linux-kernel - Re: ARM: OMPA4+: is it expected dma_coerce_mask_and_coherent(dev, DMA_BIT

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Date:	Tue, 10 Mar 2015 11:05:38 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	"Grygorii.Strashko@...aro.org" <grygorii.strashko@...aro.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>, Tejun Heo <tj@...nel.org>,
	Tony Lindgren <tony@...mide.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	Laura Abbott <lauraa@...eaurora.org>,
	open list <linux-kernel@...r.kernel.org>,
	Santosh Shilimkar <ssantosh@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Peter Ujfalusi <peter.ujfalusi@...com>
Subject: Re: ARM: OMPA4+: is it expected dma_coerce_mask_and_coherent(dev,
 DMA_BIT_MASK(64)); to fail?

On Fri, Mar 06, 2015 at 11:47:48PM +0200, Grygorii.Strashko@...aro.org wrote:
> Hi Russell,
> 
> On 03/05/2015 10:17 PM, Russell King - ARM Linux wrote:
> > On Thu, Mar 05, 2015 at 08:55:07PM +0200, Grygorii.Strashko@...aro.org wrote:
> >> The dma_coerce_mask_and_coherent() will fail in case 'Example 3' and succeed in cases 1,2.
> >> dma-mapping.c --> __dma_supported()
> >> 	if (sizeof(mask) != sizeof(dma_addr_t) && <== true for all OMAP4+
> >> 	    mask > (dma_addr_t)~0 &&		<== true for DMA_BIT_MASK(64)
> >> 	    dma_to_pfn(dev, ~0) < max_pfn) {  <== true only for Example 3
> > 
> > Hmm, I think this may make more sense to be "< max_pfn - 1" here, as
> > that would be better suited to our intention.
> > 
> > The result of dma_to_pfn(dev, ~0) is the maximum PFN which we could
> > address via DMA, but we're comparing it with the maximum PFN in the
> > system plus 1 - so we need to subtract one from it.
> 
> Ok. I'll try it.

Any news on this - I think it is a real off-by-one bug which we should
fix in any case.

Thanks.

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