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

[Li, Meng]

> -----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.
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

Thanks,
Limeng

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