Re: [PATCH] driver: adc: ltc2497: return directly after reading the adc conversion value

[Jonathan Cameron]

On Mon, 24 May 2021 02:49:01 +0000
"Li, Meng" <Meng.Li@xxxxxxxxxxxxx> wrote:

> > -----Original Message-----
> > From: Jonathan Cameron <jic23@xxxxxxxxxx>
> > Sent: Saturday, May 22, 2021 1:02 AM
> > To: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
> > Cc: Li, Meng <Meng.Li@xxxxxxxxxxxxx>; lars@xxxxxxxxxx;
> > Michael.Hennerich@xxxxxxxxxx; pmeerw@xxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx; linux-iio@xxxxxxxxxxxxxxx
> > Subject: Re: [PATCH] driver: adc: ltc2497: return directly after reading the adc
> > conversion value
> > 
> > [Please note: This e-mail is from an EXTERNAL e-mail address]
> > 
> > On Wed, 19 May 2021 11:21:04 +0200
> > Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
> >   
> > > On Wed, May 12, 2021 at 12:57:25PM +0800, Meng.Li@xxxxxxxxxxxxx  
> > wrote:  
> > > > From: Meng Li <Meng.Li@xxxxxxxxxxxxx>
> > > >
> > > > When read adc conversion value with below command:
> > > > cat /sys/.../iio:device0/in_voltage0-voltage1_raw
> > > > There is an error reported as below:
> > > > ltc2497 0-0014: i2c transfer failed: -EREMOTEIO This i2c transfer
> > > > issue is introduced by commit 69548b7c2c4f ("iio:
> > > > adc: ltc2497: split protocol independent part in a separate module").
> > > > When extract the common code into ltc2497-core.c, it change the code
> > > > logic of function ltc2497core_read(). With wrong reading sequence,
> > > > the action of enable adc channel is sent to chip again during adc
> > > > channel is in conversion status. In this way, there is no ack from
> > > > chip, and then cause i2c transfer failed.
> > > > In order to keep the code logic is the same with original ideal, it
> > > > is need to return direct after reading the adc conversion value.
> > > >
> > > > Fixes: 69548b7c2c4f ("iio: adc: ltc2497: split protocol independent
> > > > part in a separate module ")
> > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > Signed-off-by: Meng Li <Meng.Li@xxxxxxxxxxxxx>
> > > > ---
> > > >  drivers/iio/adc/ltc2497.c | 2 ++
> > > >  1 file changed, 2 insertions(+)
> > > >
> > > > diff --git a/drivers/iio/adc/ltc2497.c b/drivers/iio/adc/ltc2497.c
> > > > index 1adddf5a88a9..fd5a66860a47 100644
> > > > --- a/drivers/iio/adc/ltc2497.c
> > > > +++ b/drivers/iio/adc/ltc2497.c
> > > > @@ -41,6 +41,8 @@ static int ltc2497_result_and_measure(struct  
> > ltc2497core_driverdata *ddata,  
> > > >             }
> > > >
> > > >             *val = (be32_to_cpu(st->buf) >> 14) - (1 << 17);
> > > > +
> > > > +           return ret;  
> > >
> > > This looks wrong for me. The idea of the function
> > > ltc2497_result_and_measure is that it reads the result and starts a
> > > new measurement. I guess the problem is that
> > > ltc2497_result_and_measure is called to early, not that it does too much.
> > >
> > > But note I don't have such a system handy to actually debug this any
> > > more.  
> > 
> > @Meng Li,
> > 
> > I see from the datasheet that the device can be used with an external
> > oscillator.
> > Is that the case on your boards, because if so the timing delay of 150msecs
> > may be far too short.  If not, perhaps the part is right at the upper end of
> > timings and we just need to add 20% to the 150msecs to be sure of not
> > hitting the limit?
> >   
> 
> Hi Jonathan,
> 
> Thanks for your very professional comments.
> I check my board schematics, the pin 35 f0 is connected to GND, so I use the internal oscillator.

Bad guess :( 

> In additional, I am not very understand your comment about the case of using internal oscillator.
> Do you mean that you agree my patch and only need to change 150 into 180?
> #define LTC2497_CONVERSION_TIME_MS	150ULL
> =>  
> #define LTC2497_CONVERSION_TIME_MS	180ULL

Yes.  Just give a little more time in case the oscillator is running
a little slow.

Jonathan

> 
> Thanks,
> Limeng
> 
> > Thanks,
> > 
> > Jonathan
> > 
> >   
> > >
> > > Best regards
> > > Uwe
> > >  
>