net: dsa: sja1105: adapt to a SPI controller with a limited max transfer size

The static config of the sja1105 switch is a long stream of bytes which
is programmed to the hardware in chunks (portions with the chip select
continuously asserted) of max 256 bytes each. Each chunk is a
spi_message composed of 2 spi_transfers: the buffer with the data and a
preceding buffer with the SPI access header.

Only that certain SPI controllers, such as the spi-sc18is602 I2C-to-SPI
bridge, cannot keep the chip select asserted for that long.
The spi_max_transfer_size() and spi_max_message_size() functions are how
the controller can impose its hardware limitations upon the SPI
peripheral driver.

For the sja1105 driver to work with these controllers, both buffers must
be smaller than the transfer limit, and their sum must be smaller than
the message limit.

Regression-tested on a switch connected to a controller with no
limitations (spi-fsl-dspi) as well as with one with caps for both
max_transfer_size and max_message_size (spi-sc18is602).
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/sja1105/sja1105.h

@@ -209,6 +209,7 @@ struct sja1105_private {
 	unsigned long ucast_egress_floods;
 	unsigned long bcast_egress_floods;
 	const struct sja1105_info *info;
+	size_t max_xfer_len;
 	struct gpio_desc *reset_gpio;
 	struct spi_device *spidev;
 	struct dsa_switch *ds;

drivers/net/dsa/sja1105/sja1105_main.c

@@ -3563,6 +3563,7 @@ static int sja1105_probe(struct spi_device *spi)
 	struct sja1105_tagger_data *tagger_data;
 	struct device *dev = &spi->dev;
 	struct sja1105_private *priv;
+	size_t max_xfer, max_msg;
 	struct dsa_switch *ds;
 	int rc, port;
 
@@ -3596,6 +3597,33 @@ static int sja1105_probe(struct spi_device *spi)
 		return rc;
 	}
 
+	/* In sja1105_xfer, we send spi_messages composed of two spi_transfers:
+	 * a small one for the message header and another one for the current
+	 * chunk of the packed buffer.
+	 * Check that the restrictions imposed by the SPI controller are
+	 * respected: the chunk buffer is smaller than the max transfer size,
+	 * and the total length of the chunk plus its message header is smaller
+	 * than the max message size.
+	 * We do that during probe time since the maximum transfer size is a
+	 * runtime invariant.
+	 */
+	max_xfer = spi_max_transfer_size(spi);
+	max_msg = spi_max_message_size(spi);
+
+	/* We need to send at least one 64-bit word of SPI payload per message
+	 * in order to be able to make useful progress.
+	 */
+	if (max_msg < SJA1105_SIZE_SPI_MSG_HEADER + 8) {
+		dev_err(dev, "SPI master cannot send large enough buffers, aborting\n");
+		return -EINVAL;
+	}
+
+	priv->max_xfer_len = SJA1105_SIZE_SPI_MSG_MAXLEN;
+	if (priv->max_xfer_len > max_xfer)
+		priv->max_xfer_len = max_xfer;
+	if (priv->max_xfer_len > max_msg - SJA1105_SIZE_SPI_MSG_HEADER)
+		priv->max_xfer_len = max_msg - SJA1105_SIZE_SPI_MSG_HEADER;
+
 	priv->info = of_device_get_match_data(dev);
 
 	/* Detect hardware device */

drivers/net/dsa/sja1105/sja1105_spi.c

@@ -8,8 +8,6 @@
 #include "sja1105.h"
 
 #define SJA1105_SIZE_RESET_CMD		4
-#define SJA1105_SIZE_SPI_MSG_HEADER	4
-#define SJA1105_SIZE_SPI_MSG_MAXLEN	(64 * 4)
 
 struct sja1105_chunk {
 	u8	*buf;
@@ -40,19 +38,19 @@ static int sja1105_xfer(const struct sja1105_private *priv,
 			size_t len, struct ptp_system_timestamp *ptp_sts)
 {
 	u8 hdr_buf[SJA1105_SIZE_SPI_MSG_HEADER] = {0};
-	struct sja1105_chunk chunk = {
-		.len = min_t(size_t, len, SJA1105_SIZE_SPI_MSG_MAXLEN),
-		.reg_addr = reg_addr,
-		.buf = buf,
-	};
 	struct spi_device *spi = priv->spidev;
 	struct spi_transfer xfers[2] = {0};
 	struct spi_transfer *chunk_xfer;
 	struct spi_transfer *hdr_xfer;
+	struct sja1105_chunk chunk;
 	int num_chunks;
 	int rc, i = 0;
 
-	num_chunks = DIV_ROUND_UP(len, SJA1105_SIZE_SPI_MSG_MAXLEN);
+	num_chunks = DIV_ROUND_UP(len, priv->max_xfer_len);
+
+	chunk.reg_addr = reg_addr;
+	chunk.buf = buf;
+	chunk.len = min_t(size_t, len, priv->max_xfer_len);
 
 	hdr_xfer = &xfers[0];
 	chunk_xfer = &xfers[1];
@@ -104,7 +102,7 @@ static int sja1105_xfer(const struct sja1105_private *priv,
 		chunk.buf += chunk.len;
 		chunk.reg_addr += chunk.len / 4;
 		chunk.len = min_t(size_t, (ptrdiff_t)(buf + len - chunk.buf),
-				  SJA1105_SIZE_SPI_MSG_MAXLEN);
+				  priv->max_xfer_len);
 
 		rc = spi_sync_transfer(spi, xfers, 2);
 		if (rc < 0) {

drivers/net/dsa/sja1105/sja1105_static_config.h

@@ -9,6 +9,8 @@
 #include <linux/types.h>
 #include <asm/types.h>
 
+#define SJA1105_SIZE_SPI_MSG_HEADER			4
+#define SJA1105_SIZE_SPI_MSG_MAXLEN			(64 * 4)
 #define SJA1105_SIZE_DEVICE_ID				4
 #define SJA1105_SIZE_TABLE_HEADER			12
 #define SJA1105_SIZE_SCHEDULE_ENTRY			8