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.reding@xxxxxxxxx/ >> >> 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
