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atl1: explain 32-bit DMA restriction

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Document the fact that atl1 uses a single shared register for the high 32
bits of 64-bit DMA addresses, making 64-bit DMA more trouble than it's worth.

Signed-off-by: Chris Snook <csnook@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
This commit is contained in:
Chris Snook 2007-09-20 15:57:15 -04:00 committed by David S. Miller
parent 01faccbf86
commit cdcc520d7b
1 changed files with 8 additions and 2 deletions
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10
drivers/net/atl1/atl1_main.c
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@ -2209,8 +2209,14 @@ static int __devinit atl1_probe(struct pci_dev *pdev,
return err;
/*
* 64-bit DMA currently has data corruption problems, so let's just
* use 32-bit DMA for now. This is a big hack that is probably wrong.
* The atl1 chip can DMA to 64-bit addresses, but it uses a single
* shared register for the high 32 bits, so only a single, aligned,
* 4 GB physical address range can be used at a time.
*
* Supporting 64-bit DMA on this hardware is more trouble than it's
* worth. It is far easier to limit to 32-bit DMA than update
* various kernel subsystems to support the mechanics required by a
* fixed-high-32-bit system.
*/
err = pci_set_dma_mask(pdev, DMA_32BIT_MASK);
if (err) {