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

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Message-ID: <20090210194955.GU11872@woodchuck>
Date:	Tue, 10 Feb 2009 19:49:55 +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,

* Alexander Clouter <alex@...riz.org.uk> [2009-02-10 19:39:04+0000]:
> 
> * Herbert Xu <herbert@...dor.apana.org.au> [2009-02-10 15:57:49+1100]:
> > 
> > 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);
> }
> ----
> 
> [snipped]
> > 
> > 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.
> 
Actually, as timer_pending() just reads a field in timer_list (or so 
says the 'manual'), would it not be a waste to have a second effectively 
duplicate variable?  In a blocking read of data_presnt you would have to 
have del_timer_sync() anyway...the two approaches would be identical 
except for the rng->present variable approach would force us to waste a 
whole extra bit of ram?

Should I keep with my old original timer_pending() approach?

Of course I could be talking rubbish...

Cheers

-- 
Alexander Clouter
.sigmonster says: So many men; so little time.
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