Re: [PATCH RFC 0/3] riscv: Add DMA_COHERENT support

[Drew Fustini]

On Wed, May 19, 2021 at 01:48:23PM +0800, Guo Ren wrote:
> On Wed, May 19, 2021 at 1:20 PM Christoph Hellwig <hch@xxxxxx> wrote:
> >
> > On Wed, May 19, 2021 at 05:04:13AM +0000, guoren@xxxxxxxxxx wrote:
> > > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > >
> > > The RISC-V ISA doesn't yet specify how to query or modify PMAs, so let
> > > vendors define the custom properties of memory regions in PTE.
> >
> > Err, hell no.   The ISA needs to gets this fixed first.  Then we can
> > talk about alternatives patching things in or trapping in the SBI.
> > But if the RISC-V ISA can't get these basic done after years we can't
> > support it in Linux at all.
> 
> The patchset just leaves a configuration chance for vendors. Before
> RISC-V ISA fixes it, we should give the chance to let vendor solve
> their real chip issues.

This patch series looks like it might be useful for the StarFive JH7100
[1] [2] too as it has peripherals on a non-coherent interconnect. GMAC,
USB and SDIO require that the L2 cache must be manually flushed after
DMA operations if the data is intended to be shared with U74 cores [2].

There is the RISC-V Cache Management Operation, or CMO, task group [3]
but I am not sure if that can help the SoC's that have already been
fabbed like the the D1 and the JH7100.

thanks,
drew

[1] https://github.com/starfive-tech/beaglev_doc/blob/main/JH7100%20Data%20Sheet%20V01.01.04-EN%20(4-21-2021).pdf
[2] https://github.com/starfive-tech/beaglev_doc/blob/main/JH7100%20Cache%20Coherence%20V1.0.pdf
[3] https://github.com/riscv/riscv-CMOs