Re: [PATCH] ARM: mvebu: select OF_DMA_DEFAULT_COHERENT if MACH_MVEBU_V7

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Thorsten Leemhuis <regressions@leemhuis.info>
Cc: "Marek Behún" <kabel@kernel.org>,
	"Christoph Hellwig" <hch@lst.de>,
	"Marcin Wojtas" <mw@semihalf.com>,
	pali@kernel.org, "Robin Murphy" <robin.murphy@arm.com>,
	"Arnd Bergmann" <arnd@kernel.org>,
	"Andre Przywara" <andre.przywara@arm.com>,
	"Marc Zyngier" <maz@kernel.org>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Andrew Lunn" <andrew@lunn.ch>,
	"Gregory Clement" <gregory.clement@bootlin.com>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	iommu@lists.linux-foundation.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] ARM: mvebu: select OF_DMA_DEFAULT_COHERENT if MACH_MVEBU_V7
Date: Tue, 4 Oct 2022 10:22:06 +0100	[thread overview]
Message-ID: <Yzv7Pin2qbUPCl0A@shell.armlinux.org.uk> (raw)
In-Reply-To: <c3cf8923-26f7-3898-8e7f-be19ff9c2a08@leemhuis.info>

On Tue, Oct 04, 2022 at 11:14:55AM +0200, Thorsten Leemhuis wrote:
> On 04.10.22 10:17, Marek Behún wrote:
> > Commit ae626eb97376 ("ARM/dma-mapping: use dma-direct unconditionally")
> > caused a regression on the mvebu platform, wherein devices that are
> > dma-coherent are marked as dma-noncoherent, because although
> > mvebu_hwcc_notifier() after that commit still marks then as coherent,
> > the arm_coherent_dma_ops() function, which is called later, overwrites
> > this setting, since it is being called from drivers/of/device.c with
> > coherency parameter determined by of_dma_is_coherent(), and the
> > device-trees do not declare the 'dma-coherent' property.
> > 
> > Fix this by defaulting to dma-coherent for this platform in the
> > of_dma_is_coherent() function, if neither dma-coherent nor
> > dma-noncoherent is declared.
> > 
> > Fixes: ae626eb97376 ("ARM/dma-mapping: use dma-direct unconditionally")
> > Link: https://lore.kernel.org/linux-arm-kernel/20220930151028.0e518421@dellmb/
> 
> Thx for taking care of this. One quick note: You might want to add a
> "Cc: stable@..." on the patch to ensure it's backported in a timely manner:
> https://lore.kernel.org/lkml/YwZmu1ZTbjVqIY%2FC@kroah.com/

Sadly, this isn't a valid fix for the problem - as Christoph points
out, mvebu is part of a multiplatform kernel, and selecting options
that harm other platforms on a multiplatform kernel can't be allowed
even if it fixes a regression. It will cause a regression for other
platforms.

So, this needs fixing a different way, and there are other discussions
concerning an alternative approach.

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