net: macb: Limit maximum GEM TX length in TSO

BugLink: https://bugs.launchpad.net/bugs/1863588
[ Upstream commit f822e9c4ffa511a5c681cf866287d9383a3b6f1b ]

GEM_MAX_TX_LEN currently resolves to 0x3FF8 for any IP version supporting
TSO with full 14bits of length field in payload descriptor. But an IP
errata causes false amba_error (bit 6 of ISR) when length in payload
descriptors is specified above 16387. The error occurs because the DMA
falsely concludes that there is not enough space in SRAM for incoming
payload. These errors were observed continuously under stress of large
packets using iperf on a version where SRAM was 16K for each queue. This
errata will be documented shortly and affects all versions since TSO
functionality was added. Hence limit the max length to 0x3FC0 (rounded).

Signed-off-by: Harini Katakam <harini.katakam@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index 081f17cf67523f212e5d0de7a89478512b3426eb..95a94507cec1222e2a5e5dabb3e927e79d55d749 100644 (file)
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -73,7 +73,11 @@ struct sifive_fu540_macb_mgmt {
 /* Max length of transmit frame must be a multiple of 8 bytes */
 #define MACB_TX_LEN_ALIGN      8
 #define MACB_MAX_TX_LEN                ((unsigned int)((1 << MACB_TX_FRMLEN_SIZE) - 1) & ~((unsigned int)(MACB_TX_LEN_ALIGN - 1)))
-#define GEM_MAX_TX_LEN         ((unsigned int)((1 << GEM_TX_FRMLEN_SIZE) - 1) & ~((unsigned int)(MACB_TX_LEN_ALIGN - 1)))
+/* Limit maximum TX length as per Cadence TSO errata. This is to avoid a
+ * false amba_error in TX path from the DMA assuming there is not enough
+ * space in the SRAM (16KB) even when there is.
+ */
+#define GEM_MAX_TX_LEN         (unsigned int)(0x3FC0)
 
 #define GEM_MTU_MIN_SIZE       ETH_MIN_MTU
 #define MACB_NETIF_LSO         NETIF_F_TSO