smsc95xx: dynamically fix up TX buffer alignment with padding bytes

dwc_otg requires a 32-bit aligned buffer start address, otherwise
expensive bounce buffers are used. The LAN951x hardware can skip up to
3 bytes between the TX header and the start of frame data, which can
be used to force alignment of the URB passed to dwc_otg.

As found in https://github.com/raspberrypi/linux/issues/2924

diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
index a5bbec7..c087b6d 100644 (file)
--- a/drivers/net/usb/smsc95xx.c
+++ b/drivers/net/usb/smsc95xx.c
@@ -2082,7 +2082,9 @@ static struct sk_buff *smsc95xx_tx_fixup(struct usbnet *dev,
                                         struct sk_buff *skb, gfp_t flags)
 {
        bool csum = skb->ip_summed == CHECKSUM_PARTIAL;
-       int overhead = csum ? SMSC95XX_TX_OVERHEAD_CSUM : SMSC95XX_TX_OVERHEAD;
+       unsigned int align_bytes = -((uintptr_t)skb->data) & 0x3;
+       int overhead = csum ? SMSC95XX_TX_OVERHEAD_CSUM + align_bytes
+                               : SMSC95XX_TX_OVERHEAD + align_bytes;
        u32 tx_cmd_a, tx_cmd_b;
 
        /* We do not advertise SG, so skbs should be already linearized */
@@ -2116,16 +2118,16 @@ static struct sk_buff *smsc95xx_tx_fixup(struct usbnet *dev,
                }
        }
 
-       skb_push(skb, 4);
-       tx_cmd_b = (u32)(skb->len - 4);
+       skb_push(skb, 4 + align_bytes);
+       tx_cmd_b = (u32)(skb->len - 4 - align_bytes);
        if (csum)
                tx_cmd_b |= TX_CMD_B_CSUM_ENABLE;
        cpu_to_le32s(&tx_cmd_b);
        memcpy(skb->data, &tx_cmd_b, 4);
 
        skb_push(skb, 4);
-       tx_cmd_a = (u32)(skb->len - 8) | TX_CMD_A_FIRST_SEG_ |
-               TX_CMD_A_LAST_SEG_;
+       tx_cmd_a = (u32)(skb->len - 8 - align_bytes) | TX_CMD_A_FIRST_SEG_ |
+               (align_bytes << 16) | TX_CMD_A_LAST_SEG_;
        cpu_to_le32s(&tx_cmd_a);
        memcpy(skb->data, &tx_cmd_a, 4);
 

diff --git a/drivers/net/usb/smsc95xx.h b/drivers/net/usb/smsc95xx.h
index cfc704f..55f9f03 100644 (file)
--- a/drivers/net/usb/smsc95xx.h
+++ b/drivers/net/usb/smsc95xx.h
@@ -21,7 +21,7 @@
 #define _SMSC95XX_H
 
 /* Tx command words */
-#define TX_CMD_A_DATA_OFFSET_  (0x001F0000)    /* Data Start Offset */
+#define TX_CMD_A_DATA_OFFSET_  (0x00030000)    /* Data Start Offset */
 #define TX_CMD_A_FIRST_SEG_    (0x00002000)    /* First Segment */
 #define TX_CMD_A_LAST_SEG_     (0x00001000)    /* Last Segment */
 #define TX_CMD_A_BUF_SIZE_     (0x000007FF)    /* Buffer Size */