From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Fortini Matteo <matteo.fortini@mta.it>
Cc: "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>
Subject: Re: Recommended functions for accessing internal registers
Date: Thu, 03 Dec 2009 07:57:55 +1100 [thread overview]
Message-ID: <1259787475.2076.1160.camel@pasglop> (raw)
In-Reply-To: <4B1547E5.6050301@mta.it>
On Tue, 2009-12-01 at 17:44 +0100, Fortini Matteo wrote:
> I see that throughout the kernel source, internal PPC registers are
> accessed through [in|out]_be[32|16|8]() functions. However, they are
> translated into 3 inline assembly instructions, one of which is an
> isync, which has a huge performance hit.
> I tried using readl_be() which seems to be the right function according
> to the Documentation/ dir, but it is translated directly to in_be32(),
> so no luck.
>
> Is it really necessary to use all those instructions? I know I could use
> a (volatile u32 *) variable to avoid subsequent read/writes to be
> optimized out, but it seems to be a deprecated use.
There are good reasons why the accessors contain those barriers. What
are you doing that would be performance critical enough for those to be
a problem ?
Cheers,
Ben.
next prev parent reply other threads:[~2009-12-02 20:58 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-12-01 16:44 Recommended functions for accessing internal registers Fortini Matteo
2009-12-02 20:57 ` Benjamin Herrenschmidt [this message]
2009-12-03 11:16 ` Fortini Matteo
2009-12-03 21:10 ` Benjamin Herrenschmidt
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