
> > +
> > +		wait_event_hrtimeout(priv->xfer_wq, priv->xfer_done, timeout);
> > +	} else {
> > +		unsigned long timeout = usecs_to_jiffies(priv->timeout_usecs);
> > +
> > +		wait_event_timeout(priv->xfer_wq, priv->xfer_done, timeout);
> > +	}
> > +
> > +	return priv->xfer_done;
>
> You could eventually write this as
>
>   static bool nmk_i2c_wait_xfer_done(struct nmk_i2c_dev *priv)
>   {
> 	if (priv->timeout_usecs < jiffies_to_usecs(1)) {
> 		...
>
> 		return !wait_event_hrtimeout(...);
> 	}
>
> 	...
> 	return wait_event_timeout(...);
>   }
>
> It looks a bit cleaner to me... your choice.

The full block would become:

static bool nmk_i2c_wait_xfer_done(struct nmk_i2c_dev *priv)
{
	if (priv->timeout_usecs < jiffies_to_usecs(1)) {
		unsigned long timeout_usecs = priv->timeout_usecs;
		ktime_t timeout = ktime_set(0, timeout_usecs * NSEC_PER_USEC);

		return !wait_event_hrtimeout(priv->xfer_wq, priv->xfer_done,
					     timeout);
	}

	return wait_event_timeout(priv->xfer_wq, priv->xfer_done,
				  usecs_to_jiffies(priv->timeout_usecs));
}

Three things:

 - Deindenting the jiffy timeout case means no variable declaration
   after the if-block. This is fine from my point-of-view.

 - It means we depend on the half-mess that are return values from
   wait_event_*timeout() macros. I wanted to avoid that because it
   looks like an error when you read the above code and see one is
   negated while the other is not.

 - Also, I'm not confident in casting either return value to bool; what
   happens if either macro returns an error? This is a theoretical case
   that shouldn't happen, but behavior might change at some point or
   bugs could occur. We know priv->xfer_done will give us the right
   answer.

My preference still goes to the original version, but I'm happy we are
having a discussion about this code block.

> Reviewed-by: Andi Shyti <andi.shyti@...nel.org>

Thanks for your review Andi!

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