linux-kernel - Re: [PATCH v5 1/7] spi: Enable controllers to extend the SPI protocol with MOSI idle configuration

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Message-ID: <1d2cde40-ad55-4136-bc72-3d71515f7023@baylibre.com>
Date: Wed, 26 Jun 2024 09:57:32 -0500
From: David Lechner <dlechner@...libre.com>
To: Marcelo Schmitt <marcelo.schmitt@...log.com>, broonie@...nel.org,
 lars@...afoo.de, Michael.Hennerich@...log.com, jic23@...nel.org,
 robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org,
 nuno.sa@...log.com, corbet@....net, marcelo.schmitt1@...il.com
Cc: linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
 linux-spi@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/7] spi: Enable controllers to extend the SPI protocol
 with MOSI idle configuration

On 6/25/24 4:53 PM, Marcelo Schmitt wrote:
> diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
> index e8e1e798924f..8e50a8559225 100644
> --- a/include/linux/spi/spi.h
> +++ b/include/linux/spi/spi.h
> @@ -599,6 +599,12 @@ struct spi_controller {
>  	 * assert/de-assert more than one chip select at once.
>  	 */
>  #define SPI_CONTROLLER_MULTI_CS		BIT(7)
> +	/*
> +	 * The spi-controller is capable of keeping the MOSI line low or high
> +	 * when not clocking out data.
> +	 */
> +#define SPI_CONTROLLER_MOSI_IDLE_LOW    BIT(8)  /* Can idle MOSI low */
> +#define SPI_CONTROLLER_MOSI_IDLE_HIGH   BIT(9)  /* Can idle MOSI high */

These two flags above are still not used anywhere and are redundant with
the SPI_MOSI_IDLE_LOW/HIGH flags below so I don't think we should be adding
these.

>  
>  	/* Flag indicating if the allocation of this struct is devres-managed */
>  	bool			devm_allocated;
> diff --git a/include/uapi/linux/spi/spi.h b/include/uapi/linux/spi/spi.h
> index ca56e477d161..ee4ac812b8f8 100644
> --- a/include/uapi/linux/spi/spi.h
> +++ b/include/uapi/linux/spi/spi.h
> @@ -28,7 +28,8 @@
>  #define	SPI_RX_OCTAL		_BITUL(14)	/* receive with 8 wires */
>  #define	SPI_3WIRE_HIZ		_BITUL(15)	/* high impedance turnaround */
>  #define	SPI_RX_CPHA_FLIP	_BITUL(16)	/* flip CPHA on Rx only xfer */
> -#define SPI_MOSI_IDLE_LOW	_BITUL(17)	/* leave mosi line low when idle */
> +#define SPI_MOSI_IDLE_LOW	_BITUL(17)	/* leave MOSI line low when idle */
> +#define SPI_MOSI_IDLE_HIGH	_BITUL(18)	/* leave MOSI line high when idle */

The patch series that added SPI_MOSI_IDLE_LOW [1] also added it to spidev. Do
we need to do the same for SPI_MOSI_IDLE_HIGH?

Also, what is the plan for adding these flags to other SPI controllers. For
example, the IMX controller in [1] sounds like it should also support 
SPI_MOSI_IDLE_HIGH. And your comments on an earlier version of this series
made it sound like Raspberry Pi is always SPI_MOSI_IDLE_LOW, so should
have that flag.

[1]: https://lore.kernel.org/linux-spi/20230530141641.1155691-1-boerge.struempfel@gmail.com/

>  
>  /*
>   * All the bits defined above should be covered by SPI_MODE_USER_MASK.
> @@ -38,6 +39,6 @@
>   * These bits must not overlap. A static assert check should make sure of that.
>   * If adding extra bits, make sure to increase the bit index below as well.
>   */
> -#define SPI_MODE_USER_MASK	(_BITUL(18) - 1)
> +#define SPI_MODE_USER_MASK	(_BITUL(19) - 1)
>  
>  #endif /* _UAPI_SPI_H */