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From:   "Li, Meng" <Meng.Li@windriver.com>
To:     Jonathan Cameron <jic23@kernel.org>
CC:     "lars@metafoo.de" <lars@metafoo.de>,
        "Michael.Hennerich@analog.com" <Michael.Hennerich@analog.com>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>
Subject: RE: [PATCH] driver: adc: ltc2497: return directly after reading the
 adc conversion value
Thread-Topic: [PATCH] driver: adc: ltc2497: return directly after reading the
 adc conversion value
Thread-Index: AQHXVsiLZJqFig5jYEW95MUVkHO9fqsCeyaAgACcmKA=
Date:   Fri, 4 Jun 2021 02:16:39 +0000
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References: <20210601092805.18385-1-Meng.Li@windriver.com>
 <20210603172025.314b5ced@jic23-huawei>
In-Reply-To: <20210603172025.314b5ced@jic23-huawei>
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> -----Original Message-----
> From: Jonathan Cameron <jic23@kernel.org>
> Sent: Friday, June 4, 2021 12:20 AM
> To: Li, Meng <Meng.Li@windriver.com>
> Cc: lars@metafoo.de; Michael.Hennerich@analog.com;
> pmeerw@pmeerw.net; u.kleine-koenig@pengutronix.de; linux-
> kernel@vger.kernel.org; linux-iio@vger.kernel.org
> Subject: Re: [PATCH] driver: adc: ltc2497: return directly after reading =
the adc
> conversion value
>=20
> [Please note: This e-mail is from an EXTERNAL e-mail address]
>=20
> On Tue,  1 Jun 2021 17:28:05 +0800
> Meng.Li@windriver.com wrote:
>=20
> > From: Meng Li <Meng.Li@windriver.com>
> >
> > 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.
> >
> > v2:
> > According to ltc2497 datasheet, the max value of conversion time is
> > 149.9 ms. So, add 20% to the 150msecs so that there is enough time for
> > data conversion.
>=20
> Version change logs go below the --- as we don't want to preserve them
> forever in the git history.
>=20
> I may have lost track of the discussion, but I thought the idea was that
> perhaps the longer time period would remove the need for the early return=
?
>=20

No!
I think the ret is essential.
Because when call i2c_master_recv() to receive data from chip, there is a s=
top bit sent to chip at the last step.
When the chip receive the stop bit, it will start the next conversion opera=
tion.=20
So, if we send any command on I2C bus when the chip is in conversion status=
, there will be a i2c error occurred.

Thanks,
Limeng

> Thanks,
>=20
> Jonathan
> >
> > Fixes: 69548b7c2c4f ("iio: adc: ltc2497: split protocol independent
> > part in a separate module ")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Meng Li <Meng.Li@windriver.com>
> > ---
> >  drivers/iio/adc/ltc2497.c | 2 ++
> >  drivers/iio/adc/ltc2497.h | 2 +-
> >  2 files changed, 3 insertions(+), 1 deletion(-)
> >
> > 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 =3D (be32_to_cpu(st->buf) >> 14) - (1 << 17);
> > +
> > +             return ret;
> >       }
> >
> >       ret =3D i2c_smbus_write_byte(st->client, diff --git
> > a/drivers/iio/adc/ltc2497.h b/drivers/iio/adc/ltc2497.h index
> > d0b42dd6b8ad..e451930837d8 100644
> > --- a/drivers/iio/adc/ltc2497.h
> > +++ b/drivers/iio/adc/ltc2497.h
> > @@ -2,7 +2,7 @@
> >
> >  #define LTC2497_ENABLE                       0xA0
> >  #define LTC2497_CONFIG_DEFAULT               LTC2497_ENABLE
> > -#define LTC2497_CONVERSION_TIME_MS   150ULL
> > +#define LTC2497_CONVERSION_TIME_MS   180ULL
> >
> >  struct ltc2497core_driverdata {
> >       struct regulator *ref;