Re: [PATCH 0/6] RISC-V: Add eMMC support for TH1520 boards

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

On 04/10/2023 3:02 pm, Icenowy Zheng wrote:
[...]
>>>> I believe commit 484861e09f3e ("soc: renesas: Kconfig: Select the
>>>> required configs for RZ/Five SoC") can cause regression on all
>>>> non-dma-coherent riscv platforms with generic defconfig. This is
>>>> a common issue. The logic here is: generic riscv defconfig
>>>> selects
>>>> ARCH_R9A07G043 which selects DMA_GLOBAL_POOL, which assumes all
>>>> non-dma-coherent riscv platforms have a dma global pool, this
>>>> assumption
>>>> seems not correct. And I believe DMA_GLOBAL_POOL should not be
>>>> selected by ARCH_SOCFAMILIY, instead, only ARCH under some
>>>> specific
>>>> conditions can select it globaly, for example NOMMU ARM and so
>>>> on.
>>>>
>>>> Since this is a regression, what's proper fix? any suggestion is
>>>> appreciated.
>>
>> I think the answer is to not select DMA_GLOBAL_POOL, since that is
>> only
> 
> Well I think for RISC-V, it's not NOMMU only but applicable for every
> core that does not support Svpbmt or vendor-specific alternatives,
> because the original RISC-V priv spec does not define memory attributes
> in page table entries.
> 
> For the Renesas/Andes case I think a pool is set by OpenSBI with
> vendor-specific M-mode facility and then passed in DT, and the S-mode
> (which MMU is enabled in) just sees fixed memory attributes, in this
> case I think DMA_GLOBAL_POOL is needed.

Oh wow, is that really a thing? In that case, either you just can't 
support this platform in a multi-platform kernel, or someone needs to do 
some fiddly work in dma-direct to a) introduce the notion of an optional 
global pool, and b) make it somehow cope with DMA_DIRECT_REMAP being 
enabled but non-functional.

Thanks,
Robin.