linux-kernel - Re: [PATCH v2 1/2] dma/iommu: Add pgsize_bitmap confirmation in __iommu_dma_alloc

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Message-ID: <1460360410.18251.10.camel@mhfsdcap03>
Date:	Mon, 11 Apr 2016 15:40:10 +0800
From:	Yong Wu <yong.wu@...iatek.com>
To:	Will Deacon <will.deacon@....com>,
	Doug Anderson <dianders@...omium.org>
CC:	Joerg Roedel <joro@...tes.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Robin Murphy <robin.murphy@....com>,
	Daniel Kurtz <djkurtz@...gle.com>,
	Tomasz Figa <tfiga@...gle.com>, Arnd Bergmann <arnd@...db.de>,
	Lucas Stach <l.stach@...gutronix.de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	"moderated list:ARM/Mediatek SoC support" 
	<linux-mediatek@...ts.infradead.org>,
	<srv_heupstream@...iatek.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	<iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH v2 1/2] dma/iommu: Add pgsize_bitmap confirmation in
 __iommu_dma_alloc_pages

On Fri, 2016-04-08 at 10:34 -0700, Doug Anderson wrote:
> Hi,
> 
> On Fri, Apr 8, 2016 at 10:30 AM, Will Deacon <will.deacon@....com> wrote:
> >> > Am I barking up the wrong tree?
> >>
> >> I don't think min_order can be negative.  Certainly we could enter the
> >> loop with order == 0 and min_order == 0, though.
> >
> > ... and in that case, PageCompound will be false, and we'll call split_page
> > which won't do anything, so we break out.
> >
> >>
> >> Some examples:
> >>
> >> order = 0, min_order = 0
> >> -> Want alloc_pages _without_ __GFP_NORETRY.  OK
> >> -> If alloc_pages fails, return NULL.  OK
> >> -> If alloc pages succeeds, don't need splitting since single page.  OK
> >
> > [...]
> >
> >> I think those are all right.  Did I mess up?  You could certainly
> >> structure the loop in a different way but you need to make sure you
> >> handle all of those cases.  If you have an alternate structure that
> >> handles all those, let's consider it.
> >
> > Right, I don't think the code is broken, I just think the !order check is
> > confusing and not needed.
> 
> Ah ha!  Got it.  I didn't dig into split_page() to see that it was a
> no-op when "order == 0".  I just know that the old code didn't call
> split_page() with order == 0 so I assumed that was wise to keep.  If
> we don't need to keep that then agreed that the "if" test can simply
> be removed.  :)
> 
> -Doug

Hi Will, Doug,

Thanks very much for review this patch, and Thanks Robin's work,
Currently this one is obsoleted since it's included in [1].

[1]:http://lists.linuxfoundation.org/pipermail/iommu/2016-April/016402.html