Re: [PATCH 1/2] dma-mapping: let arch know origin of dma range passed to arch_setup_dma_ops()

[Nikita Yushchenko <nikita.yoush@cogentembedded.com>]

>>>> Hmm, I think when the dma-ranges are missing, we should either enforce
>>>> a 32-bit mask, or disallow DMA completely. It's probably too late for
>>>> the latter, I wish we had done this earlier in order to force everyone
>>>> on ARM64 to have a valid dma-ranges property for any DMA master.
>>>
>>> This can be done over time.
>>>
>>> However the very idea of this version of patch is - keep working pieces
>>> as-is, thus for now setting enforce_range to false in case of no defined
>>> dma-ranges is intentional.
>>
>> What we can do is - check bus width (as it is defined in DT) and set
>> enforce_range to true if bus is 32-bit
>>
>>> What I should re-check is - does rcar dtsi set dma-ranges, and add it if
>>> it does not.
>>
>> It does not, will have to add.
>>
>> In DT bus is defined as 64-bit. But looks like physically it is 32-bit.
>> Maybe DT needs fixing.
> 
> I think we always assumed that the lack of a dma-ranges property
> implied a 32-bit width, as that is the safe fallback as well as the
> most common case.

Yes we assumed that, but that was combined with blindly accepting wider
dma_mask per driver's request.  Later is being changed.

> AFAICT, this means you are actually fine on rcar, and all other
> platforms will keep working as we enforce it, but might get slowed
> down if they relied on the unintended behavior of allowing 64-bit
> DMA.

Yesterday Robin raised issue that a change starting to enforce default
dma_mask will break existing setups - i.e. those that depend in 64bit
DMA being implicitly supported without manually declaring such support.

In reply to that, I suggested this version of patchset that should keep
existing behavior by default.

I'm fine with both approaches regarding behavior on hw that I don't have
- but I'm not in position to make any decisions on that.