spi: designware_spi: Fix detecting FIFO depth

Current code tries to find the highest valid fifo depth by checking the value
it wrote to DW_SPI_TXFLTR. There are a few problems in current code:
1) There is an off-by-one in dws->fifo_len setting because it assumes the latest
   register write fails so the latest valid value should be fifo - 1.
2) We know the depth could be from 2 to 256 from HW spec, so it is not necessary
   to test fifo == 257. In the case fifo is 257, it means the latest valid
   setting is fifo = 256. So after the for loop iteration, we should check
   fifo == 2 case instead of fifo == 257 if detecting the FIFO depth fails.
This patch fixes above issues.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Stefan Roese <sr@denx.de>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>

diff --git a/drivers/spi/designware_spi.c b/drivers/spi/designware_spi.c
index 98c9f036f2e4f58c4f23831e2b33d18d3670be3e..0210667854569d438233802b07c0f289f8b39a51 100644 (file)
--- a/drivers/spi/designware_spi.c
+++ b/drivers/spi/designware_spi.c
@@ -164,13 +164,13 @@ static void spi_hw_init(struct dw_spi_priv *priv)
        if (!priv->fifo_len) {
                u32 fifo;
 
-               for (fifo = 2; fifo <= 257; fifo++) {
+               for (fifo = 2; fifo <= 256; fifo++) {
                        dw_writew(priv, DW_SPI_TXFLTR, fifo);
                        if (fifo != dw_readw(priv, DW_SPI_TXFLTR))
                                break;
                }
 
-               priv->fifo_len = (fifo == 257) ? 0 : fifo;
+               priv->fifo_len = (fifo == 2) ? 0 : fifo - 1;
                dw_writew(priv, DW_SPI_TXFLTR, 0);
        }
        debug("%s: fifo_len=%d\n", __func__, priv->fifo_len);