linux-kernel - Re: [PATCHv2] hw_random: add timeriomem-rng driver

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Date:	Tue, 10 Feb 2009 19:39:04 +0000
From:	Alexander Clouter <alex@...riz.org.uk>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCHv2] hw_random: add timeriomem-rng driver

Hi,

* Herbert Xu <herbert@...dor.apana.org.au> [2009-02-10 15:57:49+1100]:
> 
> On Sat, Feb 07, 2009 at 09:53:17AM +0000, Alexander Clouter wrote:
> >
> > +/*
> > + * have data return 1, however return 0 if we have nothing
> > + */
> > +static int timeriomem_rng_data_present(struct hwrng *rng, int wait)
> > +{
> > +	s32 delay;
> > +
> > +	if (rng->priv == 0)
> > +		return 1;
> > +
> > +	if (del_timer_sync(&timeriomem_rng_timer)) {
> > +		if (!wait)
> > +			return 0;
> > +
> > +		delay = timeriomem_rng_timer.expires - jiffies;
> > +
> > +		schedule_timeout_uninterruptible(delay);
> > +	}
> 
> Sorry, but this just doesn't work for O_NONBLOCK reads.  What'll
> happen is that the first failed read will return -EAGAIN, and when
> we're called again immediately (because hwrng polling support is
> non-existant), it'll just read the data right away because now
> there is no timer.
>
My original code[1] approach, with timer_pending() in there, would have 
been okay then, right?
----
if (timer_pending(&timeriomem_rng_timer)) {
  if (!wait)
    return 0;

  del_timer_sync(&timeriomem_rng_timer);
  delay = timeriomem_rng_timer.expires - jiffies;

  schedule_timeout_uninterruptible(delay);
}
----
 
> > +static int timeriomem_rng_data_read(struct hwrng *rng, u32 *data)
> > +{
> > +	unsigned long cur;
> > +	s32 delay;
> > +
> > +	*data = readl(timeriomem_rng_data->address);
> > +
> > +	if (rng->priv != 0) {
> > +		cur = jiffies;
> > +
> > +		delay = cur - timeriomem_rng_timer.expires;
> > +		delay = rng->priv - (delay % rng->priv);
> > +
> > +		timeriomem_rng_timer.expires = cur + delay;
> > +		add_timer(&timeriomem_rng_timer);
> > +	}
> 
> Instead of deleting the timer above, why not create a rng->present
> variable which you set to zero here, and the timer sets it to
> non-zero.  Then you just need to return rng->present in your
> data_present function.
> 
Sounds fair, better than the timer_pending approach and easier to 
understand.

> > +static void timeriomem_rng_trigger(unsigned long dummy)
> > +{
> > +	del_timer_sync(&timeriomem_rng_timer);
> > +}
> 
> This is going to create an infinite loop.  There is no point in
> deleting yourself.
> 
Noted.

Cheers
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