linux-kernel - Re: [PATCH v2 13/29] nios2: DMA mapping API

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Message-ID: <20140728154814.GA9179@codeaurora.org>
Date:	Mon, 28 Jul 2014 10:48:14 -0500
From:	rkuo <rkuo@...eaurora.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Ley Foon Tan <lftan@...era.com>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Chung-Lin Tang <cltang@...esourcery.com>,
	linux-hexagon@...r.kernel.org, Mark Salter <msalter@...hat.com>,
	Aurelien Jacquiot <a-jacquiot@...com>,
	linux-c6x-dev@...ux-c6x.org
Subject: Re: [PATCH v2 13/29] nios2: DMA mapping API

On Thu, Jul 24, 2014 at 02:05:16PM +0200, Arnd Bergmann wrote:
> > >> +void dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle,
> > >> +                          size_t size, enum dma_data_direction direction)
> > >> +{
> > >> +     BUG_ON(!valid_dma_direction(direction));
> > >> +
> > >> +     __dma_sync(phys_to_virt(dma_handle), size, direction);
> > >> +}
> > >> +EXPORT_SYMBOL(dma_sync_single_for_cpu);
> > >> +
> > >> +void dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle,
> > >> +                             size_t size, enum dma_data_direction direction)
> > >> +{
> > >> +     BUG_ON(!valid_dma_direction(direction));
> > >> +
> > >> +     __dma_sync(phys_to_virt(dma_handle), size, direction);
> > >> +}
> > >> +EXPORT_SYMBOL(dma_sync_single_for_device);
> > >
> > > More importantly: you do the same operation for both _for_cpu and _for_device.
> > > I assume your CPU can never do speculative cache prefetches, so it's not
> > > incorrect, but you do twice the number of invalidations and flushes that
> > > you need.
> > >
> > > Why would you do anything for _for_cpu here?
> > I am a bit confused for _for_cpu and _for_device here. I found some
> > architectures like c6x and hexagon have same operation for both
> > _for_cpu and _for_device as well.
> 
> (adding their maintainers to cc)
> 
> Yes, you are right, they seem to have the same bug and could see a noticeable
> DMA performance improvement if they change it as well.
> 

Yep that's a bug.  Thanks for pointing this out.


-Richard Kuo


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