Re: use of dma_direct_set_offset in (allwinner) drivers

[Robin Murphy <robin.murphy@arm.com>]

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.

Robin.

>> drivers/media/platform/sunxi/sun6i-csi/sun6i_csi.c
>>
>>    This driver unconditionally sets the offset.  Why can't we do this
>>    in the device tree?
>>
>> drivers/staging/media/sunxi/cedrus/cedrus_hw.c
>>
>>    Same as above.
>>
> 
> We should make those two match the previous ones, but we'll have the
> same issue here eventually. Most likely they were never ran on an SoC
> for which we have the MBUS figured out.
> 
> Maxime
> 
> 
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