On Mon, Jun 7, 2021 at 2:14 AM Nick Kossifidis [off-list ref] wrote:

Στις 2021-05-20 04:45, Guo Ren έγραψε:

quoted

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

quoted

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.

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/