netdev - Re: [PATCH 01/12] riscv: RISCV_NONSTANDARD_CACHE_OPS shouldn't depend on RISCV_DMA

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Date: Mon, 16 Oct 2023 18:16:20 +0100
From: Conor Dooley <conor@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: Conor Dooley <conor.dooley@...rochip.com>,
	Greg Ungerer <gerg@...ux-m68k.org>, iommu@...ts.linux.dev,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Magnus Damm <magnus.damm@...il.com>,
	Robin Murphy <robin.murphy@....com>,
	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, linux-riscv@...ts.infradead.org,
	linux-renesas-soc@...r.kernel.org,
	Jim Quinlan <james.quinlan@...adcom.com>
Subject: Re: [PATCH 01/12] riscv: RISCV_NONSTANDARD_CACHE_OPS shouldn't
 depend on RISCV_DMA_NONCOHERENT

On Mon, Oct 16, 2023 at 03:17:16PM +0200, Christoph Hellwig wrote:
> On Mon, Oct 16, 2023 at 01:49:41PM +0100, Conor Dooley wrote:
> > Hey,
> > 
> > On Mon, Oct 16, 2023 at 07:47:43AM +0200, Christoph Hellwig wrote:
> > > RISCV_NONSTANDARD_CACHE_OPS is also used for the pmem cache maintenance
> > > helpers, which are built into the kernel unconditionally.
> > 
> > You surely have better insight than I do here, but is this actually
> > required?
> > This patch seems to allow creation of a kernel where the cache
> > maintenance operations could be used for pmem, but would be otherwise
> > unavailable, which seems counter intuitive to me.
> >
> > Why would someone want to provide the pmem helpers with cache
> > maintenance operations, but not provide them generally?
> > 
> 
> Even if all your periphals are cache coherent (very common on server
> class hardware) you still need cache maintenance for pmem.  No need
> to force the extra text size and runtime overhead for non-coherent DMA.

Ah, right.

> > I also don't really understand what the unconditional nature of the pmem
> > helpers has to do with anything, as this patch does not unconditionally
> > provide any cache management operations, only relax the conditions under
> > which the non-standard cache management operations can be provided.
> 
> They simply were broken if a platform had non-standard cache mem but
> only coherent DMA before.  That's probably more a theoretical than
> practial case, but still worth fixing.

And this part of it makes more sense with the above use-case explained.

Acked-by: Conor Dooley <conor.dooley@...rochip.com>

Thanks,
Conor.


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