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

[Nick Kossifidis <mick@ics.forth.gr>]

Στις 2021-05-20 04:45, Guo Ren έγραψε:
> On Wed, May 19, 2021 at 2:53 PM Christoph Hellwig <hch@lst.de> 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"