[PATCH v6 5/5] i3c: master: Add optional_bytes for variable-length GET CCC validation

[tze . yee . ng]

From: Adrian Ng Ho Yin <adrian.ho.yin.ng@xxxxxxxxxx>

Add optional_bytes to struct i3c_ccc_cmd_payload so callers describe
variable-length GET CCC responses. GETMRL and GETMXDS set optional_bytes
at the call site. Extend i3c_ccc_validate_payload_len() to honour it.

Signed-off-by: Adrian Ng Ho Yin <adrian.ho.yin.ng@xxxxxxxxxx>
Signed-off-by: Tze Yee Ng <tze.yee.ng@xxxxxxxxxx>
---
Changes in v5:
- Split from v4 patch 3/3: optional_bytes and variable-length GET rules only.
- Simplify validation: drop redundant exact-length check when
  optional_bytes == 0 (per review).
- Clarify GETMXDS fallback: with optional_bytes = 3, a 2-byte success on
  the first attempt no longer needs the fallback; it remains for when the
  5-byte request fails rather than short-reading.
---
 drivers/i3c/master.c    | 29 +++++++++++++++++++++++------
 include/linux/i3c/ccc.h |  2 ++
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/drivers/i3c/master.c b/drivers/i3c/master.c
index 3b6d3ecd9d12..12ca7c929ee5 100644
--- a/drivers/i3c/master.c
+++ b/drivers/i3c/master.c
@@ -902,6 +902,7 @@ static void *i3c_ccc_cmd_dest_init(struct i3c_ccc_cmd_dest *dest, u8 addr,
 	dest->addr = addr;
 	dest->payload.len = payloadlen;
 	dest->payload.actual_len = 0;
+	dest->payload.optional_bytes = 0;
 	if (payloadlen)
 		dest->payload.data = kzalloc(payloadlen, GFP_KERNEL);
 	else
@@ -944,11 +945,19 @@ static int i3c_ccc_validate_payload_len(struct i3c_ccc_cmd *cmd)
 
 	for (i = 0; i < cmd->ndests; i++) {
 		struct i3c_ccc_cmd_payload *p = &cmd->dests[i].payload;
+		u16 min_len;
+
+		if (p->optional_bytes > p->len)
+			return -EINVAL;
 
 		if (p->actual_len > p->len)
 			return -EIO;
 
-		if (p->len && p->actual_len != p->len)
+		if (!p->len)
+			continue;
+
+		min_len = p->len - p->optional_bytes;
+		if (p->actual_len < min_len)
 			return -EIO;
 	}
 
@@ -1342,10 +1351,14 @@ static int i3c_master_getmrl_locked(struct i3c_master_controller *master,
 		return -ENOMEM;
 
 	/*
-	 * When the device does not have IBI payload GETMRL only returns 2
-	 * bytes of data.
+	 * GETMRL returns 2 bytes (max read length) when the device does not
+	 * advertise IBI payload, or 2 or 3 bytes when it does (the optional
+	 * third byte is max IBI length). Use optional_bytes to allow either
+	 * length when IBI payload is supported.
 	 */
-	if (!(info->bcr & I3C_BCR_IBI_PAYLOAD))
+	if (info->bcr & I3C_BCR_IBI_PAYLOAD)
+		dest.payload.optional_bytes = 1;
+	else
 		dest.payload.len -= 1;
 
 	i3c_ccc_cmd_init(&cmd, true, I3C_CCC_GETMRL, &dest, 1);
@@ -1414,14 +1427,18 @@ static int i3c_master_getmxds_locked(struct i3c_master_controller *master,
 	if (!getmaxds)
 		return -ENOMEM;
 
+	dest.payload.optional_bytes = 3;
+
 	i3c_ccc_cmd_init(&cmd, true, I3C_CCC_GETMXDS, &dest, 1);
 	ret = i3c_master_send_ccc_cmd_locked(master, &cmd);
 	if (ret) {
 		/*
-		 * Retry when the device does not support max read turnaround
-		 * while expecting shorter length from this CCC command.
+		 * optional_bytes = 3 accepts a 2-byte response on the first
+		 * attempt, so this fallback runs only when the 5-byte request
+		 * fails rather than returning a short read.
 		 */
 		dest.payload.len -= 3;
+		dest.payload.optional_bytes = 0;
 		i3c_ccc_cmd_init(&cmd, true, I3C_CCC_GETMXDS, &dest, 1);
 		ret = i3c_master_send_ccc_cmd_locked(master, &cmd);
 		if (ret)
diff --git a/include/linux/i3c/ccc.h b/include/linux/i3c/ccc.h
index 2506d83b8255..7ad677baf761 100644
--- a/include/linux/i3c/ccc.h
+++ b/include/linux/i3c/ccc.h
@@ -347,11 +347,13 @@ struct i3c_ccc_getxtime {
  *
  * @len: requested payload length
  * @actual_len: number of bytes received on a GET CCC (filled by the driver)
+ * @optional_bytes: GET CCCs may return up to this many fewer bytes than @len
  * @data: payload data. This buffer must be DMA-able
  */
 struct i3c_ccc_cmd_payload {
 	u16 len;
 	u16 actual_len;
+	u16 optional_bytes;
 	void *data;
 };
 
-- 
2.43.7