On Thu, Mar 15, 2012 at 03:32:40PM +0100, Jean Delvare wrote: > Hi Ville, > > On Wed, 14 Mar 2012 10:32:52 +0200, Ville Syrj?l? 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 Syrj?l? <syrjala at 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; > > 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?
Yeah that fact also occured to today. IIRC I did post an another version of the patch to some bugzilla quite a while ago that didn't suffer from this issue. Ah here [1] it is. By that time I no longer had access to the machine (a Thinkpad T400) where I initially saw the problem, so I didn't pursue it further. [1] https://bugs.freedesktop.org/show_bug.cgi?id=29787 > --- 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.) Looks good. Thanks for taking care of it. -- Ville Syrj?l? Intel OTC
