net: dsa: sja1105: Configure the Time-Aware Scheduler via tc-taprio offload

This qdisc offload is the closest thing to what the SJA1105 supports in
hardware for time-based egress shaping. The switch core really is built
around SAE AS6802/TTEthernet (a TTTech standard) but can be made to
operate similarly to IEEE 802.1Qbv with some constraints:

- The gate control list is a global list for all ports. There are 8
execution threads that iterate through this global list in parallel.
I don't know why 8, there are only 4 front-panel ports.

- Care must be taken by the user to make sure that two execution threads
never get to execute a GCL entry simultaneously. I created a O(n^4)
checker for this hardware limitation, prior to accepting a taprio
offload configuration as valid.

- The spec says that if a GCL entry's interval is shorter than the frame
length, you shouldn't send it (and end up in head-of-line blocking).
Well, this switch does anyway.

- The switch has no concept of ADMIN and OPER configurations. Because
it's so simple, the TAS settings are loaded through the static config
tables interface, so there isn't even place for any discussion about
'graceful switchover between ADMIN and OPER'. You just reset the
switch and upload a new OPER config.

- The switch accepts multiple time sources for the gate events. Right
now I am using the standalone clock source as opposed to PTP. So the
base time parameter doesn't really do much. Support for the PTP clock
source will be added in a future series.

Signed-off-by: Vladimir Oltean <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/dsa/sja1105/Kconfig

···
 	help
 	  This enables support for timestamping and PTP clock manipulations in
 	  the SJA1105 DSA driver.
+
+config NET_DSA_SJA1105_TAS
+	bool "Support for the Time-Aware Scheduler on NXP SJA1105"
+	depends on NET_DSA_SJA1105
+	help
+	  This enables support for the TTEthernet-based egress scheduling
+	  engine in the SJA1105 DSA driver, which is controlled using a
+	  hardware offload of the tc-tqprio qdisc.

drivers/net/dsa/sja1105/Makefile

···
 ifdef CONFIG_NET_DSA_SJA1105_PTP
 sja1105-objs += sja1105_ptp.o
 endif
+
+ifdef CONFIG_NET_DSA_SJA1105_TAS
+sja1105-objs += sja1105_tas.o
+endif

drivers/net/dsa/sja1105/sja1105.h

···
  */
 #define SJA1105_AGEING_TIME_MS(ms)	((ms) / 10)
 
+#include "sja1105_tas.h"
+
 /* Keeps the different addresses between E/T and P/Q/R/S */
 struct sja1105_regs {
 	u64 device_id;
···
 	 */
 	struct mutex mgmt_lock;
 	struct sja1105_tagger_data tagger_data;
+	struct sja1105_tas_data tas_data;
 };
 
 #include "sja1105_dynamic_config.h"
···
 	SPI_READ = 0,
 	SPI_WRITE = 1,
 } sja1105_spi_rw_mode_t;
+
+/* From sja1105_main.c */
+int sja1105_static_config_reload(struct sja1105_private *priv);
 
 /* From sja1105_spi.c */
 int sja1105_spi_send_packed_buf(const struct sja1105_private *priv,

drivers/net/dsa/sja1105/sja1105_main.c

···
 #include <linux/if_ether.h>
 #include <linux/dsa/8021q.h>
 #include "sja1105.h"
+#include "sja1105_tas.h"
 
 static void sja1105_hw_reset(struct gpio_desc *gpio, unsigned int pulse_len,
 			     unsigned int startup_delay)
···
  * modify at runtime (currently only MAC) and restore them after uploading,
  * such that this operation is relatively seamless.
  */
-static int sja1105_static_config_reload(struct sja1105_private *priv)
+int sja1105_static_config_reload(struct sja1105_private *priv)
 {
 	struct sja1105_mac_config_entry *mac;
 	int speed_mbps[SJA1105_NUM_PORTS];
···
 {
 	struct sja1105_private *priv = ds->priv;
 
+	sja1105_tas_teardown(ds);
 	cancel_work_sync(&priv->tagger_data.rxtstamp_work);
 	skb_queue_purge(&priv->tagger_data.skb_rxtstamp_queue);
 	sja1105_ptp_clock_unregister(priv);
···
 	return true;
 }
 
+static int sja1105_port_setup_tc(struct dsa_switch *ds, int port,
+				 enum tc_setup_type type,
+				 void *type_data)
+{
+	switch (type) {
+	case TC_SETUP_QDISC_TAPRIO:
+		return sja1105_setup_tc_taprio(ds, port, type_data);
+	default:
+		return -EOPNOTSUPP;
+	}
+}
+
 static const struct dsa_switch_ops sja1105_switch_ops = {
 	.get_tag_protocol	= sja1105_get_tag_protocol,
 	.setup			= sja1105_setup,
···
 	.port_hwtstamp_set	= sja1105_hwtstamp_set,
 	.port_rxtstamp		= sja1105_port_rxtstamp,
 	.port_txtstamp		= sja1105_port_txtstamp,
+	.port_setup_tc		= sja1105_port_setup_tc,
 };
 
 static int sja1105_check_device_id(struct sja1105_private *priv)
···
 		sp->data = tagger_data;
 	}
 	mutex_init(&priv->mgmt_lock);
+
+	sja1105_tas_setup(ds);
 
 	return dsa_register_switch(priv->ds);
 }

drivers/net/dsa/sja1105/sja1105_tas.c

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2019, Vladimir Oltean <[e-mail]>
+ */
+#include "sja1105.h"
+
+#define SJA1105_TAS_CLKSRC_DISABLED	0
+#define SJA1105_TAS_CLKSRC_STANDALONE	1
+#define SJA1105_TAS_CLKSRC_AS6802	2
+#define SJA1105_TAS_CLKSRC_PTP		3
+#define SJA1105_TAS_MAX_DELTA		BIT(19)
+#define SJA1105_GATE_MASK		GENMASK_ULL(SJA1105_NUM_TC - 1, 0)
+
+/* This is not a preprocessor macro because the "ns" argument may or may not be
+ * s64 at caller side. This ensures it is properly type-cast before div_s64.
+ */
+static s64 ns_to_sja1105_delta(s64 ns)
+{
+	return div_s64(ns, 200);
+}
+
+/* Lo and behold: the egress scheduler from hell.
+ *
+ * At the hardware level, the Time-Aware Shaper holds a global linear arrray of
+ * all schedule entries for all ports. These are the Gate Control List (GCL)
+ * entries, let's call them "timeslots" for short. This linear array of
+ * timeslots is held in BLK_IDX_SCHEDULE.
+ *
+ * Then there are a maximum of 8 "execution threads" inside the switch, which
+ * iterate cyclically through the "schedule". Each "cycle" has an entry point
+ * and an exit point, both being timeslot indices in the schedule table. The
+ * hardware calls each cycle a "subschedule".
+ *
+ * Subschedule (cycle) i starts when
+ *   ptpclkval >= ptpschtm + BLK_IDX_SCHEDULE_ENTRY_POINTS[i].delta.
+ *
+ * The hardware scheduler iterates BLK_IDX_SCHEDULE with a k ranging from
+ *   k = BLK_IDX_SCHEDULE_ENTRY_POINTS[i].address to
+ *   k = BLK_IDX_SCHEDULE_PARAMS.subscheind[i]
+ *
+ * For each schedule entry (timeslot) k, the engine executes the gate control
+ * list entry for the duration of BLK_IDX_SCHEDULE[k].delta.
+ *
+ *         +---------+
+ *         |         | BLK_IDX_SCHEDULE_ENTRY_POINTS_PARAMS
+ *         +---------+
+ *              |
+ *              +-----------------+
+ *                                | .actsubsch
+ *  BLK_IDX_SCHEDULE_ENTRY_POINTS v
+ *                 +-------+-------+
+ *                 |cycle 0|cycle 1|
+ *                 +-------+-------+
+ *                   |  |      |  |
+ *  +----------------+  |      |  +-------------------------------------+
+ *  |   .subschindx     |      |             .subschindx                |
+ *  |                   |      +---------------+                        |
+ *  |          .address |        .address      |                        |
+ *  |                   |                      |                        |
+ *  |                   |                      |                        |
+ *  |  BLK_IDX_SCHEDULE v                      v                        |
+ *  |              +-------+-------+-------+-------+-------+------+     |
+ *  |              |entry 0|entry 1|entry 2|entry 3|entry 4|entry5|     |
+ *  |              +-------+-------+-------+-------+-------+------+     |
+ *  |                                  ^                    ^  ^  ^     |
+ *  |                                  |                    |  |  |     |
+ *  |        +-------------------------+                    |  |  |     |
+ *  |        |              +-------------------------------+  |  |     |
+ *  |        |              |              +-------------------+  |     |
+ *  |        |              |              |                      |     |
+ *  | +---------------------------------------------------------------+ |
+ *  | |subscheind[0]<=subscheind[1]<=subscheind[2]<=...<=subscheind[7]| |
+ *  | +---------------------------------------------------------------+ |
+ *  |        ^              ^                BLK_IDX_SCHEDULE_PARAMS    |
+ *  |        |              |                                           |
+ *  +--------+              +-------------------------------------------+
+ *
+ *  In the above picture there are two subschedules (cycles):
+ *
+ *  - cycle 0: iterates the schedule table from 0 to 2 (and back)
+ *  - cycle 1: iterates the schedule table from 3 to 5 (and back)
+ *
+ *  All other possible execution threads must be marked as unused by making
+ *  their "subschedule end index" (subscheind) equal to the last valid
+ *  subschedule's end index (in this case 5).
+ */
+static int sja1105_init_scheduling(struct sja1105_private *priv)
+{
+	struct sja1105_schedule_entry_points_entry *schedule_entry_points;
+	struct sja1105_schedule_entry_points_params_entry
+					*schedule_entry_points_params;
+	struct sja1105_schedule_params_entry *schedule_params;
+	struct sja1105_tas_data *tas_data = &priv->tas_data;
+	struct sja1105_schedule_entry *schedule;
+	struct sja1105_table *table;
+	int schedule_start_idx;
+	s64 entry_point_delta;
+	int schedule_end_idx;
+	int num_entries = 0;
+	int num_cycles = 0;
+	int cycle = 0;
+	int i, k = 0;
+	int port;
+
+	/* Discard previous Schedule Table */
+	table = &priv->static_config.tables[BLK_IDX_SCHEDULE];
+	if (table->entry_count) {
+		kfree(table->entries);
+		table->entry_count = 0;
+	}
+
+	/* Discard previous Schedule Entry Points Parameters Table */
+	table = &priv->static_config.tables[BLK_IDX_SCHEDULE_ENTRY_POINTS_PARAMS];
+	if (table->entry_count) {
+		kfree(table->entries);
+		table->entry_count = 0;
+	}
+
+	/* Discard previous Schedule Parameters Table */
+	table = &priv->static_config.tables[BLK_IDX_SCHEDULE_PARAMS];
+	if (table->entry_count) {
+		kfree(table->entries);
+		table->entry_count = 0;
+	}
+
+	/* Discard previous Schedule Entry Points Table */
+	table = &priv->static_config.tables[BLK_IDX_SCHEDULE_ENTRY_POINTS];
+	if (table->entry_count) {
+		kfree(table->entries);
+		table->entry_count = 0;
+	}
+
+	/* Figure out the dimensioning of the problem */
+	for (port = 0; port < SJA1105_NUM_PORTS; port++) {
+		if (tas_data->offload[port]) {
+			num_entries += tas_data->offload[port]->num_entries;
+			num_cycles++;
+		}
+	}
+
+	/* Nothing to do */
+	if (!num_cycles)
+		return 0;
+
+	/* Pre-allocate space in the static config tables */
+
+	/* Schedule Table */
+	table = &priv->static_config.tables[BLK_IDX_SCHEDULE];
+	table->entries = kcalloc(num_entries, table->ops->unpacked_entry_size,
+				 GFP_KERNEL);
+	if (!table->entries)
+		return -ENOMEM;
+	table->entry_count = num_entries;
+	schedule = table->entries;
+
+	/* Schedule Points Parameters Table */
+	table = &priv->static_config.tables[BLK_IDX_SCHEDULE_ENTRY_POINTS_PARAMS];
+	table->entries = kcalloc(SJA1105_MAX_SCHEDULE_ENTRY_POINTS_PARAMS_COUNT,
+				 table->ops->unpacked_entry_size, GFP_KERNEL);
+	if (!table->entries)
+		/* Previously allocated memory will be freed automatically in
+		 * sja1105_static_config_free. This is true for all early
+		 * returns below.
+		 */
+		return -ENOMEM;
+	table->entry_count = SJA1105_MAX_SCHEDULE_ENTRY_POINTS_PARAMS_COUNT;
+	schedule_entry_points_params = table->entries;
+
+	/* Schedule Parameters Table */
+	table = &priv->static_config.tables[BLK_IDX_SCHEDULE_PARAMS];
+	table->entries = kcalloc(SJA1105_MAX_SCHEDULE_PARAMS_COUNT,
+				 table->ops->unpacked_entry_size, GFP_KERNEL);
+	if (!table->entries)
+		return -ENOMEM;
+	table->entry_count = SJA1105_MAX_SCHEDULE_PARAMS_COUNT;
+	schedule_params = table->entries;
+
+	/* Schedule Entry Points Table */
+	table = &priv->static_config.tables[BLK_IDX_SCHEDULE_ENTRY_POINTS];
+	table->entries = kcalloc(num_cycles, table->ops->unpacked_entry_size,
+				 GFP_KERNEL);
+	if (!table->entries)
+		return -ENOMEM;
+	table->entry_count = num_cycles;
+	schedule_entry_points = table->entries;
+
+	/* Finally start populating the static config tables */
+	schedule_entry_points_params->clksrc = SJA1105_TAS_CLKSRC_STANDALONE;
+	schedule_entry_points_params->actsubsch = num_cycles - 1;
+
+	for (port = 0; port < SJA1105_NUM_PORTS; port++) {
+		const struct tc_taprio_qopt_offload *offload;
+
+		offload = tas_data->offload[port];
+		if (!offload)
+			continue;
+
+		schedule_start_idx = k;
+		schedule_end_idx = k + offload->num_entries - 1;
+		/* TODO this is the base time for the port's subschedule,
+		 * relative to PTPSCHTM. But as we're using the standalone
+		 * clock source and not PTP clock as time reference, there's
+		 * little point in even trying to put more logic into this,
+		 * like preserving the phases between the subschedules of
+		 * different ports. We'll get all of that when switching to the
+		 * PTP clock source.
+		 */
+		entry_point_delta = 1;
+
+		schedule_entry_points[cycle].subschindx = cycle;
+		schedule_entry_points[cycle].delta = entry_point_delta;
+		schedule_entry_points[cycle].address = schedule_start_idx;
+
+		/* The subschedule end indices need to be
+		 * monotonically increasing.
+		 */
+		for (i = cycle; i < 8; i++)
+			schedule_params->subscheind[i] = schedule_end_idx;
+
+		for (i = 0; i < offload->num_entries; i++, k++) {
+			s64 delta_ns = offload->entries[i].interval;
+
+			schedule[k].delta = ns_to_sja1105_delta(delta_ns);
+			schedule[k].destports = BIT(port);
+			schedule[k].resmedia_en = true;
+			schedule[k].resmedia = SJA1105_GATE_MASK &
+					~offload->entries[i].gate_mask;
+		}
+		cycle++;
+	}
+
+	return 0;
+}
+
+/* Be there 2 port subschedules, each executing an arbitrary number of gate
+ * open/close events cyclically.
+ * None of those gate events must ever occur at the exact same time, otherwise
+ * the switch is known to act in exotically strange ways.
+ * However the hardware doesn't bother performing these integrity checks.
+ * So here we are with the task of validating whether the new @admin offload
+ * has any conflict with the already established TAS configuration in
+ * tas_data->offload.  We already know the other ports are in harmony with one
+ * another, otherwise we wouldn't have saved them.
+ * Each gate event executes periodically, with a period of @cycle_time and a
+ * phase given by its cycle's @base_time plus its offset within the cycle
+ * (which in turn is given by the length of the events prior to it).
+ * There are two aspects to possible collisions:
+ * - Collisions within one cycle's (actually the longest cycle's) time frame.
+ *   For that, we need to compare the cartesian product of each possible
+ *   occurrence of each event within one cycle time.
+ * - Collisions in the future. Events may not collide within one cycle time,
+ *   but if two port schedules don't have the same periodicity (aka the cycle
+ *   times aren't multiples of one another), they surely will some time in the
+ *   future (actually they will collide an infinite amount of times).
+ */
+static bool
+sja1105_tas_check_conflicts(struct sja1105_private *priv, int port,
+			    const struct tc_taprio_qopt_offload *admin)
+{
+	struct sja1105_tas_data *tas_data = &priv->tas_data;
+	const struct tc_taprio_qopt_offload *offload;
+	s64 max_cycle_time, min_cycle_time;
+	s64 delta1, delta2;
+	s64 rbt1, rbt2;
+	s64 stop_time;
+	s64 t1, t2;
+	int i, j;
+	s32 rem;
+
+	offload = tas_data->offload[port];
+	if (!offload)
+		return false;
+
+	/* Check if the two cycle times are multiples of one another.
+	 * If they aren't, then they will surely collide.
+	 */
+	max_cycle_time = max(offload->cycle_time, admin->cycle_time);
+	min_cycle_time = min(offload->cycle_time, admin->cycle_time);
+	div_s64_rem(max_cycle_time, min_cycle_time, &rem);
+	if (rem)
+		return true;
+
+	/* Calculate the "reduced" base time of each of the two cycles
+	 * (transposed back as close to 0 as possible) by dividing to
+	 * the cycle time.
+	 */
+	div_s64_rem(offload->base_time, offload->cycle_time, &rem);
+	rbt1 = rem;
+
+	div_s64_rem(admin->base_time, admin->cycle_time, &rem);
+	rbt2 = rem;
+
+	stop_time = max_cycle_time + max(rbt1, rbt2);
+
+	/* delta1 is the relative base time of each GCL entry within
+	 * the established ports' TAS config.
+	 */
+	for (i = 0, delta1 = 0;
+	     i < offload->num_entries;
+	     delta1 += offload->entries[i].interval, i++) {
+		/* delta2 is the relative base time of each GCL entry
+		 * within the newly added TAS config.
+		 */
+		for (j = 0, delta2 = 0;
+		     j < admin->num_entries;
+		     delta2 += admin->entries[j].interval, j++) {
+			/* t1 follows all possible occurrences of the
+			 * established ports' GCL entry i within the
+			 * first cycle time.
+			 */
+			for (t1 = rbt1 + delta1;
+			     t1 <= stop_time;
+			     t1 += offload->cycle_time) {
+				/* t2 follows all possible occurrences
+				 * of the newly added GCL entry j
+				 * within the first cycle time.
+				 */
+				for (t2 = rbt2 + delta2;
+				     t2 <= stop_time;
+				     t2 += admin->cycle_time) {
+					if (t1 == t2) {
+						dev_warn(priv->ds->dev,
+							 "GCL entry %d collides with entry %d of port %d\n",
+							 j, i, port);
+						return true;
+					}
+				}
+			}
+		}
+	}
+
+	return false;
+}
+
+int sja1105_setup_tc_taprio(struct dsa_switch *ds, int port,
+			    struct tc_taprio_qopt_offload *admin)
+{
+	struct sja1105_private *priv = ds->priv;
+	struct sja1105_tas_data *tas_data = &priv->tas_data;
+	int other_port, rc, i;
+
+	/* Can't change an already configured port (must delete qdisc first).
+	 * Can't delete the qdisc from an unconfigured port.
+	 */
+	if (!!tas_data->offload[port] == admin->enable)
+		return -EINVAL;
+
+	if (!admin->enable) {
+		taprio_offload_free(tas_data->offload[port]);
+		tas_data->offload[port] = NULL;
+
+		rc = sja1105_init_scheduling(priv);
+		if (rc < 0)
+			return rc;
+
+		return sja1105_static_config_reload(priv);
+	}
+
+	/* The cycle time extension is the amount of time the last cycle from
+	 * the old OPER needs to be extended in order to phase-align with the
+	 * base time of the ADMIN when that becomes the new OPER.
+	 * But of course our switch needs to be reset to switch-over between
+	 * the ADMIN and the OPER configs - so much for a seamless transition.
+	 * So don't add insult over injury and just say we don't support cycle
+	 * time extension.
+	 */
+	if (admin->cycle_time_extension)
+		return -ENOTSUPP;
+
+	if (!ns_to_sja1105_delta(admin->base_time)) {
+		dev_err(ds->dev, "A base time of zero is not hardware-allowed\n");
+		return -ERANGE;
+	}
+
+	for (i = 0; i < admin->num_entries; i++) {
+		s64 delta_ns = admin->entries[i].interval;
+		s64 delta_cycles = ns_to_sja1105_delta(delta_ns);
+		bool too_long, too_short;
+
+		too_long = (delta_cycles >= SJA1105_TAS_MAX_DELTA);
+		too_short = (delta_cycles == 0);
+		if (too_long || too_short) {
+			dev_err(priv->ds->dev,
+				"Interval %llu too %s for GCL entry %d\n",
+				delta_ns, too_long ? "long" : "short", i);
+			return -ERANGE;
+		}
+	}
+
+	for (other_port = 0; other_port < SJA1105_NUM_PORTS; other_port++) {
+		if (other_port == port)
+			continue;
+
+		if (sja1105_tas_check_conflicts(priv, other_port, admin))
+			return -ERANGE;
+	}
+
+	tas_data->offload[port] = taprio_offload_get(admin);
+
+	rc = sja1105_init_scheduling(priv);
+	if (rc < 0)
+		return rc;
+
+	return sja1105_static_config_reload(priv);
+}
+
+void sja1105_tas_setup(struct dsa_switch *ds)
+{
+}
+
+void sja1105_tas_teardown(struct dsa_switch *ds)
+{
+	struct sja1105_private *priv = ds->priv;
+	struct tc_taprio_qopt_offload *offload;
+	int port;
+
+	for (port = 0; port < SJA1105_NUM_PORTS; port++) {
+		offload = priv->tas_data.offload[port];
+		if (!offload)
+			continue;
+
+		taprio_offload_free(offload);
+	}
+}

drivers/net/dsa/sja1105/sja1105_tas.h

···
+/* SPDX-License-Identifier: GPL-2.0
+ * Copyright (c) 2019, Vladimir Oltean <[e-mail]>
+ */
+#ifndef _SJA1105_TAS_H
+#define _SJA1105_TAS_H
+
+#include <net/pkt_sched.h>
+
+#if IS_ENABLED(CONFIG_NET_DSA_SJA1105_TAS)
+
+struct sja1105_tas_data {
+	struct tc_taprio_qopt_offload *offload[SJA1105_NUM_PORTS];
+};
+
+int sja1105_setup_tc_taprio(struct dsa_switch *ds, int port,
+			    struct tc_taprio_qopt_offload *admin);
+
+void sja1105_tas_setup(struct dsa_switch *ds);
+
+void sja1105_tas_teardown(struct dsa_switch *ds);
+
+#else
+
+/* C doesn't allow empty structures, bah! */
+struct sja1105_tas_data {
+	u8 dummy;
+};
+
+static inline int sja1105_setup_tc_taprio(struct dsa_switch *ds, int port,
+					  struct tc_taprio_qopt_offload *admin)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline void sja1105_tas_setup(struct dsa_switch *ds) { }
+
+static inline void sja1105_tas_teardown(struct dsa_switch *ds) { }
+
+#endif /* IS_ENABLED(CONFIG_NET_DSA_SJA1105_TAS) */
+
+#endif /* _SJA1105_TAS_H */