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

From: Guo Ren <guoren@kernel.org>
To: Nick Kossifidis <mick@ics.forth.gr>
Cc: "Christoph Hellwig" <hch@lst.de>,
	"Drew Fustini" <drew@beagleboard.org>,
	"Anup Patel" <anup.patel@wdc.com>,
	"Palmer Dabbelt" <palmerdabbelt@google.com>,
	wefu@redhat.com, "Wei Wu (吴伟)" <lazyparser@gmail.com>,
	linux-riscv <linux-riscv@lists.infradead.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	linux-sunxi@lists.linux.dev, "Guo Ren" <guoren@linux.alibaba.com>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Benjamin Koch" <snowball@c3pb.de>,
	"Matteo Croce" <mcroce@linux.microsoft.com>,
	"Wei Fu" <tekkamanninja@gmail.com>
Subject: Re: [PATCH RFC 0/3] riscv: Add DMA_COHERENT support
Date: Wed, 9 Jun 2021 11:28:19 +0800	[thread overview]
Message-ID: <CAJF2gTQ5271AP8aw42yvfOg0LjtnmPD8j_Uza6NH2nHxVz_QgQ@mail.gmail.com> (raw)
In-Reply-To: <29733b0931d9dd6a2f0b6919067c7efe@mailhost.ics.forth.gr>

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"
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.

-- 
Best Regards
 Guo Ren

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