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Commit cdcc520d authored by Chris Snook's avatar Chris Snook Committed by David S. Miller
Browse files

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: default avatarChris Snook <csnook@redhat.com>
Signed-off-by: default avatarJeff Garzik <jeff@garzik.org>
parent 01faccbf

drivers/net/atl1/atl1_main.c

+8 −2
Original line number Diff line number Diff line
@@ -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) {