Re: IOVA allocation dependency between firmware buffer and remaining buffers

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hi Thierry,

On 24.09.2020 12:16, 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.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.

Frankly, using that would be even bigger hack than what I've proposed in 
my workaround. I see no point polluting DT with such artificial regions 
just to ensure specific IOVA space layout.

I just looking for a nice way of reusing most of the IOMMU DMA-mapping 
code with a small addition of manual IOVA space management (just to 
remap the firmware buffer at the specific IOVA address).

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland