On Wed, May 19, 2021 at 2:53 PM Christoph Hellwig [off-list ref] wrote:

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

quoted

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.

It's a very big MIPS smell. What's the attribute of the uncached
window? (uncached + strong-order/ uncached + weak, most vendors still
use AXI interconnect, how to deal with a bufferable attribute?) In
fact, customers' drivers use different ways to deal with DMA memory in
non-coherent SOC. Most riscv SOC vendors are from ARM, so giving them
the same way in DMA memory is a smart choice. So using PTE attributes
is more suitable.

See: https://github.com/riscv/virtual-memory/blob/main/specs/611-virtual-memory-diff.pdf
4.4.1
The draft supports custom attribute bits in PTE.

Although I do not agree with uncached windows, this patchset does not
conflict with SiFive uncached windows.

quoted

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

Maybe sbi_ecall (dma_sync) is enough and let the vendor deal with it
in opensbi. From a hardware view, CMO instruction only could deal with
one cache line, then CMO-trap is not a good idea.

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.

-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/