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.
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; #ifdef DEBUG if (jiffies != start && i2c_debug >= 3) pr_debug("i2c-algo-bit: needed %ld jiffies for SCL to go "
A quick hack to verify that the fix works as intended...
Do not apply.
--- drivers/i2c/algos/i2c-algo-bit.c | 6 ++++++ 1 files changed, 6 insertions(+), 0 deletions(-)
diff --git a/drivers/i2c/algos/i2c-algo-bit.c b/drivers/i2c/algos/i2c-algo-bit.c index d25112e..3f547b5 100644 --- a/drivers/i2c/algos/i2c-algo-bit.c +++ b/drivers/i2c/algos/i2c-algo-bit.c @@ -89,6 +89,7 @@ static inline void scllo(struct i2c_algo_bit_data *adap) static int sclhi(struct i2c_algo_bit_data *adap) { unsigned long start; + bool timeout = false;
setscl(adap, 1);
@@ -104,11 +105,16 @@ static int sclhi(struct i2c_algo_bit_data *adap) * are processing data internally. */ if (time_after(jiffies, start + adap->timeout)) + { + timeout = true; break; + } cond_resched(); } if (!getscl(adap)) return -ETIMEDOUT; + if (timeout) + printk(KERN_CRIT "spurious i2c scl timeout\n"); #ifdef DEBUG if (jiffies != start && i2c_debug >= 3) pr_debug("i2c-algo-bit: needed %ld jiffies for SCL to go "
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,
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 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?
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;}#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.
dri-devel@lists.freedesktop.org
-
Jean Delvare -
Ville Syrjälä -
Ville Syrjälä