atl1: explain 32-bit DMA restriction

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>

1 files changed, 8 insertions, 2 deletions

diff --git a/drivers/net/atl1/atl1_main.c b/drivers/net/atl1/atl1_main.c
index e1a9223d0c18..4c728f1169c4 100644
--- a/drivers/net/atl1/atl1_main.c
+++ b/drivers/net/atl1/atl1_main.c
@@ -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) {