linux-kernel - Re: [PATCH v3 0/5] enhance DMA CMA on x86

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Date:	Wed, 1 Oct 2014 11:05:13 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Hurley <peter@...leysoftware.com>
cc:	Akinobu Mita <akinobu.mita@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Don Dutile <ddutile@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	x86@...nel.org, iommu@...ts.linux-foundation.org,
	Greg KH <greg@...ah.com>
Subject: Re: [PATCH v3 0/5] enhance DMA CMA on x86

On Tue, 30 Sep 2014, Peter Hurley wrote:
> On 09/30/2014 07:45 PM, Thomas Gleixner wrote:
> > Whether the proposed patchset is the correct solution to support it is
> > a completely different question.
> 
> This patchset has been in mainline since 3.16 and has already caused
> regressions, so the question of whether this is the correct solution has
> already been answered.

Agreed.
 
> > So either you stop this right now and help Akinobu to find the proper
> > solution 
> 
> If this is only a test platform for ARM parts then I don't think it
> unreasonable to suggest forking x86 swiotlb support into a iommu=cma
> selector that gets DMA mapping working for this test platform and doesn't
> cause a bunch of breakage.

Breakage is not acceptable in any case.
 
> Which is different than if the plan is to ship production units for x86;
> then a general purpose solution will be required.
> 
> As to the good design of a general purpose solution for allocating and
> mapping huge order pages, you are certainly more qualified to help Akinobu
> than I am.

Fair enough. Still this does not make the case for outright rejecting
the idea of supporting that kind of device even if it is a esoteric
case. We deal with enough esoteric hardware in Linux and if done
right, it's no harm to anyone.

I'll have a look at the technical details.

Thanks,

	tglx
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