Hi Ville,
On Wed, 14 Mar 2012 10:32:52 +0200, Ville Syrjälä wrote:
When the system is under heavy load, there can be a significant delay between the getscl() and time_after() calls inside sclhi(). That delay may cause the time_after() check to trigger after SCL has gone high, causing sclhi() to return -ETIMEDOUT.
To fix the problem, double check that SCL is still low after the timeout has been reached, before deciding to return -ETIMEDOUT.
Signed-off-by: Ville Syrjälä syrjala@sci.fi
I can easily reproduce these spurious timeouts on my HP-compaq nc6000 laptop with the radeon kms driver. It's enough to have a -j2 kernel build running, and simultaneosly issue xrandr commands in a terminal. Calling xrandr will cause the driver to re-read the EDID from the display. A significant number of the EDID reads will fail. With this fix I have yet to see any failed EDID reads.
Thanks for describing a test case, I was able to reproduce the problem easily by following your instructions. The problem is real, even with the pending fixes I have to radeon's I2C implementation.
I only have one concern about your implementation:
drivers/i2c/algos/i2c-algo-bit.c | 4 +++- 1 files changed, 3 insertions(+), 1 deletions(-)
diff --git a/drivers/i2c/algos/i2c-algo-bit.c b/drivers/i2c/algos/i2c-algo-bit.c index 525c734..d25112e 100644 --- a/drivers/i2c/algos/i2c-algo-bit.c +++ b/drivers/i2c/algos/i2c-algo-bit.c @@ -104,9 +104,11 @@ static int sclhi(struct i2c_algo_bit_data *adap) * are processing data internally. */ if (time_after(jiffies, start + adap->timeout))
return -ETIMEDOUT;
break;- if (!getscl(adap))
return -ETIMEDOUT;
This means double-check even in the most common case where time_after() didn't cause the loop break. From a performance perspective, this seems undesirable. What would you think of the alternative fix below?
--- linux-3.3-rc7.orig/drivers/i2c/algos/i2c-algo-bit.c 2012-03-15 09:33:10.232176790 +0100 +++ linux-3.3-rc7/drivers/i2c/algos/i2c-algo-bit.c 2012-03-15 14:52:48.127778459 +0100 @@ -103,8 +103,14 @@ static int sclhi(struct i2c_algo_bit_dat * chips may hold it low ("clock stretching") while they * are processing data internally. */ - if (time_after(jiffies, start + adap->timeout)) + if (time_after(jiffies, start + adap->timeout)) { + /* Test one last time, as we may have been preempted + * between last check and timeout test. + */ + if (getscl(adap)) + break; return -ETIMEDOUT; + } cond_resched(); } #ifdef DEBUG
Functionally it should be equivalent to your proposal, but faster. I'll apply that (and send for stable inclusion.)
Thanks,