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

[Guo Ren]

On Wed, Jun 9, 2021 at 5:45 PM Nick Kossifidis <mick@xxxxxxxxxxxx> wrote:
>
> Στις 2021-06-09 06:28, Guo Ren έγραψε:
> > 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"
> > Agree, when our processor's mmu works in compatible mmu, we must keep
> > "Bits63–54 bit" zero in Linux.
> > So, I think this is the first version of the PTE format.
> >
> > If the "PBMT" extension proposal is approved, it will cause the second
> > version of the PTE format.
> >
> > Maybe in the future, we'll get more versions of the PTE formats.
> >
> > So, seems Linux must support multi versions of PTE formats with one
> > Image, right?
> >
> > Okay, we could stop arguing with the D1 PTE format. And talk about how
> > to let Linux support multi versions of PTE formats that come from the
> > future RISC-V privilege spec.
>
> The RISC-V ISA specs are meant to be backwards compatible, so newer PTE
> versions should work on older devices (note that the spec says that
> software must set those bits to zero for "forward compatibility" and are
> "reserved for future use" so current implementations must ignore them).
> Obviously the proposed "if any of these bits are set, a page-fault
> exception is raised" will break backwards compatibility which is why we
> need to ask for it to be removed from the draft.
>
> As an example the PBMT proposal uses bits 62:61 that on older hw should
> be ignored ("reserved for future use"), if Linux uses those bits we
> won't need a different code path for supporting older hw/older PTE
> versions, we'll just set them and older hw will ignore them. Because of
> the guarantee that ISA specs maintain backwards compatibility, the
> functionality of bits 62:61 is guaranteed to remain backwards
> compatible.
the spec says that software must set those bits to zero for "forward
compatibility". So how older hw ignore them?
If an older hw follow the current spec requires software to set those
bits to zero, how we put any PBMT bits without different Linux PTE
formats?

>
> In other words we don't need any special handling of multiple PTE
> formats, we just need to support the latest Priv. Spec and the Spec
> itself will guarantee backwards compatibility.
Nak, totally no Logically self-consistent.

-- 
Best Regards
 Guo Ren

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