Messages in this thread | | | Subject | Re: Problem with NETIF_F_HIGHDMA | From | Andi Kleen <> | Date | 22 Feb 2006 21:45:30 +0100 |
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Corey Minyard <minyard@acm.org> writes:
> I was looking at a problem with a new system we are trying to get up and > running. It has a 32-bit only PCI network device, but is a 64-bit > (x86_64) system. Looking at the code for NETIF_F_HIGHDMA (which, when > not set on a PCI network device, means that it cannot do 64-bit > accesses) in net/core/dev.c, it seems wrong to me.
> It is dependent on HIGHMEM, but HIGHMEM has nothing to do with 32/64 bit > accesses. On 64-bit systems, HIGHMEM is not set, thus the network code > will pass any address (including those >32bits) to the driver. Plus, > highmem on 32-bit systems may very well be 32-bit accessible, possibly > resulting in unecessary copies. AFAICT, the current code will only work > with i386 and PAE and is sub-optimal.
x86-64 uses the PCI DMA API to handle this. NETIF_F_HIGHDMA is only a i386 specific hack, mostly to work around the fact that not all memory might be accessible from the CPU and thus break PIO drivers. This problem doesn't exist on 64bit.
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