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From:   Guo Ren <guoren@kernel.org>
Date:   Mon, 7 Jun 2021 14:41:14 +0800
Message-ID: <CAJF2gTRZuER5YbTD=0xWLU0Np6eD8L_z3rZH0i_WXgENUD3nbQ@mail.gmail.com>
Subject: Re: [PATCH RFC 0/3] riscv: Add DMA_COHERENT support
To:     Christoph Hellwig <hch@lst.de>
Cc:     Nick Kossifidis <mick@ics.forth.gr>,
        Drew Fustini <drew@beagleboard.org>,
        Anup Patel <anup.patel@wdc.com>,
        Palmer Dabbelt <palmerdabbelt@google.com>, wefu@redhat.com,
        =?UTF-8?B?V2VpIFd1ICjlkLTkvJ8p?= <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>
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On Mon, Jun 7, 2021 at 2:27 PM Christoph Hellwig <hch@lst.de> wrote:
>
> On Mon, Jun 07, 2021 at 11:19:03AM +0800, Guo Ren wrote:
> > >From Linux non-coherency view, we need:
> >  - Non-cache + Strong Order PTE attributes to deal with drivers' DMA descriptors
> >  - Non-cache + weak order to deal with framebuffer drivers
> >  - CMO dma_sync to sync cache with DMA devices
>
> This is not strictly true.  At the very minimum you only need cache
> invalidation and writeback instructions.  For example early parisc
> CPUs and some m68knommu SOCs have no support for uncached areas at all,
> and Linux works.  But to be fair this is very painful and supports only
> very limited periphals.  So for modern full Linux support some uncahed
> memory is advisable.  But that doesn't have to be using PTE attributes.
> It could also be physical memory regions that are either totally fixed
Double/Triple the size of physical memory regions can't be accepted by
SOC vendors, because it wastes HW resources.
Some cost-down soc interconnects only have 32bit~34bit width of
physical address, are you sure you could force them to expand it? (I
can't)

> or somewhat dynamic.
How can HW implement with dynamic modifying PMA? What's the granularity?


-- 
Best Regards
 Guo Ren

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