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 13:47:05 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Stefano Panella <stefano.panella@...rix.com>
CC:	<linux-kernel@...r.kernel.org>, <konrad.wilk@...cle.com>,
	<xen-devel@...ts.xensource.com>
Subject: Re: [Xen-devel] [PATCH 1/1] XEN: Use correct masking in	xen_swiotlb_alloc_coherent.

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...

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)

>  	phys = virt_to_phys(ret);
>  	dev_addr = xen_phys_to_bus(phys);

David
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