2020-02-03 14:54:29

by Harini Katakam

Subject: [PATCH v2 2/2] net: macb: Limit maximum GEM TX length in TSO

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 <[email protected]>
---
v2:
Use 0x3FC0 by default

drivers/net/ethernet/cadence/macb_main.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index 2e418b8..cca321c 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -73,7 +73,7 @@ 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)))
+#define GEM_MAX_TX_LEN (unsigned int)(0x3FC0)

#define GEM_MTU_MIN_SIZE ETH_MIN_MTU
#define MACB_NETIF_LSO NETIF_F_TSO
--
2.7.4

2020-02-04 09:39:23

by David Miller

Subject: Re: [PATCH v2 2/2] net: macb: Limit maximum GEM TX length in TSO

From: Harini Katakam <[email protected]>
Date: Mon, 3 Feb 2020 18:48:02 +0530

> 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 <[email protected]>
> ---
> v2:
> Use 0x3FC0 by default

You should add a comment above the definition which explains how this
value was derived. It looks magic currently.

2020-02-04 10:28:08

by Harini Katakam

Subject: Re: [PATCH v2 2/2] net: macb: Limit maximum GEM TX length in TSO

Hi David,

<snip>
> > > v2:
> > > Use 0x3FC0 by default
> >
> > You should add a comment above the definition which explains how this value
> > was derived. It looks magic currently.

I'll add a comment. The value was specified in the errata document that Cadence
provided to us.

Regards,
Harini