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    SubjectRe: use of dma_direct_set_offset in (allwinner) drivers
    On Wed, Nov 04, 2020 at 10:15:49AM +0000, Robin Murphy wrote:
    > On 2020-11-04 08:14, Maxime Ripard wrote:
    > > Hi Christoph,
    > >
    > > On Tue, Nov 03, 2020 at 10:55:38AM +0100, Christoph Hellwig wrote:
    > > > Linux 5.10-rc1 switched from having a single dma offset in struct device
    > > > to a set of DMA ranges, and introduced a new helper to set them,
    > > > dma_direct_set_offset.
    > > >
    > > > This in fact surfaced that a bunch of drivers that violate our layering
    > > > and set the offset from drivers, which meant we had to reluctantly
    > > > export the symbol to set up the DMA range.
    > > >
    > > > The drivers are:
    > > >
    > > > drivers/gpu/drm/sun4i/sun4i_backend.c
    > > >
    > > > This just use dma_direct_set_offset as a fallback. Is there any good
    > > > reason to not just kill off the fallback?
    > > >
    > > > drivers/media/platform/sunxi/sun4i-csi/sun4i_csi.c
    > > >
    > > > Same as above.
    > >
    > > So, the history of this is:
    > >
    > > - We initially introduced the support for those two controllers
    > > assuming that there was a direct mapping between the physical and
    > > DMA addresses. It turns out it didn't and the DMA accesses were
    > > going through a secondary, dedicated, bus that didn't have the same
    > > mapping of the RAM than the CPU.
    > >
    > > 4690803b09c6 ("drm/sun4i: backend: Offset layer buffer address by DRAM starting address")
    > >
    > > - This dedicated bus is undocumented and barely used in the vendor
    > > kernel so this was overlooked, and it's fairly hard to get infos on
    > > it for all the SoCs we support. We added the DT support for it
    > > though on some SoCs we had enough infos to do so:
    > >
    > > c43a4469402f ("dt-bindings: interconnect: Add a dma interconnect name")
    > > 22f88e311399 ("ARM: dts: sun5i: Add the MBUS controller")
    > >
    > > This explains the check on the interconnect property
    > >
    > > - However, due to the stable DT rule, we still need to operate without
    > > regressions on older DTs that wouldn't have that property (and for
    > > SoCs we haven't figured out). Hence the fallback.
    >
    > How about having something in the platform code that keys off the top-level
    > SoC compatible and uses a bus notifier to create offsets for the relevant
    > devices if an MBUS description is missing? At least that way the workaround
    > could be confined to a single dedicated place and look somewhat similar to
    > other special cases like sta2x11, rather than being duplicated all over the
    > place.

    I'll give it a try, thanks for the suggestion :)

    Maxime
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