6.5-stable review patch. If anyone has any objections, please let me know.
------------------
From: Vladimir Oltean vladimir.oltean@nxp.com
[ Upstream commit 954ad9bf13c4f95a4958b5f8433301f2ab99e1f5 ]
More careful measurement of the tc-cbs bandwidth shows that the stream bandwidth (effectively idleslope) increases, there is a larger and larger discrepancy between the rate limit obtained by the software Qdisc, and the rate limit obtained by its offloaded counterpart.
The discrepancy becomes so large, that e.g. at an idleslope of 40000 (40Mbps), the offloaded cbs does not actually rate limit anything, and traffic will pass at line rate through a 100 Mbps port.
The reason for the discrepancy is that the hardware documentation I've been following is incorrect. UM11040.pdf (for SJA1105P/Q/R/S) states about IDLE_SLOPE that it is "the rate (in unit of bytes/sec) at which the credit counter is increased".
Cross-checking with UM10944.pdf (for SJA1105E/T) and UM11107.pdf (for SJA1110), the wording is different: "This field specifies the value, in bytes per second times link speed, by which the credit counter is increased".
So there's an extra scaling for link speed that the driver is currently not accounting for, and apparently (empirically), that link speed is expressed in Kbps.
I've pondered whether to pollute the sja1105_mac_link_up() implementation with CBS shaper reprogramming, but I don't think it is worth it. IMO, the UAPI exposed by tc-cbs requires user space to recalculate the sendslope anyway, since the formula for that depends on port_transmit_rate (see man tc-cbs), which is not an invariant from tc's perspective.
So we use the offload->sendslope and offload->idleslope to deduce the original port_transmit_rate from the CBS formula, and use that value to scale the offload->sendslope and offload->idleslope to values that the hardware understands.
Some numerical data points:
40Mbps stream, max interfering frame size 1500, port speed 100M ---------------------------------------------------------------
tc-cbs parameters: idleslope 40000 sendslope -60000 locredit -900 hicredit 600
which result in hardware values:
Before (doesn't work) After (works) credit_hi 600 600 credit_lo 900 900 send_slope 7500000 75 idle_slope 5000000 50
40Mbps stream, max interfering frame size 1500, port speed 1G -------------------------------------------------------------
tc-cbs parameters: idleslope 40000 sendslope -960000 locredit -1440 hicredit 60
which result in hardware values:
Before (doesn't work) After (works) credit_hi 60 60 credit_lo 1440 1440 send_slope 120000000 120 idle_slope 5000000 5
5.12Mbps stream, max interfering frame size 1522, port speed 100M -----------------------------------------------------------------
tc-cbs parameters: idleslope 5120 sendslope -94880 locredit -1444 hicredit 77
which result in hardware values:
Before (doesn't work) After (works) credit_hi 77 77 credit_lo 1444 1444 send_slope 11860000 118 idle_slope 640000 6
Tested on SJA1105T, SJA1105S and SJA1110A, at 1Gbps and 100Mbps.
Fixes: 4d7525085a9b ("net: dsa: sja1105: offload the Credit-Based Shaper qdisc") Reported-by: Yanan Yang yanan.yang@nxp.com Signed-off-by: Vladimir Oltean vladimir.oltean@nxp.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/net/dsa/sja1105/sja1105_main.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-)
diff --git a/drivers/net/dsa/sja1105/sja1105_main.c b/drivers/net/dsa/sja1105/sja1105_main.c index 3529a565b4aaf..fe7f09519653a 100644 --- a/drivers/net/dsa/sja1105/sja1105_main.c +++ b/drivers/net/dsa/sja1105/sja1105_main.c @@ -2157,6 +2157,7 @@ static int sja1105_setup_tc_cbs(struct dsa_switch *ds, int port, { struct sja1105_private *priv = ds->priv; struct sja1105_cbs_entry *cbs; + s64 port_transmit_rate_kbps; int index;
if (!offload->enable) @@ -2174,9 +2175,17 @@ static int sja1105_setup_tc_cbs(struct dsa_switch *ds, int port, */ cbs->credit_hi = offload->hicredit; cbs->credit_lo = abs(offload->locredit); - /* User space is in kbits/sec, hardware in bytes/sec */ - cbs->idle_slope = offload->idleslope * BYTES_PER_KBIT; - cbs->send_slope = abs(offload->sendslope * BYTES_PER_KBIT); + /* User space is in kbits/sec, while the hardware in bytes/sec times + * link speed. Since the given offload->sendslope is good only for the + * current link speed anyway, and user space is likely to reprogram it + * when that changes, don't even bother to track the port's link speed, + * but deduce the port transmit rate from idleslope - sendslope. + */ + port_transmit_rate_kbps = offload->idleslope - offload->sendslope; + cbs->idle_slope = div_s64(offload->idleslope * BYTES_PER_KBIT, + port_transmit_rate_kbps); + cbs->send_slope = div_s64(abs(offload->sendslope * BYTES_PER_KBIT), + port_transmit_rate_kbps); /* Convert the negative values from 64-bit 2's complement * to 32-bit 2's complement (for the case of 0x80000000 whose * negative is still negative).