netdev - RE: [PATCH net] net: dsa: microchip: Select SPI

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Message-ID:
 <DM3PR11MB87367B6B13B1497C5994884BEC0EA@DM3PR11MB8736.namprd11.prod.outlook.com>
Date: Wed, 10 Sep 2025 22:10:01 +0000
From: <Tristram.Ha@...rochip.com>
To: <bastien.curutchet@...tlin.com>
CC: <thomas.petazzoni@...tlin.com>, <miquel.raynal@...tlin.com>,
	<Woojung.Huh@...rochip.com>, <pascal.eberhard@...com>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<UNGLinuxDriver@...rochip.com>, <andrew@...n.ch>, <olteanv@...il.com>,
	<kuba@...nel.org>, <davem@...emloft.net>, <edumazet@...gle.com>,
	<pabeni@...hat.com>
Subject: RE: [PATCH net] net: dsa: microchip: Select SPI_MODE 0 for KSZ8463

> KSZ8463 expects the SPI clock to be low on idle and samples data on
> rising edges. This fits SPI mode 0 (CPOL = 0 / CPHA = 0) but the SPI
> mode is set to 3 for all the switches supported by the driver. This
> can lead to invalid read/write on the SPI bus.
> 
> Set SPI mode to 0 for the KSZ8463.
> Leave SPI mode 3 as default for the other switches.
> 
> Signed-off-by: Bastien Curutchet (Schneider Electric)
> <bastien.curutchet@...tlin.com>
> Fixes: 84c47bfc5b3b ("net: dsa: microchip: Add KSZ8463 switch support to KSZ DSA
> driver")
> ---
>  drivers/net/dsa/microchip/ksz_spi.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/dsa/microchip/ksz_spi.c
> b/drivers/net/dsa/microchip/ksz_spi.c
> index
> d8001734b05741446fa78a1e88c2f82e894835ce..dcc0dbddf7b9d70fbfb31d4b260b80
> ca78a65975 100644
> --- a/drivers/net/dsa/microchip/ksz_spi.c
> +++ b/drivers/net/dsa/microchip/ksz_spi.c
> @@ -139,6 +139,7 @@ static int ksz_spi_probe(struct spi_device *spi)
>         const struct regmap_config *regmap_config;
>         const struct ksz_chip_data *chip;
>         struct device *ddev = &spi->dev;
> +       u32 spi_mode = SPI_MODE_3;
>         struct regmap_config rc;
>         struct ksz_device *dev;
>         int i, ret = 0;
> @@ -155,8 +156,10 @@ static int ksz_spi_probe(struct spi_device *spi)
>         dev->chip_id = chip->chip_id;
>         if (chip->chip_id == KSZ88X3_CHIP_ID)
>                 regmap_config = ksz8863_regmap_config;
> -       else if (chip->chip_id == KSZ8463_CHIP_ID)
> +       else if (chip->chip_id == KSZ8463_CHIP_ID) {
>                 regmap_config = ksz8463_regmap_config;
> +               spi_mode = SPI_MODE_0;
> +       }
>         else if (chip->chip_id == KSZ8795_CHIP_ID ||
>                  chip->chip_id == KSZ8794_CHIP_ID ||
>                  chip->chip_id == KSZ8765_CHIP_ID)
> @@ -185,7 +188,7 @@ static int ksz_spi_probe(struct spi_device *spi)
>                 dev->pdata = spi->dev.platform_data;
> 
>         /* setup spi */
> -       spi->mode = SPI_MODE_3;
> +       spi->mode = spi_mode;
>         ret = spi_setup(spi);
>         if (ret)
>                 return ret;
> 
> ---
> base-commit: c65e2aee8971eb9d4bc2b8edc3a3a62dc98f0410
> change-id: 20250910-fix-omap-spi-d7c64f2416df

Actually it is best to completely remove the code.  The SPI mode should
be dictated by spi-cpol and spi-cpha settings in the device tree.  I do
not know why that code was there from the beginning.

All KSZ switches can use SPI mode 0 and 3, and 3 is recommended for high
SPI frequency.  Sometimes a bug/quirk in the SPI bus driver prevents the
very first SPI transfer to be successful in mode 3 because of a missed
rising edge clock signal, so it is forced to use mode 0.  (The Atmel SPI
bus driver has this issue in some old kernel versions.)

As for KSZ8463 I have always used mode 3 and do not know of any issue of
using that mode.