On 18.09.2017 07:24, Vignesh R wrote:
>
>
> On Saturday 16 September 2017 05:01 AM, Strashko, Grygorii wrote:
>>
>>
>> On 09/14/2017 10:39 AM, Claudio Foellmi wrote:
>>> A very conservative check for bus activity (to prevent interference
>>> in multimaster setups) prevented the bus recovery methods from being
>>> triggered in the case that SDA or SCL was stuck low.
>>> This defeats the purpose of the recovery mechanism, which was introduced
>>> for exactly this situation (a slave device keeping SDA pulled down).
>>>
>>> Note that bus lockups can persist across reboots. The only other options
>>> are to reset or power cycle the offending slave device, and many i2c
>>> slaves do not even have a reset pin.
>>>
>>> If we see that one of the lines is low for the entire timeout duration,
>>> we can actually be sure that there is no other master driving the bus.
>>> It is therefore save for us to attempt a bus recovery.
>>>
>>
>> Reviewed-by: Grygorii Strashko <grygorii.strashko@xxxxxx>
>>
>>> Signed-off-by: Claudio Foellmi <claudio.foellmi@xxxxxxxx>
>>> ---
>>> Caveat: It turns out I don't have the hardware to fully test the
>>> recovery mechanism. My faulty i2c slave device actually pulls down SCL,
>>> not SDA (so the recovery will not succeed in my case).
>
> Maybe, you could detect SCL stuck low case by reading status of SCL line
> from OMAP_I2C_SYSTEST_REG and then call IP reset (there is nothing much
> that can be done) instead of bus recovery.
>
I plan on posting a related patch soon, that will print better messages
if the generic recovery fails. If SCL is stuck low, I think the best we
can do is make the problem visible in the kernel log.
>>> But by directly connecting SDA to ground, I could at least make sure
>>> the recovery function gets called after applying this patch.
>>>
>
> I had seen flood of XRDY & RRDY interrupts as soon as TMODE is set to
> 0x3 as part of omap_i2c_prepare_recovery() leading to unusable system.
> Did you observe this behavior on your system? Could you mention the
> platform on which this experiment done?
>
So attempting bus recovery is dangerous on some platforms?
I did not notice any obvious problems (assuming an 'unusable system'
would be hard to miss), but then again I only have one target to test on.
I'm working with a TI AM3352, the slave is a NXP NT3H2211 on i2c-1.
>>> drivers/i2c/busses/i2c-omap.c | 7 ++++++-
>>> 1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/i2c/busses/i2c-omap.c b/drivers/i2c/busses/i2c-omap.c
>>> index 1ebb5e9..4b25fd1 100644
>>> --- a/drivers/i2c/busses/i2c-omap.c
>>> +++ b/drivers/i2c/busses/i2c-omap.c
>>> @@ -563,8 +563,13 @@ static int omap_i2c_wait_for_bb_valid(struct omap_i2c_dev *omap)
>>> }
>>>
>>> if (time_after(jiffies, timeout)) {
>>> + /*
>>> + * SDA or SCL were low for the entire timeout without
>>> + * any activity detected. Most likely, a slave is
>>> + * locking up the bus with no master driving the clock.
>>> + */
>>> dev_warn(omap->dev, "timeout waiting for bus ready\n");
>>> - return -ETIMEDOUT;
>>> + return i2c_recover_bus(&omap->adapter);
>>> }
>>>
>>> msleep(1);
>>>
>>
>
--
Regards
Claudio
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