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SubjectRe: [PATCH v2] i2c: rk3x: Increase wait timeout to 1 second
On Mon, May 11, 2015 at 12:44:28PM -0700, Doug Anderson wrote:
> Although unlikely, it is remotely possible for an i2c command to need
> more than 200ms complete. Unlike smbus, i2c devices can clock stretch
> for an unspecified amount of time. The longest time I've seen
> specified for a device is 144ms (bq27541 battery gas), but one could
> imagine a device taking a bit slower. 1 second "ought to be enough for
> anyone."
>
> The above is not the only justifcation for going above 200ms for a
> timeout, though. It turns out that if you've got a large number of
> printks going out to a serial console, interrupts on a CPU can be
> disabled for hundreds of milliseconds. That's not a great situation to
> be in to start with (maybe we should put a cap in vprintk_emit()) but
> it's pretty annoying to start seeing unexplained i2c timeouts.
>
> Note that to understand why we can timeout when printk has interrupts
> disabled, you need to understand that on current Linux ARM kernels
> interrupts are routed to a single CPU in a multicore system. Thus,
> you can get:
>
> 1. CPU1 is running rk3x_i2c_xfer()
> 2. CPU0 calls vprintk_emit(), which disables all IRQs on CPU0.
> 3. I2C interrupt is ready but is set to only run on CPU0, where IRQs
> are disabled.
> 4. CPU1 timeout expires. I2C interrupt is still ready, but CPU0 is
> still sitting in the same vprintk_emit()
> 5. CPU1 sees that no interrupt happened in 200ms, so timeout.
>
> A normal system shouldn't see i2c timeouts anyway, so increasing the
> timeout should help people debugging without hurting other people
> excessively.
>
> Signed-off-by: Doug Anderson <dianders@chromium.org>
> Tested-by: Caesar Wang <wxt@rock-chips.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

Thanks
Uwe

--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | http://www.pengutronix.de/ |


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