On 2020-09-24 11:47, Marek Szyprowski wrote:
Hi Robin,

On 24.09.2020 12:40, Robin Murphy wrote:
On 2020-09-24 11:16, Thierry Reding wrote:
On Thu, Sep 24, 2020 at 10:46:46AM +0200, Marek Szyprowski wrote:
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.red...@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.

It can't easily be a *direct* mapping though - if the driver has to
use the DMA masks to ensure that everything stays within the
addressable range, then (as far as I'm aware) there's no physical
memory that low down to equal the DMA addresses.

TBH I'm not convinced that this is a sufficiently common concern to
justify new APIs, or even to try to make overly generic. I think just
implementing a new DMA attribute to say "please allocate/map this
particular request at the lowest DMA address possible" would be good
enough. Such a thing could also serve PCI drivers that actually care
about SAC/DAC to give us more of a chance of removing the "try a
32-bit mask first" trick from everyone's hotpath...

Hmm, I like the idea of such DMA attribute! It should make things really
simple, especially in the drivers. Thanks for the great idea! I will try
to implement it then instead of the workarounds I've proposed in
s5p-mfc/exynos4-is drivers.

Right, I think it's fair to draw a line and say that anyone who wants a *specific* address needs to manage their own IOMMU domain.

In the backend I suspect it's going to be cleanest to implement a dedicated iova_alloc_low() (or similar) function in the IOVA API that sidesteps all of the existing allocation paths and goes straight to the rbtree.

Robin.
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