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From:   Marcin Wojtas <mw@semihalf.com>
Date:   Mon, 24 Oct 2022 11:11:12 +0200
Message-ID: <CAPv3WKeL30nhrFdVTgAhpYUk7v2zR4F9E3NrBrsGvYtFU-mzRA@mail.gmail.com>
Subject: Re: [PATCH] ARM: dts: armada-38x: Mark devices as dma-coherent
To:     "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc:     =?UTF-8?B?TWFyZWsgQmVow7pu?= <kabel@kernel.org>,
        Andrew Lunn <andrew@lunn.ch>,
        linux-arm-kernel@lists.infradead.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, gregory.clement@bootlin.com,
        sebastian.hesselbarth@gmail.com, robh+dt@kernel.org,
        krzysztof.kozlowski+dt@linaro.org, hch@lst.de, jaz@semihalf.com
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pon., 24 pa=C5=BA 2022 o 09:51 Russell King (Oracle)
<linux@armlinux.org.uk> napisa=C5=82(a):
>
> On Mon, Oct 24, 2022 at 08:51:02AM +0200, Marek Beh=C3=BAn wrote:
> > > Sure. In such a case the description of 380 variant (single core)
> > > should remain untouched.
> > >
> > > We need to decide what to do with dual-CPU, i.e. Armada 385/388. How =
about:
> > > - Don't change current behavior, i.e. perform a necessary kernel
> > > configuration in "arm,pl310-cache" driver,
> > > arch/arm/mach-mvebu/coherency.c + &coherencyfab:node in DT
> > > - Satisfy of_dma_is_coherent() by adding `dma-coherent;` in
> > > armada-385.dtsi only (IMO this would describe HW properly)
> > > ?
> >
> > It will describe HW properly, but someone running older kernel compiled
> > with no SMP support will see a performance drop. I wonder how many
> > people do that.
>
> If the kernel is built without SMP support, the page table entries will
> not have the shared bit set, and the system will _not_ be DMA-coherent.
> Having DT mark devices as "dma-coherent" in this case will lead to data
> corruption, because the DMA API will believe them to be DMA-coherent
> when the page tables are not setup for that to work.
>

Thanks, for the explanation. Since we're heavily dependent on what
happens in the kernel we boot, it will be easier to just drop this
patch and keep using the DT as-is.

Best regards,
Marcin