linux-kernel - Re: [PATCH v2 2/2] hwrng: iproc-rng200

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Message-ID: <54EF75F0.7000901@broadcom.com>
Date:	Thu, 26 Feb 2015 11:37:20 -0800
From:	Scott Branden <sbranden@...adcom.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	<bcm-kernel-feedback-list@...adcom.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	"Ian Campbell" <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Matt Mackall <mpm@...enic.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Grant Likely <grant.likely@...aro.org>,
	Ray Jui <rjui@...adcom.com>,
	Jonathan Richardson <jonathar@...adcom.com>,
	Dmitry Torokhov <dtor@...gle.com>,
	Anatol Pomazao <anatol@...gle.com>,
	<devicetree@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] hwrng: iproc-rng200 - Add Broadcom IPROC RNG driver

Response inline.

On 15-02-25 11:17 AM, Arnd Bergmann wrote:
> On Wednesday 25 February 2015 10:16:24 Scott Branden wrote:
>> This adds a driver for random number generator present on Broadcom
>> IPROC devices.
>>
>> Reviewed-by: Ray Jui <rjui@...adcom.com>
>> Signed-off-by: Scott Branden <sbranden@...adcom.com>
>
> The driver looks reasonable overall, I have just one question about
> something that sticks out:
>
>> +	while ((num_remaining > 0) && time_before(jiffies, idle_endtime)) {
> ...
>> +
>> +		/* Are there any random numbers available? */
>> +		if ((ioread32(rng_base + RNG_FIFO_COUNT_OFFSET) &
>> +				RNG_FIFO_COUNT_RNG_FIFO_COUNT_MASK) > 0) {
> ...
>> +		} else {
>> +			if (!wait)
>> +				/* Cannot wait, return immediately */
>> +				return max - num_remaining;
>> +
>> +			/* Can wait, give others chance to run */
>> +			cpu_relax();
>> +		}
>> +	}
>> +
>
> It looks like you do a busy-loop around cpu_relax here if asked to wait.
> Is this intentional? I would normally expect either cond_resched() or
> some msleep() instead.

This code was following examples of other open source drivers - bcm2835 
and exynos both use cpu_relax.  I'll have to look into this more to 
understand.

>
> 	Arnd
>

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