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From:   Fabio Estevam <festevam@gmail.com>
Date:   Fri, 4 Sep 2020 09:35:38 -0300
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Subject: Re: [PATCH] spi-imx: remove num-cs support, set num_chipselect to 4
To:     Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Cc:     Mark Brown <broonie@kernel.org>, Shawn Guo <shawnguo@kernel.org>,
        Sascha Hauer <s.hauer@pengutronix.de>,
        Pengutronix Kernel Team <kernel@pengutronix.de>,
        NXP Linux Team <linux-imx@nxp.com>,
        linux-spi <linux-spi@vger.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@lists.infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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Hi Matthias,

On Thu, Sep 3, 2020 at 11:40 AM Matthias Schiffer
<matthias.schiffer@ew.tq-group.com> wrote:
>
> The num-cs property is not considered useful, and no in-tree Device
> Trees define it for spi-imx.
>
> The default value to be used when no cs-gpios are defined is set to 4 to
> give access to all native CS pins of modern i.MX SoCs (i.MX6 and newer).
>
> In older SoCs, the number of CS pins varies (for example the i.MX27 has 3
> CS pins on CSPI1 and CSPI2, and only a single CS on CSPI3). Attempting
> to use the nonexisting CS pin would be an easy to notice DT
> misconfiguration; making the driver catch this doesn't seem worthwhile.
>
> Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
> ---
>  drivers/spi/spi-imx.c | 13 +------------
>  1 file changed, 1 insertion(+), 12 deletions(-)
>
> diff --git a/drivers/spi/spi-imx.c b/drivers/spi/spi-imx.c
> index 197f60632072..aece8482739b 100644
> --- a/drivers/spi/spi-imx.c
> +++ b/drivers/spi/spi-imx.c
> @@ -1581,7 +1581,6 @@ static int spi_imx_probe(struct platform_device *pdev)
>         const struct spi_imx_devtype_data *devtype_data = of_id ? of_id->data :
>                 (struct spi_imx_devtype_data *)pdev->id_entry->driver_data;
>         bool slave_mode;
> -       u32 val;
>
>         slave_mode = devtype_data->has_slavemode &&
>                         of_property_read_bool(np, "spi-slave");
> @@ -1605,6 +1604,7 @@ static int spi_imx_probe(struct platform_device *pdev)
>         master->bits_per_word_mask = SPI_BPW_RANGE_MASK(1, 32);
>         master->bus_num = np ? -1 : pdev->id;
>         master->use_gpio_descriptors = true;
> +       master->num_chipselect = 4;

On an imx6q-sabresd, which only has one SPI chip-select via GPIO, this
makes the SPI core to understand that it has 4 chip selects.

From spi_get_gpio_descs() in drivers/spi/spi.c:

ctlr->num_chipselect = max_t(int, nb, ctlr->num_chipselect);

It is 4 now after your patch, it was 3 after 8cdcd8aeee28 ("spi:
imx/fsl-lpspi: Convert to GPIO descriptors") and 1 before such commit.

Couldn't we just remove master->num_chipselect from the spi-imx.c driver?