platform/surface: aggregator: Improve documentation and handling of message target and source IDs

The `tid_in` and `tid_out` fields of the serial hub protocol command
struct (struct ssh_command) are actually source and target IDs,
indicating the peer from which the message originated and the peer for
which it is intended.

Change the naming of those fields accordingly and improve the protocol
documentation. Additionally, introduce an enum containing all currently
known peers, i.e. targets and sources.

Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/20221202223327.690880-3-luzmaximilian@gmail.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

Documentation/driver-api/surface_aggregator/client.rst

@@ -191,7 +191,7 @@ data received from it is converted from little-endian to host endianness.
             *       they do not correspond to an actual SAM/EC request.
             */
            rqst.target_category = SSAM_SSH_TC_SAM;
-           rqst.target_id = 0x01;
+           rqst.target_id = SSAM_SSH_TID_SAM;
            rqst.command_id = 0x02;
            rqst.instance_id = 0x03;
            rqst.flags = SSAM_REQUEST_HAS_RESPONSE;
@@ -241,7 +241,7 @@ one of the generator macros, for example via:
 
    SSAM_DEFINE_SYNC_REQUEST_W(__ssam_tmp_perf_mode_set, __le32, {
            .target_category = SSAM_SSH_TC_TMP,
-           .target_id       = 0x01,
+           .target_id       = SSAM_SSH_TID_SAM,
            .command_id      = 0x03,
            .instance_id     = 0x00,
    });

Documentation/driver-api/surface_aggregator/ssh.rst

@@ -13,6 +13,7 @@
 .. |DATA_NSQ| replace:: ``DATA_NSQ``
 .. |TC| replace:: ``TC``
 .. |TID| replace:: ``TID``
+.. |SID| replace:: ``SID``
 .. |IID| replace:: ``IID``
 .. |RQID| replace:: ``RQID``
 .. |CID| replace:: ``CID``
@@ -219,13 +220,13 @@ following fields, packed together and in order:
      - |u8|
      - Target category.
 
-   * - |TID| (out)
+   * - |TID|
      - |u8|
-     - Target ID for outgoing (host to EC) commands.
+     - Target ID for commands/messages.
 
-   * - |TID| (in)
+   * - |SID|
      - |u8|
-     - Target ID for incoming (EC to host) commands.
+     - Source ID for commands/messages.
 
    * - |IID|
      - |u8|
@@ -286,19 +287,20 @@ general, however, a single target category should map to a single reserved
 event request ID.
 
 Furthermore, requests, responses, and events have an associated target ID
-(``TID``). This target ID is split into output (host to EC) and input (EC to
-host) fields, with the respecting other field (e.g. output field on incoming
-messages) set to zero. Two ``TID`` values are known: Primary (``0x01``) and
-secondary (``0x02``). In general, the response to a request should have the
-same ``TID`` value, however, the field (output vs. input) should be used in
-accordance to the direction in which the response is sent (i.e. on the input
-field, as responses are generally sent from the EC to the host).
+(``TID``) and source ID (``SID``). These two fields indicate where a message
+originates from (``SID``) and what the intended target of the message is
+(``TID``). Note that a response to a specific request therefore has the source
+and target IDs swapped when compared to the original request (i.e. the request
+target is the response source and the request source is the response target).
+See (:c:type:`enum ssh_request_id <ssh_request_id>`) for possible values of
+both.
 
-Note that, even though requests and events should be uniquely identifiable
-by target category and command ID alone, the EC may require specific
-target ID and instance ID values to accept a command. A command that is
-accepted for ``TID=1``, for example, may not be accepted for ``TID=2``
-and vice versa.
+Note that, even though requests and events should be uniquely identifiable by
+target category and command ID alone, the EC may require specific target ID and
+instance ID values to accept a command. A command that is accepted for
+``TID=1``, for example, may not be accepted for ``TID=2`` and vice versa. While
+this may not always hold in reality, you can think of different target/source
+IDs indicating different physical ECs with potentially different feature sets.
 
 
 Limitations and Observations

drivers/platform/surface/aggregator/controller.c

@@ -994,7 +994,7 @@ static void ssam_handle_event(struct ssh_rtl *rtl,
 
 	item->rqid = get_unaligned_le16(&cmd->rqid);
 	item->event.target_category = cmd->tc;
-	item->event.target_id = cmd->tid_in;
+	item->event.target_id = cmd->sid;
 	item->event.command_id = cmd->cid;
 	item->event.instance_id = cmd->iid;
 	memcpy(&item->event.data[0], data->ptr, data->len);
@@ -1779,35 +1779,35 @@ EXPORT_SYMBOL_GPL(ssam_request_sync_with_buffer);
 
 SSAM_DEFINE_SYNC_REQUEST_R(ssam_ssh_get_firmware_version, __le32, {
 	.target_category = SSAM_SSH_TC_SAM,
-	.target_id       = 0x01,
+	.target_id       = SSAM_SSH_TID_SAM,
 	.command_id      = 0x13,
 	.instance_id     = 0x00,
 });
 
 SSAM_DEFINE_SYNC_REQUEST_R(ssam_ssh_notif_display_off, u8, {
 	.target_category = SSAM_SSH_TC_SAM,
-	.target_id       = 0x01,
+	.target_id       = SSAM_SSH_TID_SAM,
 	.command_id      = 0x15,
 	.instance_id     = 0x00,
 });
 
 SSAM_DEFINE_SYNC_REQUEST_R(ssam_ssh_notif_display_on, u8, {
 	.target_category = SSAM_SSH_TC_SAM,
-	.target_id       = 0x01,
+	.target_id       = SSAM_SSH_TID_SAM,
 	.command_id      = 0x16,
 	.instance_id     = 0x00,
 });
 
 SSAM_DEFINE_SYNC_REQUEST_R(ssam_ssh_notif_d0_exit, u8, {
 	.target_category = SSAM_SSH_TC_SAM,
-	.target_id       = 0x01,
+	.target_id       = SSAM_SSH_TID_SAM,
 	.command_id      = 0x33,
 	.instance_id     = 0x00,
 });
 
 SSAM_DEFINE_SYNC_REQUEST_R(ssam_ssh_notif_d0_entry, u8, {
 	.target_category = SSAM_SSH_TC_SAM,
-	.target_id       = 0x01,
+	.target_id       = SSAM_SSH_TID_SAM,
 	.command_id      = 0x34,
 	.instance_id     = 0x00,
 });

drivers/platform/surface/aggregator/ssh_msgb.h

@@ -189,8 +189,8 @@ static inline void msgb_push_cmd(struct msgbuf *msgb, u8 seq, u16 rqid,
 
 	__msgb_push_u8(msgb, SSH_PLD_TYPE_CMD);		/* Payload type. */
 	__msgb_push_u8(msgb, rqst->target_category);	/* Target category. */
-	__msgb_push_u8(msgb, rqst->target_id);		/* Target ID (out). */
-	__msgb_push_u8(msgb, 0x00);			/* Target ID (in). */
+	__msgb_push_u8(msgb, rqst->target_id);		/* Target ID. */
+	__msgb_push_u8(msgb, SSAM_SSH_TID_HOST);	/* Source ID. */
 	__msgb_push_u8(msgb, rqst->instance_id);	/* Instance ID. */
 	__msgb_push_u16(msgb, rqid);			/* Request ID. */
 	__msgb_push_u8(msgb, rqst->command_id);		/* Command ID. */

drivers/platform/surface/aggregator/ssh_request_layer.c

@@ -920,13 +920,14 @@ static void ssh_rtl_rx_command(struct ssh_ptl *p, const struct ssam_span *data)
 	 * Check if the message was intended for us. If not, drop it.
 	 *
 	 * Note: We will need to change this to handle debug messages. On newer
-	 * generation devices, these seem to be sent to tid_out=0x03. We as
-	 * host can still receive them as they can be forwarded via an override
-	 * option on SAM, but doing so does not change tid_out=0x00.
+	 * generation devices, these seem to be sent to SSAM_SSH_TID_DEBUG. We
+	 * as host can still receive them as they can be forwarded via an
+	 * override option on SAM, but doing so does not change the target ID
+	 * to SSAM_SSH_TID_HOST.
 	 */
-	if (command->tid_out != 0x00) {
+	if (command->tid != SSAM_SSH_TID_HOST) {
 		rtl_warn(rtl, "rtl: dropping message not intended for us (tid = %#04x)\n",
-			 command->tid_out);
+			 command->tid);
 		return;
 	}
 

include/linux/surface_aggregator/controller.h

@@ -912,10 +912,10 @@ enum ssam_event_mask {
 	})
 
 #define SSAM_EVENT_REGISTRY_SAM	\
-	SSAM_EVENT_REGISTRY(SSAM_SSH_TC_SAM, 0x01, 0x0b, 0x0c)
+	SSAM_EVENT_REGISTRY(SSAM_SSH_TC_SAM, SSAM_SSH_TID_SAM, 0x0b, 0x0c)
 
 #define SSAM_EVENT_REGISTRY_KIP	\
-	SSAM_EVENT_REGISTRY(SSAM_SSH_TC_KIP, 0x02, 0x27, 0x28)
+	SSAM_EVENT_REGISTRY(SSAM_SSH_TC_KIP, SSAM_SSH_TID_KIP, 0x27, 0x28)
 
 #define SSAM_EVENT_REGISTRY_REG(tid)\
 	SSAM_EVENT_REGISTRY(SSAM_SSH_TC_REG, tid, 0x01, 0x02)

include/linux/surface_aggregator/serial_hub.h

@@ -83,23 +83,21 @@ enum ssh_payload_type {
 
 /**
  * struct ssh_command - Payload of a command-type frame.
- * @type:    The type of the payload. See &enum ssh_payload_type. Should be
- *           SSH_PLD_TYPE_CMD for this struct.
- * @tc:      Command target category.
- * @tid_out: Output target ID. Should be zero if this an incoming (EC to host)
- *           message.
- * @tid_in:  Input target ID. Should be zero if this is an outgoing (host to
- *           EC) message.
- * @iid:     Instance ID.
- * @rqid:    Request ID. Used to match requests with responses and differentiate
- *           between responses and events.
- * @cid:     Command ID.
+ * @type: The type of the payload. See &enum ssh_payload_type. Should be
+ *        SSH_PLD_TYPE_CMD for this struct.
+ * @tc:   Command target category.
+ * @tid:  Target ID. Indicates the target of the message.
+ * @sid:  Source ID. Indicates the source of the message.
+ * @iid:  Instance ID.
+ * @rqid: Request ID. Used to match requests with responses and differentiate
+ *        between responses and events.
+ * @cid:  Command ID.
  */
 struct ssh_command {
 	u8 type;
 	u8 tc;
-	u8 tid_out;
-	u8 tid_in;
+	u8 tid;
+	u8 sid;
 	u8 iid;
 	__le16 rqid;
 	u8 cid;
@@ -280,6 +278,22 @@ struct ssam_span {
 	size_t len;
 };
 
+/**
+ * enum ssam_ssh_tid - Target/source IDs for Serial Hub messages.
+ * @SSAM_SSH_TID_HOST:     We as the kernel Serial Hub driver.
+ * @SSAM_SSH_TID_SAM:      The Surface Aggregator EC.
+ * @SSAM_SSH_TID_KIP:      Keyboard and perihperal controller.
+ * @SSAM_SSH_TID_DEBUG:    Debug connector.
+ * @SSAM_SSH_TID_SURFLINK: SurfLink connector.
+ */
+enum ssam_ssh_tid {
+	SSAM_SSH_TID_HOST     = 0x00,
+	SSAM_SSH_TID_SAM      = 0x01,
+	SSAM_SSH_TID_KIP      = 0x02,
+	SSAM_SSH_TID_DEBUG    = 0x03,
+	SSAM_SSH_TID_SURFLINK = 0x04,
+};
+
 /*
  * Known SSH/EC target categories.
  *