linux-kernel - Re: [Xen-devel] [PATCH 1/1] XEN: Use correct masking in xen_swiotlb_alloc

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Date:	Fri, 31 Aug 2012 12:40:11 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Stefano Panella <stefano.panella@...rix.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com
Subject: Re: [Xen-devel] [PATCH 1/1] XEN: Use correct masking in
 xen_swiotlb_alloc_coherent.

On Fri, Aug 31, 2012 at 01:47:05PM +0100, David Vrabel wrote:
> On 31/08/12 10:57, Stefano Panella wrote:
> > When running 32-bit pvops-dom0 and a driver tries to allocate a coherent
> > DMA-memory the xen swiotlb-implementation returned memory beyond 4GB.
> > 
> > This caused for example not working sound on a system with 4 GB and a 64-bit
> > compatible sound-card with sets the DMA-mask to 64bit.
> > 
> > On bare-metal and the forward-ported xen-dom0 patches from OpenSuse a coherent
> > DMA-memory is always allocated inside the 32-bit address-range by calling
> > dma_alloc_coherent_mask.
> 
> We should have the same behaviour under Xen as bare metal so:
> 
> Acked-By: David Vrabel <david.vrabel@...rix.com>
> 
> This does limit the DMA mask to 32-bits by passing it through an
> unsigned long, which seems a bit sneaky...

so is the issue that we are not casting it from 'u64' to 'u32'
(unsigned long) on 32-bit?

> 
> Presumably the sound card is capable of handling 64 bit physical
> addresses (or it would break under 64-bit kernels) so it's not clear why
> this sound driver requires this restriction.
> 
> Is there a bug in the sound driver or sound subsystem where it's
> truncating a dma_addr_t by assigning it to an unsigned long or similar?
> 
> > --- a/drivers/xen/swiotlb-xen.c
> > +++ b/drivers/xen/swiotlb-xen.c
> > @@ -232,7 +232,7 @@ xen_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
> >  		return ret;
> >  
> >  	if (hwdev && hwdev->coherent_dma_mask)
> > -		dma_mask = hwdev->coherent_dma_mask;
> > +		dma_mask = dma_alloc_coherent_mask(hwdev, flags);
> 
> Suggest
> 
>     if (hwdev)
>         dma_mask = dma_alloc_coherent_mask(hwdev, flags)

Isn't that code just doing this:
atic inline unsigned long dma_alloc_coherent_mask(struct device *dev,
                                                    gfp_t gfp)
{
        unsigned long dma_mask = 0;

        dma_mask = dev->coherent_dma_mask;
        if (!dma_mask)
                dma_mask = (gfp & GFP_DMA) ? DMA_BIT_MASK(24) :
DMA_BIT_MASK(32);

        return dma_mask;
}

and in our code, the dma_mask by default is DMA_BIT_MASK(32):

u64 dma_mask = DMA_BIT_MASK(32);

So what I am missing?
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