spi: dw: Auto-detect number of native CS

Merge series from Serge Semin <fancer.lancer@gmail.com>:

The main goal of the short series is to provide a procedure implementing
the auto-detection of the number of native Chip-Select signals supported
by the controller. The suggested algorithm is straightforward. It relies
on the fact that the SER register writable flags reflects the actual
number of available native chip-select signals. So the DW APB/AHB SSI
driver now tests the SER register for having the writable bits,
calculates the number of CS signals based on the number of set flags and
then initializes the num_cs private data field based on that, which then
will be passed to the SPI-core subsystem indicating the number of
supported hardware chip-selects. The implemented procedure will be useful
for the DW SSI device nodes not having the explicitly set "num-cs"
property. In case if the property is specified it will be utilized instead
of the auto-detection procedure.

Besides of that a small cleanup patch is introduced in the head of the
series. It converts the driver to using the BITS_TO_BYTES() macro instead
of the hard-coded DIV_ROUND_UP()-based calculation of the number of
bytes-per-transfer-word.

drivers/spi/spi-dw-core.c

@@ -6,6 +6,7 @@
  */
 
 #include <linux/bitfield.h>
+#include <linux/bitops.h>
 #include <linux/dma-mapping.h>
 #include <linux/interrupt.h>
 #include <linux/module.h>
@@ -421,10 +422,7 @@ static int dw_spi_transfer_one(struct spi_controller *host,
 	int ret;
 
 	dws->dma_mapped = 0;
-	dws->n_bytes =
-		roundup_pow_of_two(DIV_ROUND_UP(transfer->bits_per_word,
-						BITS_PER_BYTE));
-
+	dws->n_bytes = roundup_pow_of_two(BITS_TO_BYTES(transfer->bits_per_word));
 	dws->tx = (void *)transfer->tx_buf;
 	dws->tx_len = transfer->len / dws->n_bytes;
 	dws->rx = transfer->rx_buf;
@@ -836,6 +834,20 @@ static void dw_spi_hw_init(struct device *dev, struct dw_spi *dws)
 			DW_SPI_GET_BYTE(dws->ver, 1));
 	}
 
+	/*
+	 * Try to detect the number of native chip-selects if the platform
+	 * driver didn't set it up. There can be up to 16 lines configured.
+	 */
+	if (!dws->num_cs) {
+		u32 ser;
+
+		dw_writel(dws, DW_SPI_SER, 0xffff);
+		ser = dw_readl(dws, DW_SPI_SER);
+		dw_writel(dws, DW_SPI_SER, 0);
+
+		dws->num_cs = hweight16(ser);
+	}
+
 	/*
 	 * Try to detect the FIFO depth if not set by interface driver,
 	 * the depth could be from 2 to 256 from HW spec

drivers/spi/spi-dw-mmio.c

@@ -320,7 +320,6 @@ static int dw_spi_mmio_probe(struct platform_device *pdev)
 	struct resource *mem;
 	struct dw_spi *dws;
 	int ret;
-	int num_cs;
 
 	dwsmmio = devm_kzalloc(&pdev->dev, sizeof(struct dw_spi_mmio),
 			GFP_KERNEL);
@@ -364,11 +363,8 @@ static int dw_spi_mmio_probe(struct platform_device *pdev)
 				     &dws->reg_io_width))
 		dws->reg_io_width = 4;
 
-	num_cs = 4;
-
-	device_property_read_u32(&pdev->dev, "num-cs", &num_cs);
-
-	dws->num_cs = num_cs;
+	/* Rely on the auto-detection if no property specified */
+	device_property_read_u32(&pdev->dev, "num-cs", &dws->num_cs);
 
 	init_func = device_get_match_data(&pdev->dev);
 	if (init_func) {

drivers/spi/spi-dw.h

@@ -164,8 +164,8 @@ struct dw_spi {
 	u32			max_freq;	/* max bus freq supported */
 
 	u32			reg_io_width;	/* DR I/O width in bytes */
+	u32			num_cs;		/* chip select lines */
 	u16			bus_num;
-	u16			num_cs;		/* supported slave numbers */
 	void (*set_cs)(struct spi_device *spi, bool enable);
 
 	/* Current message transfer state info */