Re: [RFC 2/3] iommu/samsung: Introduce Exynos sysmmu-v8 driver

[Sam Protsenko <semen.protsenko@linaro.org>]

On Wed, 22 Jun 2022 at 12:57, Marek Szyprowski <m.szyprowski@samsung.com> wrote:
>
>
> On 22.06.2022 11:14, Robin Murphy wrote:
> > On 2022-06-21 20:57, Sam Protsenko wrote:
> >> Hi Marek,
> >>
> >> On Fri, 21 Jan 2022 at 14:31, Marek Szyprowski
> >> <m.szyprowski@samsung.com> wrote:
> >>
> >> [snip]
> >>
> >>>
> >>> Well, for starting point the existing exynos-iommu driver is really
> >>> enough. I've played a bit with newer Exyos SoCs some time ago. If I
> >>> remember right, if you limit the iommu functionality to the essential
> >>> things like mapping pages to IO-virtual space, the hardware differences
> >>> between SYSMMU v5 (already supported by the exynos-iommu driver) and v7
> >>> are just a matter of changing a one register during the initialization
> >>> and different bits the page fault reason decoding. You must of course
> >>> rely on the DMA-mapping framework and its implementation based on
> >>> mainline DMA-IOMMU helpers. All the code for custom iommu group(s)
> >>> handling or extended fault management are not needed for the initial
> >>> version.
> >>>
> >>
> >> Thanks for the advice! Just implemented some testing driver, which
> >> uses "Emulated Translation" registers available on SysMMU v7. That's
> >> one way to verify the IOMMU driver with no actual users of it. It
> >> works fine with vendor SysMMU driver I ported to mainline earlier, and
> >> now I'm trying to use it with exynos-sysmmu driver (existing
> >> upstream). If you're curious -- I can share the testing driver
> >> somewhere on GitHub.
> >>
> >> I believe the register you mentioned is PT_BASE one, so I used
> >> REG_V7_FLPT_BASE_VM = 0x800C instead of REG_V5_PT_BASE_PFN. But I
> >> didn't manage to get that far, unfortunately, as
> >> exynos_iommu_domain_alloc() function fails in my case, with BUG_ON()
> >> at this line:
> >>
> >>      /* For mapping page table entries we rely on dma == phys */
> >>      BUG_ON(handle != virt_to_phys(domain->pgtable));
> >>
> >> One possible explanation for this BUG is that "dma-ranges" property is
> >> not provided in DTS (which seems to be the case right now for all
> >> users of "samsung,exynos-sysmmu" driver). Because of that the SWIOTLB
> >> is used for dma_map_single() call (in exynos_iommu_domain_alloc()
> >> function), which in turn leads to that BUG. At least that's what
> >> happens in my case. The call chain looks like this:
> >>
> >>      exynos_iommu_domain_alloc()
> >>          v
> >>      dma_map_single()
> >>          v
> >>      dma_map_single_attrs()
> >>          v
> >>      dma_map_page_attrs()
> >>          v
> >>      dma_direct_map_page()  // dma_capable() == false
> >>          v
> >>      swiotlb_map()
> >>          v
> >>      swiotlb_tbl_map_single()
> >>
> >> And the last call of course always returns the address different than
> >> the address for allocated pgtable. E.g. in my case I see this:
> >>
> >>      handle = 0x00000000fbfff000
> >>      virt_to_phys(domain->pgtable) = 0x0000000880d0c000
> >>
> >> Do you know what might be the reason for that? I just wonder how the
> >> SysMMU driver work for all existing Exynos platforms right now. I feel
> >> I might be missing something, like some DMA option should be enabled
> >> so that SWIOTLB is not used, or something like that. Please let me
> >> know if you have any idea on possible cause. The vendor's SysMMU
> >> driver is kinda different in that regard, as it doesn't use
> >> dma_map_single(), so I don't see such issue there.
> >
> > If this SysMMU version is capable of addressing more than 32 bits,
> > then exynos_iommu_probe_device() should set its DMA masks appropriately.
>
> Well, Sam's question was about the Exynos SYSMMU own platform device,
> not the device for which Exynos SYSMMU manages the IO virtual address
> space.
>
> Simply add something like
>
> dma_set_mask(dev, DMA_BIT_MASK(36));
>

Yep, that one worked, thanks! Just submitted a patch, with a bit of
additions: [1].

[1] https://lkml.org/lkml/2022/7/2/269

> to the beginning of the exynos_sysmmu_probe(). This will disable SWIOTLB
> and switch to DMA-direct for the Exynos SYSMMU platform device.
>
>
> > (as a side note since I looked, the use of PAGE_SIZE/PAGE_SHIFT in the
> > driver looks wrong, since I can't imagine that the hardware knows
> > whether Linux is using 4KB, 16KB or 64KB and adjusts itself
> > accordingly...)
>
> Right, this has to be cleaned up. This driver was never used on systems
> with kernel configuration for non-4KB pages.
>
> Best regards
> --
> Marek Szyprowski, PhD
> Samsung R&D Institute Poland
>