linux-kernel - Re: [PATCH 2/2] iommu/arm-smmu: Add support for non-coherent page table mappings

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Date:   Tue, 22 Jan 2019 05:43:28 +0000
From:   Will Deacon <will.deacon@....com>
To:     Vivek Gautam <vivek.gautam@...eaurora.org>
Cc:     linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/2] iommu/arm-smmu: Add support for non-coherent page
 table mappings

On Mon, Jan 21, 2019 at 11:35:30AM +0530, Vivek Gautam wrote:
> On Sun, Jan 20, 2019 at 5:31 AM Will Deacon <will.deacon@....com> wrote:
> > On Thu, Jan 17, 2019 at 02:57:18PM +0530, Vivek Gautam wrote:
> > > Adding a device tree option for arm smmu to enable non-cacheable
> > > memory for page tables.
> > > We already enable a smmu feature for coherent walk based on
> > > whether the smmu device is dma-coherent or not. Have an option
> > > to enable non-cacheable page table memory to force set it for
> > > particular smmu devices.
> >
> > Hmm, I must be missing something here. What is the difference between this
> > new property, and simply omitting dma-coherent on the SMMU?
> 
> So, this is what I understood from the email thread for Last level
> cache support -
> Robin pointed to the fact that we may need to add support for setting
> non-cacheable
> mappings in the TCR.
> Currently, we don't do that for SMMUs that omit dma-coherent.
> We rely on the interconnect to handle the configuration set in TCR,
> and let interconnect
> ignore the cacheability if it can't support.

I think that's a bug. With that fixed, can you get what you want by omitting
"dma-coherent"?

Will