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@xxxxxx>
> ---
> 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;
> cond_resched();
> }
> + 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,
--
Jean Delvare
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