When GPIO chip-select is used nothing prevents any available SPI
controllers to work with both CS-high and traditional CS-low modes.
In fact the SPI bus core code already does it, so we don't need to
introduce any modification there. But spi_setup() still fails to
switch the interface settings if CS-high flag is set for the case
of GPIO-driven slave chip-select when the SPI controller doesn't
support the hardwired CS-inversion. Lets fix it by clearing the
SPI_CS_HIGH flag out from bad_bits (unsupported by controller) when
client chip is selected by GPIO.
This feature is useful for slave devices, which in accordance with
communication protocol can work with both active-high and active-low
chip-selects. I am aware of one such device. It is MMC-SPI interface,
when at init sequence the driver needs to perform a read operation with
low and high chip-select sequentially (requirement of 74 clock cycles
with both chipselect, see the mmc_spi driver for details).
Signed-off-by: Serge Semin <fancer.lan...@gmail.com>
---
drivers/spi/spi.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 93986f879b09..49808892ef35 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -2943,6 +2943,11 @@ int spi_setup(struct spi_device *spi)
* so it is ignored here.
*/
bad_bits = spi->mode & ~(spi->controller->mode_bits | SPI_CS_WORD);
+ /* nothing prevents from working with active-high CS in case if it
+ * is driven by GPIO.
+ */
+ if (gpio_is_valid(spi->cs_gpio))
+ bad_bits &= ~SPI_CS_HIGH;
ugly_bits = bad_bits &
(SPI_TX_DUAL | SPI_TX_QUAD | SPI_TX_OCTAL |
SPI_RX_DUAL | SPI_RX_QUAD | SPI_RX_OCTAL);
--
2.21.0
