[PATCH v2 2/3] soundwire: cadence: Remove wasted space in response_buf

[Richard Fitzgerald <rf@opensource.cirrus.com>]

The response_buf was declared much larger (128 entries) than the number
of responses that could ever be written into it. The Cadence IP is
configurable up to a maximum of 32 entries, and the datasheet says
that RX_FIFO_AVAIL can be 2 larger than this. So allow up to 34
responses.

Also add checking in cdns_read_response() to prevent overflowing
reponse_buf if RX_FIFO_AVAIL contains an unexpectedly large number.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
---
Changes since v1:
- Make response_buf large enough for the maximum IP config of 32 plus
  the mysterious extra 2.
- If RX_FIFO_AVAIL is too big trim it to the size of response_buf
  instead of CDNS_MCP_CMD_LEN.
---
 drivers/soundwire/cadence_master.c |  7 +++++++
 drivers/soundwire/cadence_master.h | 13 ++++++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/soundwire/cadence_master.c b/drivers/soundwire/cadence_master.c
index 27699f341f2c..a6635f7f350e 100644
--- a/drivers/soundwire/cadence_master.c
+++ b/drivers/soundwire/cadence_master.c
@@ -774,8 +774,15 @@ static void cdns_read_response(struct sdw_cdns *cdns)
 	u32 num_resp, cmd_base;
 	int i;
 
+	/* RX_FIFO_AVAIL can be 2 entries more than the FIFO size */
+	BUILD_BUG_ON(ARRAY_SIZE(cdns->response_buf) < CDNS_MCP_CMD_LEN + 2);
+
 	num_resp = cdns_readl(cdns, CDNS_MCP_FIFOSTAT);
 	num_resp &= CDNS_MCP_RX_FIFO_AVAIL;
+	if (num_resp > ARRAY_SIZE(cdns->response_buf)) {
+		dev_warn(cdns->dev, "RX AVAIL %d too long\n", num_resp);
+		num_resp = ARRAY_SIZE(cdns->response_buf);
+	}
 
 	cmd_base = CDNS_MCP_CMD_BASE;
 
diff --git a/drivers/soundwire/cadence_master.h b/drivers/soundwire/cadence_master.h
index 0434d70d4b1f..e0a64b28c6b9 100644
--- a/drivers/soundwire/cadence_master.h
+++ b/drivers/soundwire/cadence_master.h
@@ -8,6 +8,12 @@
 #define SDW_CADENCE_GSYNC_KHZ		4 /* 4 kHz */
 #define SDW_CADENCE_GSYNC_HZ		(SDW_CADENCE_GSYNC_KHZ * 1000)
 
+/*
+ * The Cadence IP supports up to 32 entries in the FIFO, though implementations
+ * can configure the IP to have a smaller FIFO.
+ */
+#define CDNS_MCP_IP_MAX_CMD_LEN		32
+
 /**
  * struct sdw_cdns_pdi: PDI (Physical Data Interface) instance
  *
@@ -117,7 +123,12 @@ struct sdw_cdns {
 	struct sdw_bus bus;
 	unsigned int instance;
 
-	u32 response_buf[0x80];
+	/*
+	 * The datasheet says the RX FIFO AVAIL can be 2 entries more
+	 * than the FIFO capacity, so allow for this.
+	 */
+	u32 response_buf[CDNS_MCP_IP_MAX_CMD_LEN + 2];
+
 	struct completion tx_complete;
 	struct sdw_defer *defer;
 
-- 
2.30.2