Re: [PATCH] i2c-omap: close suspected race between omap_i2c_idle() and omap_i2c_isr()

[Tony Lindgren <tony@atomide.com>]

* Paul Walmsley <paul@pwsan.com> [080715 22:04]:
> 
> omap_i2c_idle() sets an internal flag, "dev->idle", instructing its
> ISR to decline interrupts.  It sets this flag before it actually masks
> the interrupts on the I2C controller.  This is problematic, since an
> I2C interrupt could arrive after dev->idle is set, but before the
> interrupt source is masked.  When this happens, Linux disables the I2C
> controller's IRQ, causing all future transactions on the bus to fail.
> 
> Symptoms, happening on about 7% of boots:
> 
>    irq 56: nobody cared (try booting with the "irqpoll" option)
>    <warning traceback here>
>    Disabling IRQ #56
>    i2c_omap i2c_omap.1: controller timed out
> 
> In omap_i2c_idle(), this patch sets dev->idle only after the interrupt
> mask write to the I2C controller has left the ARM write buffer.
> That's probably the major offender.  For additional prophylaxis, in
> omap_i2c_unidle(), the patch clears the dev->idle flag before
> interrupts are enabled, rather than afterwards.
> 
> The patch has survived twenty-two reboots on the 3430SDP here without
> wedging I2C1.  Not absolutely dispositive, but promising!

Great, pushing today.

Tony

> 
> 
> Signed-off-by: Paul Walmsley <paul@pwsan.com>
> ---
> 
>  drivers/i2c/busses/i2c-omap.c |   10 ++++++++--
>  1 files changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-omap.c b/drivers/i2c/busses/i2c-omap.c
> index 55779f5..ed7e9ad 100644
> --- a/drivers/i2c/busses/i2c-omap.c
> +++ b/drivers/i2c/busses/i2c-omap.c
> @@ -209,22 +209,28 @@ static void omap_i2c_unidle(struct omap_i2c_dev *dev)
>  	if (dev->iclk != NULL)
>  		clk_enable(dev->iclk);
>  	clk_enable(dev->fclk);
> +	dev->idle = 0;
>  	if (dev->iestate)
>  		omap_i2c_write_reg(dev, OMAP_I2C_IE_REG, dev->iestate);
> -	dev->idle = 0;
>  }
>  
>  static void omap_i2c_idle(struct omap_i2c_dev *dev)
>  {
>  	u16 iv;
>  
> -	dev->idle = 1;
>  	dev->iestate = omap_i2c_read_reg(dev, OMAP_I2C_IE_REG);
>  	omap_i2c_write_reg(dev, OMAP_I2C_IE_REG, 0);
>  	if (dev->rev1)
>  		iv = omap_i2c_read_reg(dev, OMAP_I2C_IV_REG);
>  	else
>  		omap_i2c_write_reg(dev, OMAP_I2C_STAT_REG, dev->iestate);
> +	/*
> +	 * The wmb() is to ensure that the I2C interrupt mask write
> +	 * reaches the I2C controller before the dev->idle store
> +	 * occurs.
> +	 */
> +	wmb();
> +	dev->idle = 1;
>  	clk_disable(dev->fclk);
>  	if (dev->iclk != NULL)
>  		clk_disable(dev->iclk);
> --
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