linux-kernel - Re: [PATCH 1/3] spi: dw: select SS0 when gpio cs is used

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Date:   Fri, 9 Dec 2022 14:46:25 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Edmund Berenson <edmund.berenson@...ix.com>
Cc:     Lukasz Zemla <Lukasz.Zemla@...dward.com>,
        Mark Brown <broonie@...nel.org>, linux-spi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] spi: dw: select SS0 when gpio cs is used

Hello Edmund

On Fri, Dec 02, 2022 at 10:48:59AM +0100, Edmund Berenson wrote:
> SER register contains only 4-bit bit-field for enabling 4 SPI chip selects.
> If gpio cs are used the cs number may be >= 4. To ensure we do not write
> outside of the valid area, we choose SS0 in case of gpio cs to start
> spi transfer.

Next time please don't forget to add me to the whole series Cc-list. I
am missing the patch #2 in my inbox.

> 
> Co-developed-by: Lukasz Zemla <Lukasz.Zemla@...dward.com>
> Signed-off-by: Lukasz Zemla <Lukasz.Zemla@...dward.com>
> Signed-off-by: Edmund Berenson <edmund.berenson@...ix.com>
> ---
>  drivers/spi/spi-dw-core.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/spi/spi-dw-core.c b/drivers/spi/spi-dw-core.c
> index 99edddf9958b..57c9e384d6d4 100644
> --- a/drivers/spi/spi-dw-core.c
> +++ b/drivers/spi/spi-dw-core.c
> @@ -94,6 +94,10 @@ void dw_spi_set_cs(struct spi_device *spi, bool enable)
>  {
>  	struct dw_spi *dws = spi_controller_get_devdata(spi->controller);
>  	bool cs_high = !!(spi->mode & SPI_CS_HIGH);
> +	u8 enable_cs = 0;
> +
> +	if (!spi->cs_gpiod)
> +		enable_cs = spi->chip_select;
>  
>  	/*
>  	 * DW SPI controller demands any native CS being set in order to
> @@ -103,7 +107,7 @@ void dw_spi_set_cs(struct spi_device *spi, bool enable)
>  	 * support active-high or active-low CS level.
>  	 */
>  	if (cs_high == enable)

> -		dw_writel(dws, DW_SPI_SER, BIT(spi->chip_select));
> +		dw_writel(dws, DW_SPI_SER, BIT(enable_cs));

No, it's not that easy. By applying this patch we'll get a regression
for the platforms which make use of both the GPIO-based and native
chip-selects. Consider the next platform setup:
 +--------------+
 | SoC X        |
 |              |    +-------------+
 |   DW SSI CS0-+----+ SPI-slave 0 |
 |              |    +-------------+
 |              |    +-------------+
 |        GPIOn-+----+ SPI-slave 1 |
 |              |    +-------------+
 +--------------+

If we apply this patch then the communications targeted to the
SPI-slave 1 will also reach the SPI-slave 0 device, which isn't what
we'd want.

That's why currently the DW SSI driver activates the native CS line
with the corresponding ID irrespective whether it is a GPIO-based
CS or a native one.

-Serge(y)

>  	else
>  		dw_writel(dws, DW_SPI_SER, 0);
>  }
> -- 
> 2.37.4
>