linux-kernel - [RFC PATCH v2 04/12] greybus: cpc: pack cport ID in Greybus header

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Message-ID: <20251114150738.32426-5-damien.riegel@silabs.com>
Date: Fri, 14 Nov 2025 10:07:30 -0500
From: Damien Riégel <damien.riegel@...abs.com>
To: greybus-dev@...ts.linaro.org, Johan Hovold <johan@...nel.org>,
        Alex Elder <elder@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Cc: Silicon Labs Kernel Team <linux-devel@...abs.com>,
        Damien Riégel <damien.riegel@...abs.com>
Subject: [RFC PATCH v2 04/12] greybus: cpc: pack cport ID in Greybus header

Take advantage of the padding bytes present in the Greybus header to
store the CPort ID and minize overhead. This technique is already used
by the es2 driver.

Signed-off-by: Damien Riégel <damien.riegel@...abs.com>
---
 drivers/greybus/cpc/cpc.h   |  3 +++
 drivers/greybus/cpc/cport.c | 29 +++++++++++++++++++++++++++++
 drivers/greybus/cpc/host.c  | 13 ++++++++++++-
 drivers/greybus/cpc/host.h  |  2 +-
 4 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/drivers/greybus/cpc/cpc.h b/drivers/greybus/cpc/cpc.h
index 7e032f6cf50..0f2d204d86d 100644
--- a/drivers/greybus/cpc/cpc.h
+++ b/drivers/greybus/cpc/cpc.h
@@ -24,6 +24,9 @@ struct cpc_cport {
 struct cpc_cport *cpc_cport_alloc(u16 cport_id, gfp_t gfp_mask);
 void cpc_cport_release(struct cpc_cport *cport);
 
+void cpc_cport_pack(struct gb_operation_msg_hdr *gb_hdr, u16 cport_id);
+u16 cpc_cport_unpack(struct gb_operation_msg_hdr *gb_hdr);
+
 int cpc_cport_transmit(struct cpc_cport *cport, struct sk_buff *skb);
 
 struct cpc_skb_cb {
diff --git a/drivers/greybus/cpc/cport.c b/drivers/greybus/cpc/cport.c
index ed0b8e8b0d7..0fc4ff0c5bb 100644
--- a/drivers/greybus/cpc/cport.c
+++ b/drivers/greybus/cpc/cport.c
@@ -3,6 +3,9 @@
  * Copyright (c) 2025, Silicon Laboratories, Inc.
  */
 
+#include <linux/unaligned.h>
+#include <linux/skbuff.h>
+
 #include "cpc.h"
 #include "host.h"
 
@@ -31,6 +34,27 @@ void cpc_cport_release(struct cpc_cport *cport)
 	kfree(cport);
 }
 
+/**
+ * cpc_cport_pack() - Pack CPort ID into Greybus Operation Message header.
+ * @gb_hdr: Greybus operation message header.
+ * @cport_id: CPort ID to pack.
+ */
+void cpc_cport_pack(struct gb_operation_msg_hdr *gb_hdr, u16 cport_id)
+{
+	put_unaligned_le16(cport_id, gb_hdr->pad);
+}
+
+/**
+ * cpc_cport_unpack() - Unpack CPort ID from Greybus Operation Message header.
+ * @gb_hdr: Greybus operation message header.
+ *
+ * Return: CPort ID packed in the header.
+ */
+u16 cpc_cport_unpack(struct gb_operation_msg_hdr *gb_hdr)
+{
+	return get_unaligned_le16(gb_hdr->pad);
+}
+
 /**
  * cpc_cport_transmit() - Transmit skb over cport.
  * @cport: cport.
@@ -39,6 +63,11 @@ void cpc_cport_release(struct cpc_cport *cport)
 int cpc_cport_transmit(struct cpc_cport *cport, struct sk_buff *skb)
 {
 	struct cpc_host_device *cpc_hd = cport->cpc_hd;
+	struct gb_operation_msg_hdr *gb_hdr;
+
+	/* Inject cport ID in Greybus header */
+	gb_hdr = (struct gb_operation_msg_hdr *)skb->data;
+	cpc_cport_pack(gb_hdr, cport->id);
 
 	return cpc_hd_send_skb(cpc_hd, skb);
 }
diff --git a/drivers/greybus/cpc/host.c b/drivers/greybus/cpc/host.c
index 2ca938c2b48..1d81c624dd6 100644
--- a/drivers/greybus/cpc/host.c
+++ b/drivers/greybus/cpc/host.c
@@ -204,8 +204,19 @@ void cpc_hd_message_sent(struct sk_buff *skb, int status)
 }
 EXPORT_SYMBOL_GPL(cpc_hd_message_sent);
 
-void cpc_hd_rcvd(struct cpc_host_device *cpc_hd, u16 cport_id, u8 *data, size_t length)
+void cpc_hd_rcvd(struct cpc_host_device *cpc_hd, u8 *data, size_t length)
 {
+	struct gb_operation_msg_hdr *gb_hdr;
+	u16 cport_id;
+
+	/* Prevent an out-of-bound access if called with non-sensical parameters. */
+	if (!data || length < sizeof(*gb_hdr))
+		return;
+
+	/* Retrieve cport ID that was packed in Greybus header */
+	gb_hdr = (struct gb_operation_msg_hdr *)data;
+	cport_id = cpc_cport_unpack(gb_hdr);
+
 	greybus_data_rcvd(cpc_hd->gb_hd, cport_id, data, length);
 }
 EXPORT_SYMBOL_GPL(cpc_hd_rcvd);
diff --git a/drivers/greybus/cpc/host.h b/drivers/greybus/cpc/host.h
index 104d61e3bc5..86d205fcb59 100644
--- a/drivers/greybus/cpc/host.h
+++ b/drivers/greybus/cpc/host.h
@@ -45,7 +45,7 @@ struct cpc_host_device *cpc_hd_create(struct cpc_hd_driver *driver, struct devic
 int cpc_hd_add(struct cpc_host_device *cpc_hd);
 void cpc_hd_put(struct cpc_host_device *cpc_hd);
 void cpc_hd_del(struct cpc_host_device *cpc_hd);
-void cpc_hd_rcvd(struct cpc_host_device *cpc_hd, u16 cport_id, u8 *data, size_t length);
+void cpc_hd_rcvd(struct cpc_host_device *cpc_hd, u8 *data, size_t length);
 void cpc_hd_message_sent(struct sk_buff *skb, int status);
 
 int cpc_hd_send_skb(struct cpc_host_device *cpc_hd, struct sk_buff *skb);
-- 
2.49.0