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

[Guo Ren]

On Mon, Jun 7, 2021 at 2:14 AM Nick Kossifidis <mick@xxxxxxxxxxxx> wrote:
>
> Στις 2021-05-20 04:45, Guo Ren έγραψε:
> > On Wed, May 19, 2021 at 2:53 PM Christoph Hellwig <hch@xxxxxx> wrote:
> >>
> >> 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.
> > 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.
> >
>
> Not only it doesn't support custom attributes on PTEs:
>
> "Bits63–54 are reserved for future standard use and must be zeroed by
> software for forward compatibility."
>
> It also goes further to say that:
>
> "if any of these bits are set, a page-fault exception is raised"

In RISC-V VM TG, A C-bit discussion is raised. So it's a comm idea to
support it.

Let Linux support custom PTE attributes won't get any side effect in practice.

IMO:
We needn't waste a bit in PTE, but the custom idea in PTE reserved
bits is necessary. Because Allwinner D1 needs custom PTE bits in
reserved bits to work around.
So I recommend just remove the "C" bit in PTE, but allow vendors to
define their own PTE attributes in reserved bits. I've found a way to
compact different PTE attributes of different vendors during the Linux
boot stage. That means we still could use One Image for all vendors in
Linux



-- 
Best Regards
 Guo Ren

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