default values for pci dma_mask and coherent_dma_mask

default values for pci dma_mask and coherent_dma_mask

[Manfred Spraul]

Hi,

 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).

--
    Manfred

Re: default values for pci dma_mask and coherent_dma_mask

[Lee Revell]

On Sun, 2005-07-31 at 12:36 +0200, Manfred Spraul wrote:
> 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",

Please don't do this.  Instead, add a DMA_40BIT_MASK define to
dma_mapping.h.

Lee