spi: dw: Add a number of native CS auto-detection

Aside with the FIFO depth and DFS field size it's possible to auto-detect
a number of native chip-select synthesized in the DW APB/AHB SSI IP-core.
It can be done just by writing ones to the SER register. The number of
writable flags in the register is limited by the SSI_NUM_SLAVES IP-core
synthesize parameter. All the upper flags are read-only and wired to zero.
Based on that let's add the number of native CS auto-detection procedure
so the low-level platform drivers wouldn't need to manually set it up
unless it's required to set a constraint due to platform-specific reasons
(for instance, due to a hardware bug).
Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://lore.kernel.org/r/20240424150657.9678-3-fancer.lancer@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-dw-core.c

@@ -834,6 +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