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

[Christoph Hellwig <hch@lst.de>]

On Tue, May 18, 2021 at 11:44:35PM -0700, Drew Fustini wrote:
> 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].

Not too much, given that the SiFive lineage CPUs have an uncached
window, that is a totally different way to allocate uncached memory.

> 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.

It does, because unimplemented instructions trap into M-mode, where they
can be emulated.

Or to summarize things.  Non-coherent DMA (and not coherent as title in
this series) requires two things:

 1) allocating chunks of memory that is marked as not cachable
 2) instructions to invalidate and/or writeback cache lines

none of which currently exists in RISV-V.  Hacking vendor specific
cruft into the kernel doesn't scale, as shown perfectly by this
series which requires to hard code vendor specific non-standardized
extensions in a kernel that makes it specific to that implementation.

What we need to do is to standardize a way to do this properly, and then
after that figure out a way to quirk in non-compliant implementations
in a way that does not harm the general kernel.