default values for pci dma_mask and coherent_dma_mask

From: Manfred Spraul
Date: Sun Jul 31 2005 - 05:37:32 EST


From Documentation/DMA-mapping.txt:

pci_alloc_consistent() by default will return 32-bit DMA addresses.
PCI-X specification requires PCI-X devices to support 64-bit
addressing (DAC) for all transactions. And at least one platform (SGI
SN2) requires 64-bit consistent allocations to operate correctly when
the IO bus is in PCI-X mode. Therefore, like with pci_set_dma_mask(),
it's good practice to call pci_set_consistent_dma_mask() to set the
appropriate mask even if your device only supports 32-bit DMA
(default) and especially if it's a PCI-X device.

What does "good practice" mean? Is the default 32-bit on all archs or not?
Recent nForce nics support 32-bit for the (coherent) ring buffer and 40-bit for the rx/tx data buffers. I intend to add these lines to the driver:

+ if (pci_set_dma_mask(pci_dev, 0x0000007fffffffffULL)) {
+ printk(KERN_INFO "forcedeth: 64-bit DMA failed, using 32-bit addressing for device %s.\n",
+ pci_name(pci_dev));
+ }

Is this sufficient, or is it mandatory to add pci_set_dma_mask(,DMA_32BIT_MASK) and pci_set_consistent_dma_mask (with error handling - IMHO 10 useless lines).


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