Date:	Thu, 4 Aug 2016 16:37:49 -0700
From:	Alison Schofield <amsfield22@...il.com>
To:	Matt Ranostay <mranostay@...il.com>
Cc:	Peter Meerwald-Stadler <pmeerw@...erw.net>,
	Jonathan Cameron <jic23@...nel.org>,
	Hartmut Knaack <knaack.h@....de>,
	Lars-Peter Clausen <lars@...afoo.de>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iio: humidity: hdc100x: use i2c_master_recv to read
 sensor data

On Thu, Aug 04, 2016 at 03:21:13PM -0700, Matt Ranostay wrote:
> On Thu, Aug 4, 2016 at 8:35 AM, Alison Schofield <amsfield22@...il.com> wrote:
> > On Wed, Aug 03, 2016 at 10:50:54PM -0700, Matt Ranostay wrote:
> >> On Wed, Aug 3, 2016 at 10:19 PM, Peter Meerwald-Stadler <pmeerw@...erw.net>
> >> wrote:
> >>
> >> >
> >> > > > Replace the i2c_smbus_read_byte commmands used to retrieve the sensor
> >> > > > data with an i2c_master_recv command.
> >> > > >
> >> > > > The smbus read byte method fails because the device does not expect a
> >> > > > stop condition after sending the first byte. When we issue the second
> >> > > > read, we are getting the first byte again. Net effect is that of the 14
> >> > > > bits used for the measurement, the 8 most significant bits are correct,
> >> > > > the lower 6 are not.
> >> > > >
> >> > > > None of the smbus read protocols follow the pattern this device
> >> > requires
> >> > > > (S Addr Rd [A] Data [A] Data NA P), hence the switch to an i2c receive
> >> > > > transaction.
> >> > > >
> >> > > > Signed-off-by: Alison Schofield <amsfield22@...il.com>
> >> > > > Cc: Daniel Baluta <daniel.baluta@...il.com>
> >> > > > ---
> >> > > >  drivers/iio/humidity/hdc100x.c | 27 +++++++--------------------
> >> > > >  1 file changed, 7 insertions(+), 20 deletions(-)
> >> > > >
> >> > > > diff --git a/drivers/iio/humidity/hdc100x.c
> >> > b/drivers/iio/humidity/hdc100x.c
> >> > > > index a03832a..643a42d 100644
> >> > > > --- a/drivers/iio/humidity/hdc100x.c
> >> > > > +++ b/drivers/iio/humidity/hdc100x.c
> >> > > > @@ -142,7 +142,7 @@ static int hdc100x_get_measurement(struct
> >> > hdc100x_data *data,
> >> > > >         struct i2c_client *client = data->client;
> >> > > >         int delay = data->adc_int_us[chan->address];
> >> > > >         int ret;
> >> > > > -       int val;
> >> > > > +       u8 buf[2];
> >> >
> >> > __le16 val;
> >> >
> >> > > >
> >> > > >         /* start measurement */
> >> > > >         ret = i2c_smbus_write_byte(client, chan->address);
> >> > > > @@ -154,26 +154,13 @@ static int hdc100x_get_measurement(struct
> >> > hdc100x_data *data,
> >> > > >         /* wait for integration time to pass */
> >> > > >         usleep_range(delay, delay + 1000);
> >> > > >
> >> > > > -       /*
> >> > > > -        * i2c_smbus_read_word_data cannot() be used here due to the
> >> > command
> >> > > > -        * value not being understood and causes NAKs preventing any
> >> > reading
> >> > > > -        * from being accessed.
> >> > > > -        */
> >> > > > -       ret = i2c_smbus_read_byte(client);
> >> > > > +       /* read the 2 byte measurement */
> >> > > > +       ret = i2c_master_recv(data->client, buf, 2);
> >> > > >         if (ret < 0) {
> >> > > > -               dev_err(&client->dev, "cannot read high byte
> >> > measurement");
> >> > > > +               dev_err(&client->dev, "cannot read sensor data\n");
> >> > > >                 return ret;
> >> > > >         }
> >> > > > -       val = ret << 8;
> >> > > > -
> >> > > > -       ret = i2c_smbus_read_byte(client);
> >> > > > -       if (ret < 0) {
> >> > > > -               dev_err(&client->dev, "cannot read low byte
> >> > measurement");
> >> > > > -               return ret;
> >> > > > -       }
> >> > > > -       val |= ret;
> >> > > > -
> >> > > > -       return val;
> >> > > > +       return (int)(buf[0] << 8 | buf[1]);
> >> > >
> >> > > Pretty sure you don't need to cast to int type here.
> >> >
> >> > return le16_to_cpu(val);
> >> >
> >> >
> >> You mean le16_to_cpu(&buf)  I assume?
> >>
> >>
> >> > >
> >> > > >  }
> >> > > >
> >> > > >  static int hdc100x_get_heater_status(struct hdc100x_data *data)
> >> > > > @@ -272,8 +259,8 @@ static int hdc100x_probe(struct i2c_client *client,
> >> > > >         struct iio_dev *indio_dev;
> >> > > >         struct hdc100x_data *data;
> >> > > >
> >> > > > -       if (!i2c_check_functionality(client->adapter,
> >> > > > -                               I2C_FUNC_SMBUS_WORD_DATA |
> >> > I2C_FUNC_SMBUS_BYTE))
> >> > > > +       if (!i2c_check_functionality(client->adapter,
> >> > I2C_FUNC_SMBUS_WORD_DATA |
> >> > > > +                                    I2C_FUNC_SMBUS_BYTE |
> >> > I2C_FUNC_I2C))
> >> > >
> >> > > Not sure we want to kill smbus support for this device...  iwe should
> >> > > have two read methods for i2c and smbus (see the PulsedLight LIDAR
> >> > > driver) in this case.
> >> > >
> >> > > Ideally I wouldn't have written it with only smbus in mind, but can
> >> > > kill backwards compatibility unless we have a good reason.
> >> > >
> >> > > Thanks,
> >> > >
> >> > > Matt
> >
> > Thanks for the reviews. Let me clarify:
> >
> > This is a fix for a bug in the current driver.  See the changelog.
> >
> > The relationship between this patch and my triggered-buffer work
> > is that I found this bug while trying to do just what you say above.
> > I tried to have 2 methods for reading data - smbus and plain i2c.
> > That's the point where I found that smbus reads do not work, never did.
> >
> > I know we want to stay on the smbus, so I'm looking for suggestions.
> > As I noted in changelog and my 'smbus help' email - every defined smbus
> > read command fails.
> 
> Of course this depends on what dev board you are using and if the i2c
> controller supports both.
> 
> Ideally you check for i2c support first then point to that xfer
> transfer function for it, and then check for smbus support. Suspect
> you did something similar to ->
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/iio/proximity/pulsedlight-lidar-lite-v2.c?id=refs/tags/v4.7#n274
> , correct?

No, that's not what this bug & patch is about.

This patch is about the fact that we are reading the temp &
humid data registers incorrectly in the current driver and
we are exposing incorrect numbers via sysfs raw reads.

I have tried by testing and inspection each read protocol.
It NAKs all of them - except for read_byte. Problem with
read_byte is that we are always getting the MSB.   We do 2
consecutive read_bytes, expecting MSB,LSB, but get MSB,MSB.

I have been doing msmts w this driver for quite awhile, and
to my human eye they looked reasonable in integer format.
But, once I started looking at the byte level, to test moving
those bytes into buffers, I saw the unmistakable pattern: 
MSB == LSB always.

Sorry to be the bearer of bad news.  I still hold out hope that
we can fix this with some smbus magic and not 'kill backward
compatibility'.  

Who's the resident SMBUS expert(s)?

I don't have a session logged, but I can set up and send a 
demo if wanted.  Let me know.

alisons

> 
> Thanks,
> 
> Matt > 
> >
> > alisons
> >
> >> > >
> >> > > >                 return -EOPNOTSUPP;
> >> > > >
> >> > > >         indio_dev = devm_iio_device_alloc(&client->dev, sizeof(*data));
> >> > > > --
> >> > > > 2.1.4
> >> > > >
> >> > >
> >> >
> >> > --
> >> >
> >> > Peter Meerwald-Stadler
> >> > +43-664-2444418 (mobile)
> >> >

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