linux-kernel - Re: [PATCH] Staging: nvec: nvec: fixed two usleep

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Message-Id: <CZ6HDVZIGHOE.2SGOQNXEGENY3@gmail.com>
Date: Fri, 16 Feb 2024 12:53:18 +0100
From: "Thierry Reding" <thierry.reding@...il.com>
To: "Nam Cao" <namcao@...utronix.de>, "Moritz C. Weber"
 <mo.c.weber@...il.com>
Cc: <marvin24@....de>, <ac100@...ts.launchpad.net>,
 <linux-tegra@...r.kernel.org>, <linux-staging@...ts.linux.dev>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Staging: nvec: nvec: fixed two usleep_range is
 preferred over udelay warnings

On Mon Feb 12, 2024 at 3:21 PM CET, Nam Cao wrote:
> On 12/Feb/2024 Moritz C. Weber wrote:
> > Fixed a code style issue raised by checkpatch.
> > ---
> >  drivers/staging/nvec/nvec.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/staging/nvec/nvec.c b/drivers/staging/nvec/nvec.c
> > index 2823cacde..18c5471d5 100644
> > --- a/drivers/staging/nvec/nvec.c
> > +++ b/drivers/staging/nvec/nvec.c
> > @@ -627,7 +627,7 @@ static irqreturn_t nvec_interrupt(int irq, void *dev)
> >  		break;
> >  	case 2:		/* first byte after command */
> >  		if (status == (I2C_SL_IRQ | RNW | RCVD)) {
> > -			udelay(33);
> > +			usleep_range(32, 33);
> >  			if (nvec->rx->data[0] != 0x01) {
> >  				dev_err(nvec->dev,
> >  					"Read without prior read command\n");
> > @@ -714,7 +714,7 @@ static irqreturn_t nvec_interrupt(int irq, void *dev)
> >  	 * We experience less incomplete messages with this delay than without
> >  	 * it, but we don't know why. Help is appreciated.
> >  	 */
> > -	udelay(100);
> > +	usleep_range(99, 100);
> >  
> >  	return IRQ_HANDLED;
> >  }
>
> I have zero knowledge about this driver, but nvec_interrupt() seems to be
> a hard interrupt handler, and sleeping in an interrupt handler is a big no
> no. So I think this change breaks the driver.
>
> Delaying like the driver is currently doing doesn't break things, but it is
> not very nice because this is interrupt handler and the processor cannot
> switch to other tasks, so delaying is wasting processor's cycles here. The
> better fix would be to figure out how to remove the delay entirely, or
> switch to threaded interrupt handler and then we can use usleep_range() in
> there, but you need actual hardware to test such changes.

Also, pay attention to what else is being said in the timers-howto.rst
documentation. It specifically mentions that usleep_range() uses a range
in order to give the scheduler some leeway in coalescing with other
wakeups, so choosing a range of 32-33 us or 99-100 us isn't very useful.

Thierry

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