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

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

Στις 2021-06-07 03:04, Guo Ren έγραψε:
> On Mon, Jun 7, 2021 at 2:14 AM Nick Kossifidis <mick@ics.forth.gr> 
> wrote:
>> 
>> 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"
> 
> In RISC-V VM TG, A C-bit discussion is raised. So it's a comm idea to
> support it.
> 

The C-bit was recently dropped, there is a new proposal for Page Based 
Memory Attributes (PBMT) that we can work on / push for.

> 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

The spec is clear, those attributes are for standard use only, not for 
custom/platform use. It's one thing to implement custom CMOs where the 
ISA doesn't have anything for it and doesn't prevent you to do so (so 
you are not violating anything, it's just a custom extension), and we 
can hide them behind SBI calls etc, and another to violate the current 
Privilege Spec by using bits on PTEs that are reserved for standard use 
only. The intentions of the VM TG are clear, not only those bits are 
reserved but if software uses them the hw will result a page fault in 
future revisions of the spec. What's the idea here, to support 
non-compliant implementations on mainline ? I'm sure you have a good 
idea on how to make this work, but as long as it violates the spec it 
can't go in IMHO.