[PATCH v2 1/1] spi: dw: revisit FIFO size detection again

[PATCH v2 1/1] spi: dw: revisit FIFO size detection again

[Andy Shevchenko]

The commit d297933cc7fc (spi: dw: Fix detecting FIFO depth) tries to fix the
logic of the FIFO detection based on the description on the comments. However,
there is a slight difference between numbers in TX Level and TX FIFO size.

So, by specification the FIFO size would be in a range 2-256 bytes. From TX
Level prospective it means we can set threshold in the range 0-(FIFO size - 1)
bytes. Hence there are currently two issues:
  a) FIFO size 2 bytes is actually skipped since TX Level is 1 bit and could be
     either 0 or 1 byte;
  b) FIFO size is incorrectly decreased by 1 which already done by meaning of
     TX Level register.

This patch fixes it eventually right.

Fixes: d297933cc7fc (spi: dw: Fix detecting FIFO depth)
Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Reviewed-by: Axel Lin <axel.lin-8E1dMatC8ynQT0dZR+AlfA@public.gmane.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
---
Changelog v2:
- address Alex' comment
 drivers/spi/spi-dw.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/spi/spi-dw.c b/drivers/spi/spi-dw.c
index 0257dd1..bf3b7bc 100644
--- a/drivers/spi/spi-dw.c
+++ b/drivers/spi/spi-dw.c
@@ -494,14 +494,14 @@ static void spi_hw_init(struct device *dev, struct dw_spi *dws)
 	if (!dws->fifo_len) {
 		u32 fifo;
 
-		for (fifo = 2; fifo <= 256; fifo++) {
+		for (fifo = 1; fifo < 256; fifo++) {
 			dw_writew(dws, DW_SPI_TXFLTR, fifo);
 			if (fifo != dw_readw(dws, DW_SPI_TXFLTR))
 				break;
 		}
 		dw_writew(dws, DW_SPI_TXFLTR, 0);
 
-		dws->fifo_len = (fifo == 2) ? 0 : fifo - 1;
+		dws->fifo_len = (fifo == 1) ? 0 : fifo;
 		dev_dbg(dev, "Detected FIFO size: %u bytes\n", dws->fifo_len);
 	}
 }
-- 
2.1.4

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Re: [PATCH v2 1/1] spi: dw: revisit FIFO size detection again

[Mark Brown]

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On Wed, Feb 25, 2015 at 11:39:36AM +0200, Andy Shevchenko wrote:
> The commit d297933cc7fc (spi: dw: Fix detecting FIFO depth) tries to fix the
> logic of the FIFO detection based on the description on the comments. However,
> there is a slight difference between numbers in TX Level and TX FIFO size.

Applied, thanks.

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