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.

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

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