linux-kernel - Re: [RFC PATCH] iommu/dma: Allow drivers to reserve an iova range

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Date:   Wed, 19 Feb 2020 12:06:28 -0800
From:   isaacm@...eaurora.org
To:     Will Deacon <will@...nel.org>
Cc:     Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@...radead.org>, pratikp@...eaurora.org,
        linux-kernel@...r.kernel.org, Liam Mark <lmark@...eaurora.org>,
        iommu@...ts.linux-foundation.org, kernel-team@...roid.com
Subject: Re: [RFC PATCH] iommu/dma: Allow drivers to reserve an iova range

On 2020-02-19 03:15, Will Deacon wrote:
> On Tue, Feb 18, 2020 at 05:57:18PM -0800, isaacm@...eaurora.org wrote:
>> On 2020-02-17 07:50, Robin Murphy wrote:
>> > On 17/02/2020 8:01 am, Christoph Hellwig wrote:
>> > > On Fri, Feb 14, 2020 at 02:58:16PM -0800, Isaac J. Manjarres wrote:
>> > > > From: Liam Mark <lmark@...eaurora.org>
>> > > >
>> > > > Some devices have a memory map which contains gaps or holes.
>> > > > In order for the device to have as much IOVA space as possible,
>> > > > allow its driver to inform the DMA-IOMMU layer that it should
>> > > > not allocate addresses from these holes.
>> > >
>> > > Layering violation.  dma-iommu is the translation layer between the
>> > > DMA API and the IOMMU API.  And calls into it from drivers performing
>> > > DMA mappings need to go through the DMA API (and be documented there).
>> >
>> > +1
>> >
>> > More than that, though, we already have "holes in the address space"
>> > support for the sake of PCI host bridge windows - assuming this is the
>> > same kind of thing (i.e. the holes are between memory regions and
>> > other resources in PA space, so are only relevant once address
>> > translation comes into the picture), then this is IOMMU API level
>> To make sure that we're on the same page, this support alludes to the
>> handling in
>> dma-iommu.c that reserves portions of the IOVA space for the PCI host 
>> bridge
>> windows,
>> correct? If so, then yes, this is similar.
>> > stuff, so even a DMA API level interface would be inappropriate.
>> Does this mean that the driver should be managing the IOVA space and
>> mappings for this device using the IOMMU API? If so, is the rationale 
>> for
>> this because the device driver can have the information of what IOVA 
>> ranges
>> can and cannot be used? Shouldn't there be a generic way of informing 
>> an
>> IOMMU driver about these reserved ranges? Perhaps through a device 
>> tree
>> property, instead of deferring this type of management to the driver?
> 
> Before we dive into designing that, can you please clarify whether the
> reserved IOVA range applies to all DMA masters mastering through a
> particular SMMU, or whether it's just about one specific master? I was
> assuming the former, but wanted to be sure.
> 
This situation currently applies to one master.
> Thanks,
> 
> Will

Thanks,
Isaac