linux-kernel - Re: [PATCH 1/2] hw_random: core updates to allow more efficient drivers

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Message-Id: <200912011338.15763.rusty@rustcorp.com.au>
Date:	Tue, 1 Dec 2009 13:38:15 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Ian Molton <ian.molton@...labora.co.uk>,
	linux-kernel@...r.kernel.org,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Jeff Garzik <jeff@...zik.org>
Subject: Re: [PATCH 1/2] hw_random: core updates to allow more efficient drivers

On Tue, 1 Dec 2009 05:14:16 am Matt Mackall wrote:
> On Mon, 2009-11-30 at 10:28 +0000, Ian Molton wrote:
> > Rusty Russell wrote:
> > 
> > > And might as well just #defube RNG_BUFFSIZE SMP_CACHE_BYTES (or use
> > > SMP_CACHE_BYTES here and sizeof() elsewhere).
> > 
> > This can lead to a rather small (4 byte) buffer on some systems, however
> > I don't know if in practice a tiny buffer or a big one would be better
> > for performance on those machines. I guess if its a problem someone can
> > patch the code to allocate a minimum of (say) 16 bytes in future...
> 
> Hmmm, I think this was bad advice from Rusty.

Me too.  But really, it's because we're using cacheline alignment
as a proxy for something else, and it's not a good fit.

But we're bikeshedding, so apply or revert.

	/* A buffer which can hold any fundamental type: drivers are fussy. */
	static u8 rng_buffer[SMP_CACHE_BYTES < 8 ? 8 : SMP_CACHE_BYTES]
		__cacheline_aligned;

Either way, both patches: Acked-by: Rusty Russell <rusty@...tcorp.com.au>

Thanks,
Rusty.
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