linux-kernel - Re: [PATCH] ARM: dts: armada-38x: Mark devices as dma-coherent

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Date:   Mon, 24 Oct 2022 08:51:34 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Marek Behún <kabel@...nel.org>
Cc:     Marcin Wojtas <mw@...ihalf.com>, Andrew Lunn <andrew@...n.ch>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, gregory.clement@...tlin.com,
        sebastian.hesselbarth@...il.com, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, hch@....de, jaz@...ihalf.com
Subject: Re: [PATCH] ARM: dts: armada-38x: Mark devices as dma-coherent

On Mon, Oct 24, 2022 at 08:51:02AM +0200, Marek Behún 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.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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