spi: Add Flag to Enable Slave Select with GPIO Chip Select.

Some SPI masters require slave selection before the transfer
can begin [1]. The SPI framework currently selects the chip using
either 1) the internal CS mechanism or 2) the GPIO CS, but not both.

This patch adds a new master->flags define to indicate both the GPIO
CS and the internal chip select mechanism should be used.

Tested On:
    Altera CycloneV development kit
    Compile tested for build errors on x86_64 (allyesconfigs)

[1] DesignWare dw_apb_ssi Databook, Version 3.20a (page 39)

Signed-off-by: Thor Thayer <tthayer@opensource.altera.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

2 files changed, 8 insertions, 2 deletions

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 8146ccd35a1a..8708da7c8140 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -697,10 +697,15 @@ static void spi_set_cs(struct spi_device *spi, bool enable)
         if (spi->mode & SPI_CS_HIGH)
                 enable = !enable;
 
-        if (gpio_is_valid(spi->cs_gpio))
+        if (gpio_is_valid(spi->cs_gpio)) {
                 gpio_set_value(spi->cs_gpio, !enable);
-        else if (spi->master->set_cs)
+                /* Some SPI masters need both GPIO CS & slave_select */
+                if ((spi->master->flags & SPI_MASTER_GPIO_SS) &&
+                    spi->master->set_cs)
+                        spi->master->set_cs(spi, !enable);
+        } else if (spi->master->set_cs) {
                 spi->master->set_cs(spi, !enable);
+        }
 }
 
 #ifdef CONFIG_HAS_DMA
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 4b743ac35396..75c6bd0ac605 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -442,6 +442,7 @@ struct spi_master {
 #define SPI_MASTER_NO_TX        BIT(2)          /* can't do buffer write */
 #define SPI_MASTER_MUST_RX      BIT(3)          /* requires rx */
 #define SPI_MASTER_MUST_TX      BIT(4)          /* requires tx */
+#define SPI_MASTER_GPIO_SS      BIT(5)          /* GPIO CS must select slave */
 
         /*
          * on some hardware transfer / message size may be constrained