Re: [PATCH 1/5] iommu: Replace uses of IOMMU_CAP_CACHE_COHERENCY with dev_is_dma_coherent()

From: Christoph Hellwig <hch@lst.de>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>,
	"Tian, Kevin" <kevin.tian@intel.com>,
	Christoph Hellwig <hch@lst.de>,
	Alex Williamson <alex.williamson@redhat.com>,
	Lu Baolu <baolu.lu@linux.intel.com>,
	Christian Benvenuti <benve@cisco.com>,
	Cornelia Huck <cohuck@redhat.com>,
	David Woodhouse <dwmw2@infradead.org>,
	Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	Jason Wang <jasowang@redhat.com>, Joerg Roedel <joro@8bytes.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	 "linux-arm-msm@vger.kernel.org" <linux-arm-msm@vger.kernel.org>,
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	Matthew Rosato <mjrosato@linux.ibm.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Nelson Escobar <neescoba@cisco.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	Rob Clark <robdclark@gmail.com>,
	Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
	"virtualization@lists.linux-foundation.org"
	<virtualization@lists.linux-foundation.org>,
	Will Deacon <will@kernel.org>
Subject: Re: [PATCH 1/5] iommu: Replace uses of IOMMU_CAP_CACHE_COHERENCY with dev_is_dma_coherent()
Date: Thu, 7 Apr 2022 17:31:03 +0200	[thread overview]
Message-ID: <20220407153103.GA15336@lst.de> (raw)
In-Reply-To: <fb55a025-348e-800c-e368-48be075d8e9c@arm.com>

On Thu, Apr 07, 2022 at 04:17:11PM +0100, Robin Murphy wrote:
>> My take is that the drivers using this API are doing it to make sure
>> their HW blocks are setup in a way that is consistent with the DMA API
>> they are also using, and run in constrained embedded-style
>> environments that know the firmware support is present.
>>
>> So in the end it does not seem suitable right now for linking to
>> IOMMU_CACHE..
>
> That seems a pretty good summary - I think they're basically all "firmware 
> told Linux I'm coherent so I'd better act coherent" cases, but that still 
> doesn't necessarily mean that they're *forced* to respect that.

Yes. And the interface is horribly misnamed for that.  I'll see what
I can do to clean this up as I've noticed various other not very
nice things in that area.

> One of the 
> things on my to-do list is to try adding a DMA_ATTR_NO_SNOOP that can force 
> DMA cache maintenance for coherent devices, primarily to hook up in 
> Panfrost (where there is a bit of a performance to claw back on the 
> coherent AmLogic SoCs by leaving certain buffers non-cacheable).

This has been an explicit request from the amdgpu folks and thus been
on my TODO list for quite a while as well.  Note that I don't think it
should be a flag to dma_alloc_attrs, but rather for dma_alloc_pages
as the drivers that want non-snoop generally also want to actually
be able to deal with pages.

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