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From: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
To: Guo Ren <guoren@kernel.org>
Cc: "Nick Kossifidis" <mick@ics.forth.gr>,
	"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 14:05:21 +0800	[thread overview]
Message-ID: <20210609140521.3038e40e@xhacker.debian> (raw)
In-Reply-To: <CAJF2gTQ5271AP8aw42yvfOg0LjtnmPD8j_Uza6NH2nHxVz_QgQ@mail.gmail.com>

On Wed, 9 Jun 2021 11:28:19 +0800
Guo Ren <guoren@kernel.org> wrote:


> 
> 
> 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.
> > >

<snip>

> > > 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.
> 

Just my humble opinion:
When those bits(63~54) usage are standardized in future RISC-V privilege spec
generic Image can still be supported with the following solutions:

*alternative patch only fly:
If the bit is only need to be set during init, we may insert nop instruction(s)
at proper place, then patch the nop into set_the_target_bit instruction(s) by
hart's feature.

*normal check feature then use:
If the feature needs a bit complex code, we could go through the "feature check
then use". static key tech can be used here to avoid branches.

  reply	other threads:[~2021-06-09  6:05 UTC|newest]

Thread overview: 56+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-19  5:04 [PATCH RFC 0/3] riscv: Add DMA_COHERENT support guoren
2021-05-19  5:04 ` [PATCH RFC 1/3] riscv: pgtable.h: Fixup _PAGE_CHG_MASK usage guoren
2021-05-19  5:04 ` [PATCH RFC 2/3] riscv: Add DMA_COHERENT for custom PTE attributes guoren
2021-05-19  5:04 ` [PATCH RFC 3/3] riscv: Add SYNC_DMA_FOR_CPU/DEVICE for DMA_COHERENT guoren
2021-05-19  6:32   ` Guo Ren
2021-05-19  5:20 ` [PATCH RFC 0/3] riscv: Add DMA_COHERENT support Christoph Hellwig
2021-05-19  5:48   ` Guo Ren
2021-05-19  5:55     ` Christoph Hellwig
2021-05-19  6:09       ` Guo Ren
2021-05-19  6:44     ` Drew Fustini
2021-05-19  6:53       ` Christoph Hellwig
2021-05-20  1:45         ` Guo Ren
2021-05-20  5:48           ` Christoph Hellwig
2021-06-06 18:14           ` Nick Kossifidis
2021-06-07  0:04             ` Guo Ren
2021-06-07  2:16               ` Nick Kossifidis
2021-06-07  3:19                 ` Guo Ren
2021-06-07  6:27                   ` Christoph Hellwig
2021-06-07  6:41                     ` Guo Ren
2021-06-07  6:51                       ` Christoph Hellwig
2021-06-07  7:46                         ` Guo Ren
2021-06-08 15:00                     ` David Laight
2021-06-08 15:32                       ` 'Christoph Hellwig'
2021-06-08 16:11                         ` David Laight
2021-06-07  8:35                   ` Nick Kossifidis
2021-06-09  3:28             ` Guo Ren
2021-06-09  6:05               ` Jisheng Zhang [this message]
2021-06-09  9:45               ` Nick Kossifidis
2021-06-09 12:43                 ` Guo Ren
2021-05-19  6:05   ` Guo Ren
2021-05-19  6:06     ` Christoph Hellwig
2021-05-19  6:11       ` Guo Ren
2021-05-19  6:54       ` Drew Fustini
2021-05-19  6:56         ` Christoph Hellwig
2021-05-19  7:14         ` Anup Patel
2021-05-19  8:25           ` Damien Le Moal
2021-05-20  1:47           ` Guo Ren
2021-05-20  1:59             ` Guo Ren
2021-05-22  0:36           ` Guo Ren
2021-05-30  0:30             ` Palmer Dabbelt
2021-06-03  4:13               ` Palmer Dabbelt
2021-06-03  6:00                 ` Anup Patel
2021-06-03 15:39                   ` Palmer Dabbelt
2021-06-04  9:02                     ` David Laight
2021-06-04  9:53                     ` Arnd Bergmann
2021-06-04 14:47                       ` Guo Ren
2021-06-04 16:12                         ` Palmer Dabbelt
2021-06-04 21:26                           ` Arnd Bergmann
2021-06-04 22:10                             ` Palmer Dabbelt
2021-06-08 12:26                           ` Guo Ren
2021-06-06 17:11                   ` Guo Ren
2021-06-07  3:38                     ` Anup Patel
2021-06-07  4:22                       ` Guo Ren
2021-06-07  4:47                         ` Anup Patel
2021-06-07  5:08                           ` Guo Ren
2021-06-07  5:13                           ` Guo Ren

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