[PATCH 2/2] spi: meson-spicc: Use pinctrl to drive CLK line when idle

[Neil Armstrong]

On 10/08/2022 10:52, Jerome Brunet wrote:
> 
> On Tue 09 Aug 2022 at 19:20, Amjad Ouled-Ameur <aouledameur at baylibre.com> wrote:
> 
>> Between SPI transactions, all SPI pins are in HiZ state. When using the SS
>> signal from the SPICC controller it's not an issue because when the
>> transaction resumes all pins come back to the right state at the same time
>> as SS.
>>
>> The problem is when we use CS as a GPIO. In fact, between the GPIO CS
>> state change and SPI pins state change from idle, you can have a missing or
>> spurious clock transition.
>>
>> Set a bias on the clock depending on the clock polarity requested before CS
>> goes active, by passing a special "idle-low" and "idle-high" pinctrl state
>> and setting the right state at a start of a message
>>
>> Reported-by: Da Xue <da at libre.computer>
>> Signed-off-by: Neil Armstrong <narmstrong at baylibre.com>
>> Signed-off-by: Amjad Ouled-Ameur <aouledameur at baylibre.com>
>> ---
>>   arch/arm64/boot/dts/amlogic/meson-gxl.dtsi | 14 ++++++++
>>   drivers/spi/spi-meson-spicc.c              | 39 +++++++++++++++++++++-
> 
> These 2 changes should not be in the same patch.
> 
>>   2 files changed, 52 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/boot/dts/amlogic/meson-gxl.dtsi b/arch/arm64/boot/dts/amlogic/meson-gxl.dtsi
>> index c3ac531c4f84..04e9d0f1bde0 100644
>> --- a/arch/arm64/boot/dts/amlogic/meson-gxl.dtsi
>> +++ b/arch/arm64/boot/dts/amlogic/meson-gxl.dtsi
> 
> Does the problem applies only the gxl ? not gxbb, g12, axg ?

Only on GXL, starting from AXG the pins mode output state can be kept between bursts.

> 
>> @@ -429,6 +429,20 @@ mux {
>>   			};
>>   		};
>>   
>> +		spi_idle_high_pins: spi-idle-high-pins {
>> +			mux {
>> +				groups = "spi_sclk";
>> +				bias-pull-up;
>> +			};
>> +		};
>> +
>> +		spi_idle_low_pins: spi-idle-low-pins {
>> +			mux {
>> +				groups = "spi_sclk";
>> +				bias-pull-down;
> 
> Would it be safer to properly drive the pin in push-pull mode ?
> Like using gpio pinumux mode and output-high/output-low pinconf ?

The pins mux must be kept in the SPI function, thus only a bias can be applied.

> 
>> +			};
>> +		};
>> +
>>   		spi_ss0_pins: spi-ss0 {
>>   			mux {
>>   				groups = "spi_ss0";
>> diff --git a/drivers/spi/spi-meson-spicc.c b/drivers/spi/spi-meson-spicc.c
>> index 0bc7daa7afc8..d42171ee1d61 100644
>> --- a/drivers/spi/spi-meson-spicc.c
>> +++ b/drivers/spi/spi-meson-spicc.c
>> @@ -21,6 +21,7 @@
>>   #include <linux/types.h>
>>   #include <linux/interrupt.h>
>>   #include <linux/reset.h>
>> +#include <linux/pinctrl/consumer.h>
>>   
>>   /*
>>    * The Meson SPICC controller could support DMA based transfers, but is not
>> @@ -166,14 +167,31 @@ struct meson_spicc_device {
>>   	unsigned long			tx_remain;
>>   	unsigned long			rx_remain;
>>   	unsigned long			xfer_remain;
>> +	struct pinctrl			*pinctrl;
>> +	struct pinctrl_state		*pins_idle_high;
>> +	struct pinctrl_state		*pins_idle_low;
>>   };
>>   
>>   static void meson_spicc_oen_enable(struct meson_spicc_device *spicc)
>>   {
>>   	u32 conf;
>>   
>> -	if (!spicc->data->has_oen)
>> +	if (!spicc->data->has_oen) {
>> +		/* Try to get pinctrl states for idle high/low */
>> +		spicc->pins_idle_high = pinctrl_lookup_state(spicc->pinctrl,
>> +							     "idle-high");
>> +		if (IS_ERR(spicc->pins_idle_high)) {
>> +			dev_warn(&spicc->pdev->dev, "can't get idle-high pinctrl\n");
>> +			spicc->pins_idle_high = NULL;
>> +		}
>> +		spicc->pins_idle_low = pinctrl_lookup_state(spicc->pinctrl,
>> +							     "idle-low");
>> +		if (IS_ERR(spicc->pins_idle_low)) {
>> +			dev_warn(&spicc->pdev->dev, "can't get idle-low pinctrl\n");
>> +			spicc->pins_idle_low = NULL;
>> +		}
>>   		return;
>> +	}
>>   
>>   	conf = readl_relaxed(spicc->base + SPICC_ENH_CTL0) |
>>   		SPICC_ENH_MOSI_OEN | SPICC_ENH_CLK_OEN | SPICC_ENH_CS_OEN;
>> @@ -438,6 +456,16 @@ static int meson_spicc_prepare_message(struct spi_master *master,
>>   	else
>>   		conf &= ~SPICC_POL;
>>   
>> +	if (!spicc->data->has_oen) {
>> +		if (spi->mode & SPI_CPOL) {
>> +			if (spicc->pins_idle_high)
>> +				pinctrl_select_state(spicc->pinctrl, spicc->pins_idle_high);
>> +		} else {
>> +			if (spicc->pins_idle_low)
>> +				pinctrl_select_state(spicc->pinctrl, spicc->pins_idle_low);
>> +		}
>> +	}
>> +
>>   	if (spi->mode & SPI_CPHA)
>>   		conf |= SPICC_PHA;
>>   	else
>> @@ -482,6 +510,9 @@ static int meson_spicc_unprepare_transfer(struct spi_master *master)
>>   
>>   	device_reset_optional(&spicc->pdev->dev);
>>   
>> +	if (!spicc->data->has_oen)
>> +		pinctrl_select_default_state(&spicc->pdev->dev);
>> +
>>   	return 0;
>>   }
>>   
>> @@ -733,6 +764,12 @@ static int meson_spicc_probe(struct platform_device *pdev)
>>   		goto out_core_clk;
>>   	}
>>   
>> +	spicc->pinctrl = devm_pinctrl_get(&pdev->dev);
>> +	if (IS_ERR(spicc->pinctrl)) {
>> +		ret = PTR_ERR(spicc->pinctrl);
>> +		goto out_clk;
>> +	}
>> +
>>   	device_reset_optional(&pdev->dev);
>>   
>>   	master->num_chipselect = 4;
>