Linux/AM3352: McSPI chip-select issue

Configure all SPI chip-selects to active-low in the Linux SPI driver for
Sitara.

When the SPI peripheral is reset, all SPI chip-selects on the bus become
configured as active-high. When a SPI device is first accessed, its specific
chip-select polarity will be properly configured, but the other chip-selects
on the same bus may not have been configured yet. If these other chip-selects
are actually active-low, the SPI access can cause multiple active-low
chip-select signals to be asserted.

To eliminate this issue, configure _all_ chip-selects on the bus as active-low
when the SPI peripheral is configured as a master. The limitation is that no
active-high chip-slects can be supported - _all_ SPI devices must use the
active-low chip-select polarity.

NOTE: This change requires that _all_ devices on the SPI bus _must_ use
active-low chip-select polarity. Active-high chip-selects are no longer
supported.


--- a/drivers/spi/spi-omap2-mcspi.c	2016-01-06 09:52:40.742535425 -0800
+++ b/drivers/spi/spi-omap2-mcspi.c	2016-01-07 16:19:24.148534128 -0800
@@ -257,6 +257,8 @@ static void omap2_mcspi_set_master_mode(
 	struct omap2_mcspi	*mcspi = spi_master_get_devdata(master);
 	struct omap2_mcspi_regs	*ctx = &mcspi->ctx;
 	u32 l;
+	int i;
+	u32 chconf;
 
 	/*
 	 * Setup when switching from (reset default) slave mode
@@ -268,6 +270,19 @@ static void omap2_mcspi_set_master_mode(
 	mcspi_write_reg(master, OMAP2_MCSPI_MODULCTRL, l);
 
 	ctx->modulctrl = l;
+
+	/*
+	 * Configure all chip-selects on this bus to active-low
+	 *
+	 * NOTE: This requires that _all_ devices on the SPI bus _must_ use
+	 * active-low chip-selects. Active-high chip-selects are not supported!
+	 */
+	for (i = 0; i < master->num_chipselect; i++)
+	{
+		chconf = mcspi_read_reg(master, OMAP2_MCSPI_CHCONF0 + (i * 0x14));
+		chconf |= OMAP2_MCSPI_CHCONF_EPOL;
+		mcspi_write_reg(master, OMAP2_MCSPI_CHCONF0 + (i * 0x14), chconf);
+	}
 }
 
 static void omap2_mcspi_set_fifo(const struct spi_device *spi,
@@ -976,7 +991,8 @@ static int omap2_mcspi_setup(struct spi_
 		cs->base = mcspi->base + spi->chip_select * 0x14;
 		cs->phys = mcspi->phys + spi->chip_select * 0x14;
 		cs->mode = 0;
-		cs->chconf0 = 0;
+		/* Configure chip-select as active-low */
+		cs->chconf0 = OMAP2_MCSPI_CHCONF_EPOL;
 		spi->controller_state = cs;
 		/* Link this to context save list */
 		list_add_tail(&cs->node, &ctx->cs);