From: David Miller <da...@davemloft.net> Sent: Saturday, September 24, 2016
10:46 AM
> To: e...@nelint.com
> Cc: and...@lunn.ch; eduma...@google.com; Andy Duan
> <fugang.d...@nxp.com>; ota...@ossystems.com.br;
> netdev@vger.kernel.org; troy.ki...@boundarydevices.com;
> rmk+ker...@arm.linux.org.uk; cjb.sw.nos...@gmail.com; linux-arm-
> ker...@lists.infradead.org
> Subject: Re: Alignment issues with freescale FEC driver
>
> From: Eric Nelson <e...@nelint.com>
> Date: Fri, 23 Sep 2016 11:35:17 -0700
>
> > From the i.MX6DQ reference manual, bit 7 of ENET_RACC says this:
> >
> > "RX FIFO Shift-16
> >
> > When this field is set, the actual frame data starts at bit 16 of the
> > first word read from the RX FIFO aligning the Ethernet payload on a
> > 32-bit boundary."
> >
> > Same for the i.MX6UL.
> >
> > I'm not sure what it will take to use this, but it seems to be exactly
> > what we're looking for.
>
> +1
RACC[SHIFT16] just instructs the MAC to write two additional bytes in front of
each frame received into the RX FIFO to align
the Ethernet payload on a 32-bit boundary.
Eric's patch "net: fec: support RRACC_SHIFT16 to align IP header" works fine.
For the alignment issues, that is introduced by commit 1b7bde6d6 and c259c132a
in net-next tree. Before these commits, no alignment issue.
How to fix the issue:
Solution1: to enable HW RRACC_SHIFT16 feature (test pass):
Eric's patch "net: fec: support RRACC_SHIFT16 to align IP header".
Solution2: include the correct prefetch() header (test pass):
--- a/drivers/net/ethernet/freescale/fec_main.c
+++ b/drivers/net/ethernet/freescale/fec_main.c
@@ -59,7 +59,7 @@
#include <linux/pinctrl/consumer.h>
#include <linux/pm_runtime.h>
#include <linux/busfreq-imx.h>
-#include <linux/prefetch.h>
+#include <asm/processor.h>
Solution3: use __netdev_alloc_skb_ip_align() instead of netdev_alloc_skb().
Or: still use the previous method before commit 1b7bde6d6:
skb = netdev_alloc_skb(ndev, pkt_len - 4 + NET_IP_ALIGN);
skb_reserve(skb, NET_IP_ALIGN);
Comparing these solutions:
From sw effort and performance, I think these are the similar. Enable
RRACC_SHIFT16 doesn't take extra advantage.
Correct my if I am wrong. Thanks.
Regards,
Andy
