linux-kernel - Re: [PATCH] iommu/dma: Add support for DMA_ATTR_SYS

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Date:   Mon, 28 Oct 2019 15:44:20 +0000
From:   Will Deacon <will@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     isaacm@...eaurora.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, joro@...tes.org,
        m.szyprowski@...sung.com, robin.murphy@....com,
        pratikp@...eaurora.org, lmark@...eaurora.org,
        jcrouse@...eaurora.org
Subject: Re: [PATCH] iommu/dma: Add support for DMA_ATTR_SYS_CACHE

On Mon, Oct 28, 2019 at 12:37:28PM +0100, Christoph Hellwig wrote:
> On Mon, Oct 28, 2019 at 11:24:58AM +0000, Will Deacon wrote:
> > Agreed. The way I /think/ it works is that on many SoCs there is a
> > system/last-level cache (LLC) which effectively sits in front of memory for
> > all masters. Even if a device isn't coherent with the CPU caches, we still
> > want to be able to allocate into the LLC. Why this doesn't happen
> > automatically is beyond me, but it appears that on these Qualcomm designs
> > you actually have to set the memory attributes up in the page-table to
> > ensure that the resulting memory transactions are non-cacheable for the CPU
> > but cacheable for the LLC. Without any changes, the transactions are
> > non-cacheable in both of them which assumedly has a performance cost.
> > 
> > But you can see that I'm piecing things together myself here. Isaac?
> 
> If that is the case it sounds like we'd want to drive this through
> DT properties, not the driver API.  But again, without an actual consumer
> it pretty much is a moot point anyway.

I think there's certainly a DT aspect to it so that the driver knows that
the SoC is hooked up this way, but I agree that we need a consumer so that
we can figure out how dynamic this needs to be. If it's just a
fire-and-forget thing then it needn't escape the IOMMU layer, but I fear
that it's probably more flexible than that.

If nothing shows up for 5.6, I'll send a patch to remove it (since Jordan
said there was something coming soon [1])

Will

[1] http://lkml.kernel.org/r/20191024153832.GA7966@jcrouse1-lnx.qualcomm.com