netdev - Re: [PATCH 5/6] net: fec: use dma_alloc

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Date: Mon, 9 Oct 2023 14:58:44 +0200
From: Christoph Hellwig <hch@....de>
To: Robin Murphy <robin.murphy@....com>
Cc: Christoph Hellwig <hch@....de>, iommu@...ts.linux.dev,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Wei Fang <wei.fang@....com>, Shenwei Wang <shenwei.wang@....com>,
	Clark Wang <xiaoning.wang@....com>,
	NXP Linux Team <linux-imx@....com>, linux-m68k@...ts.linux-m68k.org,
	netdev@...r.kernel.org, Jim Quinlan <james.quinlan@...adcom.com>,
	Greg Ungerer <gerg@...ux-m68k.org>
Subject: Re: [PATCH 5/6] net: fec: use dma_alloc_noncoherent for m532x

On Mon, Oct 09, 2023 at 11:29:12AM +0100, Robin Murphy wrote:
> It looks a bit odd that this ends up applying to all of Coldfire, while the 
> associated cache flush only applies to the M532x platform, which implies 
> that we'd now be relying on the non-coherent allocation actually being 
> coherent on other Coldfire platforms.
>
> Would it work to do something like this to make sure dma-direct does the 
> right thing on such platforms (which presumably don't have caches?), and 
> then reduce the scope of this FEC hack accordingly, to clean things up even 
> better?

Probably.  Actually Greg comment something along the lines last
time, and mentioned something about just instruction vs instruction
and data cache.

>
> diff --git a/arch/m68k/Kconfig.cpu b/arch/m68k/Kconfig.cpu
> index b826e9c677b2..1851fa3fe077 100644
> --- a/arch/m68k/Kconfig.cpu
> +++ b/arch/m68k/Kconfig.cpu
> @@ -27,6 +27,7 @@ config COLDFIRE
>  	select CPU_HAS_NO_BITFIELDS
>  	select CPU_HAS_NO_CAS
>  	select CPU_HAS_NO_MULDIV64
> +	select DMA_DEFAULT_COHERENT if !MMU && !M523x

Although it would probably make more sense to simply not select
CONFIG_ARCH_HAS_SYNC_DMA_FOR_DEVICE and
CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU for these platforms and not
build the non-coherent code at all.  This should also include
all coldfire platforms with mmu (M54xx/M548x/M5441x).  Then
again for many of the coldfire platforms the Kconfig allows
to select CACHE_WRITETHRU/CACHE_COPYBACK which looks related.

Greg, any chance you could help out with the caching modes on
coldfire and legacy m68knommu?