Subject: Re: hw_random fixes
From: Matt Mackall <mpm@selenic.com>
To: Ian Molton <ian.molton@collabora.co.uk>
Cc: Rusty Russell <rusty@rustcorp.com.au>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
In-Reply-To: <4B0D03FC.40406@collabora.co.uk>
References: <4B080621.5000109@collabora.co.uk>
	 <4B0C18B0.2000206@collabora.co.uk> <1259084901.17871.624.camel@calx>
	 <200911251135.41871.rusty@rustcorp.com.au> <4B0D03FC.40406@collabora.co.uk>
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Date: Wed, 25 Nov 2009 13:27:32 -0600
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[cc:ing to linux-kernel, finally]

On Wed, 2009-11-25 at 10:16 +0000, Ian Molton wrote:
> Rusty Russell wrote:
> 
> > Make that:
> > 
> > 	ssize_t (*get_rng_data)(void *buf, size_t max, bool wait);
> > 
> > Then, if driver supplies that hook, use it exclusively.  Otherwise, use old
> > ones.  We can convert them gradually that way.
> 
> This doesn't quite solve things neatly, because it means one of:
> 
> 1) The core has to wait until there is nothing left before requesting
> more data, because it doesnt know the alignment requirements of the driver.

Hmm, this seems to imply you'd be calling get_rng_data multiple times
with different offsets into buf to accumulate data. I think that's more
complex than is needed. Just use buf as a nicely aligned scratch buffer
and empty it completely into the final output buffer before the next
driver request. If you end up with 1 byte of data hanging around until
the next read, that's not a problem - the next read might be 5 bytes.

You'll probably want to use cacheline alignment on buf to make Via
Padlock happy, if anything needs larger alignment (ie page) it should
handle it internally.

-- 
http://selenic.com : development and support for Mercurial and Linux


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