Re: IOVA allocation dependency between firmware buffer and remaining buffers

From: Thierry Reding <thierry.reding@gmail.com>
To: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Joerg Roedel <joro@8bytes.org>,
	Shaik Ameer Basha <shaik.samsung@gmail.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Ajay kumar <ajaynumb@gmail.com>,
	Linux IOMMU <iommu@lists.linux-foundation.org>,
	linux-mm@kvack.org, Rob Clark <robdclark@gmail.com>,
	jean-philippe@linaro.org, will@kernel.org, hch@lst.de,
	baolu.lu@linux.intel.com,
	Shaik Ameer Basha <shaik.ameer@samsung.com>
Subject: Re: IOVA allocation dependency between firmware buffer and remaining buffers
Date: Thu, 24 Sep 2020 12:16:40 +0200	[thread overview]
Message-ID: <20200924101640.GE2483160@ulmo> (raw)
In-Reply-To: <37e767b8-8ec4-ae80-ea0d-1caf3cdab8fa@samsung.com>

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On Thu, Sep 24, 2020 at 10:46:46AM +0200, Marek Szyprowski wrote:
> Hi Joerg,
> 
> On 24.09.2020 10:28, Joerg Roedel wrote:
> > On Wed, Sep 23, 2020 at 08:48:26AM +0200, Marek Szyprowski wrote:
> >> It allows to remap given buffer at the specific IOVA address, although
> >> it doesn't guarantee that those specific addresses won't be later used
> >> by the IOVA allocator. Probably it would make sense to add an API for
> >> generic IOMMU-DMA framework to mark the given IOVA range as
> >> reserved/unused to protect them.
> > There is an API for that, the IOMMU driver can return IOVA reserved
> > regions per device and the IOMMU core code will take care of mapping
> > these regions and reserving them in the IOVA allocator, so that
> > DMA-IOMMU code will not use it for allocations.
> >
> > Have a look at the iommu_ops->get_resv_regions() and
> > iommu_ops->put_resv_regions().
> 
> I know about the reserved regions IOMMU API, but the main problem here, 
> in case of Exynos, is that those reserved regions won't be created by 
> the IOMMU driver but by the IOMMU client device. It is just a result how 
> the media drivers manages their IOVA space. They simply have to load 
> firmware at the IOVA address lower than the any address of the used 
> buffers.

I've been working on adding a way to automatically add direct mappings
using reserved-memory regions parsed from device tree, see:

    https://lore.kernel.org/lkml/20200904130000.691933-1-thierry.reding@gmail.com/

Perhaps this can be of use? With that you should be able to add a
reserved-memory region somewhere in the lower range that you need for
firmware images and have that automatically added as a direct mapping
so that it won't be reused later on for dynamic allocations.

Thierry

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