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

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Message-ID: <fc6e1b6689bca7a00e6b12d2bc877d20@codeaurora.org>
Date:   Tue, 18 Feb 2020 17:57:18 -0800
From:   isaacm@...eaurora.org
To:     Robin Murphy <robin.murphy@....com>
Cc:     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 2020-02-17 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?
> 
> Robin.