netdev - RE: [Pv-drivers] [PATCH] vmxnet3: round down # of queues to power of two

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Date:	Wed, 6 Jul 2011 18:53:57 -0700
From:	Yong Wang <yongwang@...are.com>
To:	Dmitry Torokhov <dtor@...are.com>,
	"pv-drivers@...are.com" <pv-drivers@...are.com>
CC:	Shreyas Bhatewara <sbhatewara@...are.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [Pv-drivers] [PATCH] vmxnet3: round down # of queues to power
 of two

I just want to point out that rounddown_pow_of_two() returns 0 when (1) the input is 1; and (2) the __buildin_constant_p(n) path is taken.  In the vmxnet3 case, even if num_rx_queues is 1, we are safe as its value is not known to be constant at compile time and 1 will be returned by taking the other path (1UL << (fls_long(n) - 1)). The original check of power_of_two is there just to be on the safe side.

Related to this, I wonder if we should always return 1 when the input is 1 (1 is also a power of two as 2^0). I don't have the history of this API though and curious if there are any users of this API that depend on this behavior?

Thanks,
Yong

> -----Original Message-----
> From: Dmitry Torokhov [mailto:dtor@...are.com]
> Sent: Wednesday, July 06, 2011 6:09 PM
> To: pv-drivers@...are.com
> Cc: Shreyas Bhatewara; netdev@...r.kernel.org; Yong Wang
> Subject: Re: [Pv-drivers] [PATCH] vmxnet3: round down # of queues to power of
> two
> 
> On Wednesday, July 06, 2011 05:58:51 PM Shreyas Bhatewara wrote:
> 
> >
> > +	if (!is_power_of_2(num_rx_queues))
> > +		num_rx_queues = rounddown_pow_of_two(num_rx_queues);
> > +
> 
> No need to do the check, just do:
> 
> 	num_rx_queues = rounddown_pow_of_two(num_rx_queues);
> 
> Thanks,
> Dmitry
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