linux-kernel - Re: [PATCH 01/16] Blackfin SPI Driver: ensure cache coherency before doing DMA

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Message-Id: <200811201405.06811.david-b@pacbell.net>
Date:	Thu, 20 Nov 2008 14:05:06 -0800
From:	David Brownell <david-b@...bell.net>
To:	"Mike Frysinger" <vapier.adi@...il.com>
Cc:	"Bryan Wu" <cooloney@...nel.org>,
	spi-devel-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org,
	"Vitja Makarov" <vitja.makarov@...il.com>
Subject: Re: [PATCH 01/16] Blackfin SPI Driver: ensure cache coherency before doing DMA

On Thursday 20 November 2008, Mike Frysinger wrote:
> On Thu, Nov 20, 2008 at 16:47, David Brownell wrote:

> > Basically, dma_map_single(), dma_unmap_single() ... and
> > remember that the caller may have done the mappings for
> > you already.
> 
> these arent required to provide coherent memory right ?  if that's the
> case, i can take a look at getting things updated.

Right.  If caller provides dma-coherent memory to you, they
must also have provided the DMA addresses the controller
driver should be using ... so you won't need dma mapping
calls on those paths.


> > It'd be nice if one of patches snuck in a comment on that
> > point:  "Full duplex only works for non-DMA transfers."
> > Same rationale:  you may know this hardware inside out,
> > but the next person won't.
> 
> well, hopefully if they dont know they wont be touching the driver ;).
>  i'll add a comment in this code chunk.  thanks for the feedback.

The way it usually works is someone observes a problem and
then starts experimenting on relevant code.  So they learn
a bit while debugging ... and code authors/maintainers need
to keep that learning curve from being too steep.  ;)

- Dave

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