linux-kernel - Re: [PATCH 2/4] spi: spi-fsl-dspi: Use non-coherent memory for DMA

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Message-ID: <20250613165638.5d8ed000@pumpkin>
Date: Fri, 13 Jun 2025 16:56:38 +0100
From: David Laight <david.laight.linux@...il.com>
To: James Clark <james.clark@...aro.org>
Cc: Frank Li <Frank.li@....com>, Vladimir Oltean <olteanv@...il.com>, Mark
 Brown <broonie@...nel.org>, Vladimir Oltean <vladimir.oltean@....com>,
 linux-spi@...r.kernel.org, imx@...ts.linux.dev,
 linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 2/4] spi: spi-fsl-dspi: Use non-coherent memory for DMA

On Tue, 10 Jun 2025 16:46:36 +0100
James Clark <james.clark@...aro.org> wrote:

> On 10/06/2025 4:15 pm, Frank Li wrote:
> > On Mon, Jun 09, 2025 at 04:32:39PM +0100, James Clark wrote:  
> >> Using coherent memory here isn't functionally necessary.
> >> Because the
> >> change to use non-coherent memory isn't overly complex and only a few
> >> synchronization points are required, we might as well do it while fixing
> >> up some other DMA issues.  
> > 
> > Any beanfit by use on-coherent memory here?
> > 
> > Frank
> >   
> 
> Presumably less cache maintenance traffic?

I bet it only helps when cache-coherent memory has to be uncached.
Otherwise the software cache operations are pretty much guaranteed
to be more expensive than the hardware ones.

	David