From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: "Marek Behún" <kabel@kernel.org>
Cc: Marcin Wojtas <mw@semihalf.com>, 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
Subject: Re: [PATCH] ARM: dts: armada-38x: Mark devices as dma-coherent
Date: Mon, 24 Oct 2022 08:51:34 +0100 [thread overview]
Message-ID: <Y1ZEBqrxZ1PSLCU6@shell.armlinux.org.uk> (raw)
In-Reply-To: <20221024085102.15712ce9@thinkpad>
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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next prev parent reply other threads:[~2022-10-24 7:52 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-22 23:40 [PATCH] ARM: dts: armada-38x: Mark devices as dma-coherent Marcin Wojtas
2022-10-23 15:04 ` Andrew Lunn
2022-10-23 16:20 ` Russell King (Oracle)
2022-10-23 21:30 ` Marcin Wojtas
2022-10-24 6:51 ` Marek Behún
2022-10-24 7:51 ` Russell King (Oracle) [this message]
2022-10-24 9:11 ` Marcin Wojtas
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