[PATCH 03/32] spi: dw: Fix driving MOSI low while recieving

[Sean Anderson]

On 11/7/20 3:13 AM, Damien Le Moal wrote:
> Fix for the Synopsis DesignWare SPI mmio driver taken from the work
> by Sean Anderson for the U-Boot project. Sean comments:
> 
> The resting state of MOSI is high when nothing is driving it. If we
> drive it low while recieving, it looks like we are transmitting 0x00
> instead of transmitting nothing. This can confuse slaves (like SD cards)
> which allow new commands to be sent over MOSI while they are returning
> data over MISO. The return of MOSI from 0 to 1 at the end of recieving
> a byte can look like a start bit and a transmission bit to an SD card.
> This will cause the card to become out-of-sync with the SPI device, as
> it thinks the device has already started transmitting two bytes of a new
> command. The mmc-spi driver will not detect the R1 response from the SD
> card, since it is sent too early, and offset by two bits. This patch
> fixes transfer errors when using SD cards with dw spi.
> 
> Signed-off-by: Sean Anderson <seanga2 at gmail.com>
> Signed-off-by: Damien Le Moal <damien.lemoal at wdc.com>
> ---
>  drivers/spi/spi-dw-core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/spi/spi-dw-core.c b/drivers/spi/spi-dw-core.c
> index 841c85247f01..c2ef1d8d46d5 100644
> --- a/drivers/spi/spi-dw-core.c
> +++ b/drivers/spi/spi-dw-core.c
> @@ -137,7 +137,7 @@ static inline u32 rx_max(struct dw_spi *dws)
>  static void dw_writer(struct dw_spi *dws)
>  {
>  	u32 max = tx_max(dws);
> -	u16 txw = 0;
> +	u16 txw = 0xffff;
>  
>  	while (max--) {
>  		if (dws->tx) {
> 

Reviewed-by: Sean Anderson <seanga2 at gmail.com>