linux-kernel - Re: [PATCH] iommu: Remove stub functions for !CONFIG_IOMMU

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Date:	Wed, 31 Aug 2011 15:03:04 +0200
From:	"Roedel, Joerg" <Joerg.Roedel@....com>
To:	Ohad Ben-Cohen <ohad@...ery.com>
CC:	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	Stepan Moskovchenko <stepanm@...eaurora.org>,
	David Woodhouse <dwmw2@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Brown <davidb@...eaurora.org>
Subject: Re: [PATCH] iommu: Remove stub functions for !CONFIG_IOMMU_API

On Mon, Aug 29, 2011 at 09:21:54AM -0400, Ohad Ben-Cohen wrote:
> On Mon, Aug 29, 2011 at 4:05 PM, Roedel, Joerg <Joerg.Roedel@....com> wrote:
> > So in this case you can skip this whole part when CONFIG_IOMMU_API is
> > disabled
> 
> Sure, we can do something like this in the driver:
> 
> #ifdef CONFIG_IOMMU_API
> 
> int do_the_iommu_thing(..)
> {
>    ... call the IOMMU API as needed...
> }
> 
> #else
> 
> int do_the_iommu_thing(..)
> {
>     return 0;
> }
> 
> #endif
> 
> Essentially, this means implementing the !CONFIG_IOMMU_API stubs in the driver.

I expect only very few drivers need that. Most drivers (like KVM and the
upcoming VFIO) rely on the iommu-api and I expect most other drivers
using the directly will also rely on an iommu. So this change makes
still sense to me.

> Possible of course, but I think it's a bit neater to have that in one
> place, where all drivers can use. Similar to !CONFIG_HWSPINLOCK,
> !CONFIG_DEBUG_FS, ...
> 
> > This saves you even more .text size, no?
> 
> Not sure; I expect the code to be compiled out today when the IOMMU
> API is called and CONFIG_IOMMU_API is not set.

But you even save all code that would use the iommu-api in your driver.
This change forces you to compile out this code too leading to smaller
.text-size.

	Joerg

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