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Date:   Tue, 26 Mar 2019 19:11:23 +0000
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Thierry Reding <thierry.reding@...il.com>,
        Randolph Maaßen <gaireg@...reg.de>
CC:     Laxman Dewangan <ldewangan@...dia.com>,
        Mark Brown <broonie@...nel.org>,
        Jonathan Hunter <jonathanh@...dia.com>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] spi: tegra20-slink: change chip select action order

> On Tue, Mar 26, 2019 at 03:30:50PM +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.
> > 
> > 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@...reg.de>

Very minor typo in the comment below:

Fix looks good as chip select need to be selected prior to asserting during
start of transfer.

Reviewed-by: Sowjanya Komatineni <skomatineni@...dia.com>

> > ---
> >  drivers/spi/spi-tegra20-slink.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
>
> Looks good to me. Adding Sowjanya who has been looking into SPI recently.
>
> Thierry
>
> > diff --git a/drivers/spi/spi-tegra20-slink.c 
> > b/drivers/spi/spi-tegra20-slink.c index 1427f343b39a..6d4679126213 
> > 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 bevore the command register prevents
> > +	 * a spike in chip_select line 0. This selects the chip_select line
> > +	 * before changing the chip_select value.
> > +	 */

Type "before"

> >  	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
> > 

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