Re: [PATCH] i2c-cadence: Do not let signals interrupt I2C transfers

[Mike Looijmans]

ïOn 03/11/2014 05:49 AM, Suneel Garapati wrote:
> Hi Mike/Soren,
>
> >

Met vriendelijke groet / kind regards,

Mike Looijmans

TOPIC Embedded Systems
Eindhovenseweg 32-C, NL-5683 KH Best
Postbus 440, NL-5680 AK Best
Telefoon: (+31) (0) 499 33 69 79
Telefax:  (+31) (0) 499 33 69 70
E-mail: mike.looijmans@xxxxxxxx
Website: www.topic.nl

Please consider the environment before printing this e-mail

-----Original Message-----
> > From: SÃren Brinkmann [mailto:soren.brinkmann@xxxxxxxxxx]
> > Sent: Tuesday, March 11, 2014 03:30
> > To: Mike Looijmans; Michal Simek
> > Cc: git; wsa@xxxxxxxxxxxxx; linux-i2c@xxxxxxxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx
> > Subject: Re: [PATCH] i2c-cadence: Do not let signals interrupt I2C transfers
> >
> > Hi Mike,
> >
> > The cadence driver is not in mainline yet. I think for our vendor tree we can
> > pretty much take it this way.
> > Regarding getting this into mainline, I'll send another iteration of the change
> > set and include these changes.
> >
> > On Mon, 2014-03-10 at 12:12PM +0100, Mike Looijmans wrote:
> > > Pressing CTRL-C while communicating with an I2C device leads to
> > > erratic behaviour. The cause is that the controller will interrupt the
> > > I2C transfer in progress, and leave the client device in an undefined
> > > state. Many drivers do not handle error return codes on I2C transfers.
> > > The calling driver has no way of telling how much of the transfer has
> > > actually completed, so it cannot reliably determine the device's state.
> > >
> > > The best solution here is to not handle signals in the I2C bus driver
> > > at all, but always complete a transaction before returning control.
> > >
> > > See for a related patch and discussion on this topic:
> > > http://lkml.org/lkml/2014/1/9/246
> >
> > Can we get your Signed-off-by, please?

Sure, I forgot the -s, will add it in the next version.

> > > ---
> > >   drivers/i2c/busses/i2c-cadence.c |   12 ++++--------
> > >   1 file changed, 4 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/drivers/i2c/busses/i2c-cadence.c
> > > b/drivers/i2c/busses/i2c-cadence.c
> > > index 86713d6..32ce2ee 100644
> > > --- a/drivers/i2c/busses/i2c-cadence.c
> > > +++ b/drivers/i2c/busses/i2c-cadence.c
> > > @@ -452,16 +452,12 @@ static int cdns_i2c_process_msg(struct cdns_i2c
> > *id, struct i2c_msg *msg,
> > >                      cdns_i2c_msend(id);
> > >
> > >              /* Wait for the signal of completion */
> > > -           ret = wait_for_completion_interruptible_timeout(
> > > -                                                   &id->xfer_done, HZ);
> > > -           if (ret < 1) {
> > > +           ret = wait_for_completion_timeout(&id->xfer_done, HZ);
> > > +           if (ret == 0) {
> >
> > To match the style used throughout the file this should just be
> >        if (!ret) {

Will do.

> Instead of discarding Ctrl+C, Can we wait until the current msg transfer completes
> [to avoid client undefined state or re-init the host to a known state]
>
> I see da-vinci driver handling in a similar way.

My patch for the davinci should have completely ignored the signals too. I 
haven't come across a driver that actually calls the xfer function with more 
than one item anyway. If drivers check the result of the xfer at all, they 
usually don't have a clue how to handle partial transfers. Many drivers just 
ignore the return code completely and don't even report failure.

The gain is a stable I2C system. The loss here is something like <1ms more 
latency to interrupt a process in the case it was just transferring I2C data. 
From experience I can tell that the gain far outweighs that loss.

Mike.


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/