Re: [Xen-devel] Re: [Patch] Constrain DMADOM/swiotlb allocations to 1G phys to fix b44 NIC driver

[Keir Fraser <keir@xxxxxxxxxxxxx>]

On 28/11/06 15:49, "Stephen C. Tweedie" <sct@xxxxxxxxxx> wrote:

>> It turns out that this 2GB / 31-bit constraint is magically hardcoded in
>> a couple of places rather than being read correctly from config.h.  So
>> this patch fixes that; changes config.h to specify a 1GB / 30-bit
>> constraint for the DMADOM heap; and changes the x86/x86_64 swiotlb to
>> request a 30-bit constraint for allocations of the swiotlb memory.
> 
> The 30 bit constraint is enough to let b44 work.  But there are a number
> of sound cards which require a 28-bit DMA mask (256MB max physical
> addressable memory).  1GB seems reasonably safe, but I'm much less
> comfortable with restricting DMADOM to 256MB by default.  Does anyone
> have any strong feelings about shifting this?  What other users of
> DMADOM are there that might overflow if we restrict it even further?

We could make bit-width of the DMA allocator in Xen a boot parameter. And we
could make the bit-width of the SWIOTLB allocations a Linux boot parameter.
Then those that care can do the necessary cmdline hacking.

I'd be tempted to recommend this for b44 users too, and leave the default
bit-width as 31. I'm not convinced there can be that many who are using Xen.
But maybe the chipset is more common than I think.

 -- Keir


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