[PATCH v2] spi: tegra20-slink: change chip select action order

[PATCH v2] spi: tegra20-slink: change chip select action order

[Randolph Maaßen]

To transfer via SPI the tegra20-slink driver first sets the command
register, which contains the chip select value, and after that the
command2 register, which contains the chip select line. This leads to a
small spike in the chip selct 0 line between the set of the value and
the selection of the chip select line.

This commit changes the order of the register writes so that first the
chip select line is chosen and then the value is set, removing the
spike.

Signed-off-by: Randolph Maaßen <gaireg@gaireg.de>
Reviewed-by: Sowjanya Komatineni <skomatineni@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
---
v2: fixed typo in comment: bevore -> before
---
 drivers/spi/spi-tegra20-slink.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/spi/spi-tegra20-slink.c b/drivers/spi/spi-tegra20-slink.c
index 1427f343b39a..2d173fadeb19 100644
--- a/drivers/spi/spi-tegra20-slink.c
+++ b/drivers/spi/spi-tegra20-slink.c
@@ -717,9 +717,6 @@ static int tegra_slink_start_transfer_one(struct spi_device *spi,
 	command2 = tspi->command2_reg;
 	command2 &= ~(SLINK_RXEN | SLINK_TXEN);
 
-	tegra_slink_writel(tspi, command, SLINK_COMMAND);
-	tspi->command_reg = command;
-
 	tspi->cur_direction = 0;
 	if (t->rx_buf) {
 		command2 |= SLINK_RXEN;
@@ -729,9 +726,18 @@ static int tegra_slink_start_transfer_one(struct spi_device *spi,
 		command2 |= SLINK_TXEN;
 		tspi->cur_direction |= DATA_DIR_TX;
 	}
+
+	/*
+	 * Writing to the command2 register before the command register prevents
+	 * a spike in chip_select line 0. This selects the chip_select line
+	 * before changing the chip_select value.
+	 */
 	tegra_slink_writel(tspi, command2, SLINK_COMMAND2);
 	tspi->command2_reg = command2;
 
+	tegra_slink_writel(tspi, command, SLINK_COMMAND);
+	tspi->command_reg = command;
+
 	if (total_fifo_words > SLINK_FIFO_DEPTH)
 		ret = tegra_slink_start_dma_based_transfer(tspi, t);
 	else
-- 
2.11.0

Re: [PATCH v2] spi: tegra20-slink: change chip select action order

[Mark Brown]

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On Fri, Mar 29, 2019 at 06:44:11PM +0100, Randolph Maaßen wrote:
> To transfer via SPI the tegra20-slink driver first sets the command
> register, which contains the chip select value, and after that the
> command2 register, which contains the chip select line. This leads to a
> small spike in the chip selct 0 line between the set of the value and
> the selection of the chip select line.

Please do not submit new versions of already applied patches, please
submit incremental updates to the existing code.  Modifying existing
commits creates problems for other users building on top of those
commits so it's best practice to only change pubished git commits if
absolutely essential.

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