linux-kernel - Re: [PATCH v4 1/4] i2c: imx: only poll for bus busy in multi master mode

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Message-ID: <6378933c8d30bec2c3084e41c83b9d129d0d40d1.camel@pengutronix.de>
Date: Wed, 02 Oct 2024 16:40:32 +0200
From: Lucas Stach <l.stach@...gutronix.de>
To: Stefan Eichenberger <eichest@...il.com>, o.rempel@...gutronix.de, 
	kernel@...gutronix.de, andi.shyti@...nel.org, shawnguo@...nel.org, 
	s.hauer@...gutronix.de, festevam@...il.com, Frank.Li@....com
Cc: imx@...ts.linux.dev, linux-kernel@...r.kernel.org, Stefan Eichenberger
	 <stefan.eichenberger@...adex.com>, linux-i2c@...r.kernel.org, 
	francesco.dolcini@...adex.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 1/4] i2c: imx: only poll for bus busy in multi master
 mode

Am Mittwoch, dem 02.10.2024 um 13:19 +0200 schrieb Stefan Eichenberger:
> From: Stefan Eichenberger <stefan.eichenberger@...adex.com>
> 
> According to the i.MX8M Mini reference manual chapter "16.1.4.2
> Generation of Start" it is only necessary to poll for bus busy and
> arbitration lost in multi master mode. This helps to avoid rescheduling
> while the i2c bus is busy and avoids SMBus devices to timeout.
> 
This is a backward incompatible change, as far as I can see. Until now
the driver would properly handle a multi-mastered bus, without any
specific configuration. Now it requires the new multi-master DT
property to be set, which isn't even documented in the binding to be
understood by this driver.

Are you sure that every single instance of a i.MX i2c bus is only
single mastered?

If this is a worthwhile performance improvement I guess you need to
flip the logic around by adding a new single-master DT property (or
something along those lines), which should go through proper DT binding
review. You can then use this property for boards/busses to opt into
skipping the arbitration lost check.

Regards,
Lucas

> Signed-off-by: Stefan Eichenberger <stefan.eichenberger@...adex.com>
> Reviewed-by: Frank Li <Frank.Li@....com>
> Acked-by: Oleksij Rempel <o.rempel@...gutronix.de>
> ---
>  drivers/i2c/busses/i2c-imx.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
> index 98539313cbc97..fbacdfaf6b28e 100644
> --- a/drivers/i2c/busses/i2c-imx.c
> +++ b/drivers/i2c/busses/i2c-imx.c
> @@ -216,6 +216,8 @@ struct imx_i2c_struct {
>  	struct i2c_client	*slave;
>  	enum i2c_slave_event last_slave_event;
>  
> +	bool			multi_master;
> +
>  	/* For checking slave events. */
>  	spinlock_t     slave_lock;
>  	struct hrtimer slave_timer;
> @@ -481,6 +483,9 @@ static int i2c_imx_bus_busy(struct imx_i2c_struct *i2c_imx, int for_busy, bool a
>  	unsigned long orig_jiffies = jiffies;
>  	unsigned int temp;
>  
> +	if (!i2c_imx->multi_master)
> +		return 0;
> +
>  	while (1) {
>  		temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2SR);
>  
> @@ -540,8 +545,8 @@ static int i2c_imx_trx_complete(struct imx_i2c_struct *i2c_imx, bool atomic)
>  		return -ETIMEDOUT;
>  	}
>  
> -	/* check for arbitration lost */
> -	if (i2c_imx->i2csr & I2SR_IAL) {
> +	/* In multi-master mode check for arbitration lost */
> +	if (i2c_imx->multi_master && (i2c_imx->i2csr & I2SR_IAL)) {
>  		dev_dbg(&i2c_imx->adapter.dev, "<%s> Arbitration lost\n", __func__);
>  		i2c_imx_clear_irq(i2c_imx, I2SR_IAL);
>  
> @@ -1468,6 +1473,8 @@ static int i2c_imx_probe(struct platform_device *pdev)
>  		goto rpm_disable;
>  	}
>  
> +	i2c_imx->multi_master = of_property_read_bool(pdev->dev.of_node, "multi-master");
> +
>  	/* Set up clock divider */
>  	i2c_imx->bitrate = I2C_MAX_STANDARD_MODE_FREQ;
>  	ret = of_property_read_u32(pdev->dev.of_node,