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

[Icenowy Zheng <uwu@icenowy.me>]

在 2023-10-04星期三的 15:18 +0100，Robin Murphy写道：
> 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 

Emmmm thus RZ/Five should `depends on NONPORTABLE`?

> 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.