linux-kernel - Re: For the problem when using swiotlb

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Message-ID: <2530749.roRsteyaXx@wuerfel>
Date:	Fri, 21 Nov 2014 18:04:28 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ding Tianhong <dingtianhong@...wei.com>
Subject: Re: For the problem when using swiotlb

On Friday 21 November 2014 16:57:09 Catalin Marinas wrote:
> On Fri, Nov 21, 2014 at 12:48:09PM +0000, Arnd Bergmann wrote:
> > On Friday 21 November 2014 09:35:10 Catalin Marinas wrote:
> > > +static inline int dma_set_coherent_mask(struct device *dev, u64 mask)
> > > +{
> > > +	if (!dma_supported(dev, mask))
> > > +		return -EIO;
> > > +	if (mask > dev->coherent_dma_mask)
> > > +		mask &= of_dma_get_range_mask(dev);
> > > +	dev->coherent_dma_mask = mask;
> > > +	return 0;
> > > +}
> > 
> > There is an interesting side problem here: the dma mask points to
> > coherent_dma_mask for all devices probed from DT, so this breaks
> > if we have any driver that sets them to different values. It is a
> > preexisting problem them.
> 
> Such drivers would have to set both masks separately (or call
> dma_set_mask_and_coherent). What we assume though is that dma-ranges
> refers to both dma_mask and coherent_dma_mask. I don't think that would
> be a problem for ARM systems.

Right, I'm just saying that we don't have a way to detect drivers that
break this assumption, not that we have a serious problem already.

> > >  EXPORT_SYMBOL_GPL(of_dma_get_range);
> > >  
> > > +u64 of_dma_get_range_mask(struct device *dev)
> > > +{
> > > +	u64 dma_addr, paddr, size;
> > > +
> > > +	/* no dma mask limiting if no of_node or no dma-ranges property */
> > > +	if (!dev->of_node ||
> > > +	    of_dma_get_range(dev->of_node, &dma_addr, &paddr, &size) < 0)
> > > +		return DMA_BIT_MASK(64);
> > 
> > If no dma-ranges are present, we should assume that the bus only supports
> > 32-bit DMA, or we could make it architecture specific. It would probably
> > be best for arm64 to require a dma-ranges property for doing any DMA
> > at all, but we can't do that on arm32 any more now.
> 
> I thought about this but it could break some existing arm64 DT files if
> we mandate dma-ranges property (we could try though). The mask limiting
> is arch-specific anyway.

Yes, this has taken far too long to get addressed, we should have added
the properties right from the start. If we have a function in architecture
specific code, maybe we can just check for the short list of already
supported platforms that need backwards compatibility and require the
mask for everything else?

> > > diff --git a/drivers/of/platform.c b/drivers/of/platform.c
> > > index 3b64d0bf5bba..50d1ac4739e6 100644
> > > --- a/drivers/of/platform.c
> > > +++ b/drivers/of/platform.c
> > > @@ -200,6 +200,10 @@ static void of_dma_configure(struct device *dev)
> > >  	/* DMA ranges found. Calculate and set dma_pfn_offset */
> > >  	dev->dma_pfn_offset = PFN_DOWN(paddr - dma_addr);
> > >  	dev_dbg(dev, "dma_pfn_offset(%#08lx)\n", dev->dma_pfn_offset);
> > > +
> > > +	/* limit the coherent_dma_mask to the dma-ranges size property */
> > 
> > I would change the comment to clarify that we are actually changing
> > the dma_mask here as well.
> > 
> > > +	if (size < (1ULL << 32))
> > > +		dev->coherent_dma_mask = DMA_BIT_MASK(ilog2(size));
> > >  }
> > 
> > As you mentioned in another mail in this thread, we wouldn't be
> > able to suuport this case on arm64.
> 
> The mask would still be valid and even usable if an IOMMU is present. Do
> you mean we should not limit it at all here?

The code is certainly correct on arm32, as long as we have an appropriate
DMA zone.

> There is a scenario where smaller mask would work on arm64. For example
> Juno, you can have 2GB of RAM in the 32-bit phys range (starting at
> 0x80000000). A device with 31-bit mask and a dma_pfn_offset of
> 0x80000000 would still work (there isn't any but just as an example). So
> the check in dma_alloc_coherent() would be something like:
> 
> 	phys_to_dma(top of ZONE_DMA) - dma_pfn_offset <= coherent_dma_mask
> 
> (or assuming RAM starts at 0 and ignoring dma_pfn_offset for now)
> 
> If the condition above fails, dma_alloc_coherent() would no longer fall
> back to swiotlb but issue a dev_warn() and return NULL.

Ah, that looks like it should work on all architectures, very nice.
How about checking this condition, and then printing a small warning
(dev_warn, not WARN_ON) and setting the dma_mask pointer to NULL?

	Arnd
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