linux-kernel - Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev

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Message-ID: <20160121020409.GA7792@sha-win-210.asiapac.arm.com>
Date:	Thu, 21 Jan 2016 10:04:11 +0800
From:	Huang Shijie <shijie.huang@....com>
To:	Robin Murphy <robin.murphy@....com>
CC:	Joerg Roedel <joro@...tes.org>, <iommu@...ts.linux-foundation.org>,
	<will.deacon@....com>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is NULL

On Wed, Jan 20, 2016 at 02:46:34PM +0000, Robin Murphy wrote:
> On 20/01/16 13:34, Huang Shijie wrote:
> >On Wed, Jan 20, 2016 at 01:02:25PM +0100, Joerg Roedel wrote:
> >>On Tue, Jan 12, 2016 at 10:15:05AM +0800, Huang Shijie wrote:
> >>>This patch adds a shortcut for the code when the @device_node is NULL.
> >>>In my juno-r1 board, the boot time can be faster by 0.004014s.
> >>
> >>How have you made sure this number is reliable and not just noise in the
> >>boot process?
> >In the boot process, there are 5 or more modules whose @dev_node are
> >NULL. Without the patch, the kernel will waste some cycles to do the
> >meaningless calculations for all these modules.
>
> With a quick counting hack, booting 4.4 on my r1 indeed shows 5 calls where
> dev_node is null. Plus 68 calls in which we waste cycles doing meaningless
> calculations when dev_node is non-null. The fundamental issue at hand is

The 68 calls are just compare instructions, such as "b.eq".
But if the code runs to the followng branch when the @dev_node is NULL, the
instructions executed will more the 68 calls (even including
some read memory instructions when try to grab the locks).

> that the "platform bus" is a rubbish abstraction.
This is just a small trivial patch, please ignore it if we think it is
no use. :)

thanks
Huang Shijie
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