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/
