Re: [PATCH v2] i2c: busses: i2c-bcm2835: limits cdiv to allowed values

From: Eric Anholt <eric-WhKQ6XTQaPysTnJN9+BGXg@public.gmane.org>
To: wsa-z923LK4zBo2bacvFa/9K2g@public.gmane.org
Cc: linux-i2c-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Silvan Wicki <linux_wi-ADq4ffItWIY@public.gmane.org>,
	linux-rpi-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org
Subject: Re: [PATCH v2] i2c: busses: i2c-bcm2835: limits cdiv to allowed values
Date: Mon, 01 Jun 2015 11:07:07 -0700	[thread overview]
Message-ID: <87pp5f5mlw.fsf@eliezer.anholt.net> (raw)
In-Reply-To: <1432895204-19924-1-git-send-email-linux_wi-ADq4ffItWIY@public.gmane.org>

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Silvan Wicki <linux_wi-ADq4ffItWIY@public.gmane.org> writes:

> Adds: make sure bits 16-31 of DIV register are always 0
> Adds: assume minimal divider of 2 if divider resulted in 0
>       (bcm2835 sets divider to 32768 if cdiv is set to 0)
>
> See page 33/34 of BCM2835-ARM-Peripherals.pdf for the DIV register.
> https://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
>
> Signed-off-by: Silvan Wicki <linux_wi-ADq4ffItWIY@public.gmane.org>
> ---
>  drivers/i2c/busses/i2c-bcm2835.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/drivers/i2c/busses/i2c-bcm2835.c b/drivers/i2c/busses/i2c-bcm2835.c
> index c9336a3..8195b04 100644
> --- a/drivers/i2c/busses/i2c-bcm2835.c
> +++ b/drivers/i2c/busses/i2c-bcm2835.c
> @@ -50,6 +50,9 @@
>  #define BCM2835_I2C_S_CLKT	BIT(9)
>  #define BCM2835_I2C_S_LEN	BIT(10) /* Fake bit for SW error reporting */
>  
> +#define BCM2835_I2C_CDIV_MIN	0x0002
> +#define BCM2835_I2C_CDIV_MAX	0xFFFE
> +
>  #define BCM2835_I2C_TIMEOUT (msecs_to_jiffies(1000))
>  
>  struct bcm2835_i2c_dev {
> @@ -252,12 +255,22 @@ static int bcm2835_i2c_probe(struct platform_device *pdev)
>  
>  	divider = DIV_ROUND_UP(clk_get_rate(i2c_dev->clk), bus_clk_rate);
>  	/*
> +	 * Divider results in 0 by extremely high bus_clk_rate values
> +	 * such as bus_clk_rate >= 4044967297 and core_clock = 250MHz.
> +	 * In such a case assume the minimal possible divider since
> +	 * bcm2835 chip sets divisor internally to 32768 if cdiv is 0.
> +	 */
> +	if (divider < BCM2835_I2C_CDIV_MIN)
> +		divider = BCM2835_I2C_CDIV_MIN;
> +	/*
>  	 * Per the datasheet, the register is always interpreted as an even
>  	 * number, by rounding down. In other words, the LSB is ignored. So,
>  	 * if the LSB is set, increment the divider to avoid any issue.
>  	 */
>  	if (divider & 1)
>  		divider++;
> +	if (divider > BCM2835_I2C_CDIV_MAX)
> +		divider = BCM2835_I2C_CDIV_MAX;
>  	bcm2835_i2c_writel(i2c_dev, BCM2835_I2C_DIV, divider);

I'm not clear on the motivation of this patch. If the bus_clk_rate is
something totally unreasonable (125MHz?  3.8KHz?) it seems like
something is misconfigured and we should at best just fail the probe.
Does this fix some particular problem you've run into?

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