netdev - Re: [PATCH 04/12] soc: sifive: ccache: Add non-coherent DMA handling

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Message-ID: <Y/Nc+u2tP07zjdn5@wendy>
Date:   Mon, 20 Feb 2023 11:43:54 +0000
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     Emil Renner Berthing <emil.renner.berthing@...onical.com>
CC:     Conor Dooley <conor@...nel.org>,
        Cristian Ciocaltea <cristian.ciocaltea@...labora.com>,
        Lee Jones <lee@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "Paolo Abeni" <pabeni@...hat.com>,
        Emil Renner Berthing <kernel@...il.dk>,
        "Palmer Dabbelt" <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Richard Cochran <richardcochran@...il.com>,
        Sagar Kadam <sagar.kadam@...ive.com>,
        Yanhong Wang <yanhong.wang@...rfivetech.com>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <netdev@...r.kernel.org>, <linux-riscv@...ts.infradead.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <linux-arm-kernel@...ts.infradead.org>, <kernel@...labora.com>,
        <daire.mcnamara@...rochip.com>, <atishp@...shpatra.org>
Subject: Re: [PATCH 04/12] soc: sifive: ccache: Add non-coherent DMA handling

On Sun, Feb 19, 2023 at 10:32:52PM +0100, Emil Renner Berthing wrote:
> On Thu, 16 Feb 2023 at 19:51, Conor Dooley <conor@...nel.org> wrote:
> >
> > Emil,
> >
> > +CC Daire
> >
> > On Sat, Feb 11, 2023 at 05:18:13AM +0200, Cristian Ciocaltea wrote:
> > > From: Emil Renner Berthing <kernel@...il.dk>
> > >
> > > Add functions to flush the caches and handle non-coherent DMA.
> > >
> > > Signed-off-by: Emil Renner Berthing <kernel@...il.dk>
> > > [replace <asm/cacheflush.h> with <linux/cacheflush.h>]
> > > Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@...labora.com>
> > > ---
> >
> > > +void *sifive_ccache_set_uncached(void *addr, size_t size)
> > > +{
> > > +     phys_addr_t phys_addr = __pa(addr) + uncached_offset;
> > > +     void *mem_base;
> > > +
> > > +     mem_base = memremap(phys_addr, size, MEMREMAP_WT);
> > > +     if (!mem_base) {
> > > +             pr_err("%s memremap failed for addr %p\n", __func__, addr);
> > > +             return ERR_PTR(-EINVAL);
> > > +     }
> > > +
> > > +     return mem_base;
> > > +}
> >
> > The rest of this I either get b/c we did it, or will become moot so I
> > amn't worried about it, but can you please explain this, in particular
> > the memremap that you're doing here?
> 
> No, I can't really. As we talked about it's also based on a prototype
> by Atish. I'm sure you know that the general idea is that we want to
> return a pointer that accesses the same physical memory, but through
> the uncached alias.

Yah, I follow all the rest of what's going on - it's just this bit of it
that I don't.

> I can't tell you exactly why it's done this way
> though, sorry.

I had a bit of a look on lore, but don't really see anything there that
contained any discussion of what was going on here.

Adding Atish in the off-chance that he remembers!

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