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

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Message-ID: <3062825.7b1oYP4yuL@wuerfel>
Date:	Wed, 25 Feb 2015 20:17:42 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Scott Branden <sbranden@...adcom.com>
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

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.

	Arnd
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