netdev - Re: [PATCH] irda: irda-usb: use msecs_to

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Message-ID: <20150520014408.GA19321@roeck-us.net>
Date:	Tue, 19 May 2015 18:44:08 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Joe Perches <joe@...ches.com>
Cc:	David Miller <davem@...emloft.net>, hofrat@...dl.org,
	samuel@...tiz.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] irda: irda-usb: use msecs_to_jiffies for conversions

On Tue, May 19, 2015 at 02:09:07PM -0700, Joe Perches wrote:
> On Tue, 2015-05-19 at 16:45 -0400, David Miller wrote:
> > From: Joe Perches <joe@...ches.com>
> > Date: Tue, 19 May 2015 07:32:15 -0700
> > 
> > > On Tue, 2015-05-19 at 12:51 +0200, Nicholas Mc Guire wrote:
> > >> Converting milliseconds to jiffies by "val * HZ / 1000" is technically
> > >> is not a clean solution as it does not handle all corner cases correctly.
> > >> By changing the conversion to use msecs_to_jiffies(val) conversion is
> > >> correct in all cases.
> > >> 
> > >> in the current code: 
> > >>   mod_timer(&self->rx_defer_timer, jiffies + (10 * HZ / 1000));
> > >> for HZ < 100 (e.g. CONFIG_HZ == 64|32 in alpha) this effectively results 
> > >> in no delay at all.
> > > []
> > >> diff --git a/drivers/net/irda/irda-usb.c b/drivers/net/irda/irda-usb.c
> > > []
> > >> @@ -848,7 +848,9 @@ static void irda_usb_receive(struct urb *urb)
> > >>  		 * Jean II */
> > >>  		self->rx_defer_timer.function = irda_usb_rx_defer_expired;
> > >>  		self->rx_defer_timer.data = (unsigned long) urb;
> > >> -		mod_timer(&self->rx_defer_timer, jiffies + (10 * HZ / 1000));
> > >> +		mod_timer(&self->rx_defer_timer,
> > >> +			  jiffies + (msecs_to_jiffies(10)));
> > > 
> > > The unnecessary () around msecs_to_jiffies could be removed.
> 
> The other thing that could be done is to use
> 	max(1ul, msecs_to_jiffies())
> so that there's always some delay even if HZ <= 50
> 
I may be mistaken, but I am quite sure that msecs_to_jiffies() always returns
at least 1 in such situations. Otherwise there would be lots of converstions
to 0 in the kernel.

Guenter
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