netdev - [PATCH] r8169: default to 64-bit DMA on systems without memory below 4 GB

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Date:	Wed, 11 May 2016 09:47:49 +0200
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	netdev@...r.kernel.org
Cc:	ricardo.salveti@...aro.org, leo.duran@....com,
	graeme.gregory@...aro.org,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Realtek linux nic maintainers <nic_swsd@...ltek.com>
Subject: [PATCH] r8169: default to 64-bit DMA on systems without memory below 4 GB

The current logic around the 'use_dac' module parameter prevents the
r81969 driver from being loadable on 64-bit systems without any RAM
below 4 GB when the parameter is left at its default value.
So introduce a new default value -1 which indicates that 64-bit DMA
should be enabled implicitly, but only if setting a 32-bit DMA mask
has failed earlier. This should prevent any regressions like the ones
caused by previous attempts to change this code.

Cc: Realtek linux nic maintainers <nic_swsd@...ltek.com> 
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
---
 drivers/net/ethernet/realtek/r8169.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/realtek/r8169.c b/drivers/net/ethernet/realtek/r8169.c
index 94f08f1e841c..a49e8a58e539 100644
--- a/drivers/net/ethernet/realtek/r8169.c
+++ b/drivers/net/ethernet/realtek/r8169.c
@@ -345,7 +345,7 @@ static const struct pci_device_id rtl8169_pci_tbl[] = {
 MODULE_DEVICE_TABLE(pci, rtl8169_pci_tbl);
 
 static int rx_buf_sz = 16383;
-static int use_dac;
+static int use_dac = -1;
 static struct {
 	u32 msg_enable;
 } debug = { -1 };
@@ -859,7 +859,8 @@ struct rtl8169_private {
 MODULE_AUTHOR("Realtek and the Linux r8169 crew <netdev@...r.kernel.org>");
 MODULE_DESCRIPTION("RealTek RTL-8169 Gigabit Ethernet driver");
 module_param(use_dac, int, 0);
-MODULE_PARM_DESC(use_dac, "Enable PCI DAC. Unsafe on 32 bit PCI slot.");
+MODULE_PARM_DESC(use_dac,
+	"Enable PCI DAC. Unsafe on 32 bit PCI slot (default -1: enable on 64-bit archs only if needed");
 module_param_named(debug, debug.msg_enable, int, 0);
 MODULE_PARM_DESC(debug, "Debug verbosity level (0=none, ..., 16=all)");
 MODULE_LICENSE("GPL");
@@ -8226,12 +8227,16 @@ static int rtl_init_one(struct pci_dev *pdev, const struct pci_device_id *ent)
 
 	tp->cp_cmd = 0;
 
+	/* Tentatively set the DMA mask to 32 bits. This may fail
+	 * on 64-bit architectures without any RAM below 4 GB.
+	 */
+	rc = pci_set_dma_mask(pdev, DMA_BIT_MASK(32));
 	if ((sizeof(dma_addr_t) > 4) &&
-	    !pci_set_dma_mask(pdev, DMA_BIT_MASK(64)) && use_dac) {
+	    (use_dac == 1 || (use_dac == -1 && rc < 0)) &&
+	    !pci_set_dma_mask(pdev, DMA_BIT_MASK(64))) {
 		tp->cp_cmd |= PCIDAC;
 		dev->features |= NETIF_F_HIGHDMA;
 	} else {
-		rc = pci_set_dma_mask(pdev, DMA_BIT_MASK(32));
 		if (rc < 0) {
 			netif_err(tp, probe, dev, "DMA configuration failed\n");
 			goto err_out_free_res_3;
-- 
2.7.4