linux-kernel - Re: [Outreachy kernel] [PATCH v4] staging: iio: ade7753: Replace mlock with driver private lock

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Message-ID: <CALrZqyNP-8CeAGRr3uDuAhBweT1Sp3H4bWPy8y__7tvnzxR=zw@mail.gmail.com>
Date:   Tue, 28 Mar 2017 22:55:17 +0530
From:   SIMRAN SINGHAL <singhalsimran0@...il.com>
To:     Alison Schofield <amsfield22@...il.com>
Cc:     Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Greg KH <gregkh@...uxfoundation.org>,
        linux-iio@...r.kernel.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org,
        outreachy-kernel <outreachy-kernel@...glegroups.com>
Subject: Re: [Outreachy kernel] [PATCH v4] staging: iio: ade7753: Replace
 mlock with driver private lock

On Fri, Mar 24, 2017 at 12:51 AM, Alison Schofield <amsfield22@...il.com> wrote:
> On Fri, Mar 24, 2017 at 12:05:20AM +0530, simran singhal wrote:
>> The IIO subsystem is redefining iio_dev->mlock to be used by
>> the IIO core only for protecting device operating mode changes.
>> ie. Changes between INDIO_DIRECT_MODE, INDIO_BUFFER_* modes.
>>
>> In this driver, mlock was being used to protect hardware state
>> changes.  Replace it with a lock in the devices global data.
>
> Hi Simran,
>
> Please post all revision histories below the --- not just the most
> recent.
>
Sorry, will not repeat this.

> Does this lock enforce the needed "atomicity" in the write_frequency
> function? I read Jonathans comment on a previous revision about
> "ensuring the spi bus frequency and sampling frequency of the device
> are changed in an atomic fashion"
>

By introducing another lock I am protecting read_modify_write and
in this way also protecting the designated register that we are about
to write.

> Is it possible for another spi bus transaction (read or write) to
> occur between the read and write in write_frequency?
>

Gargi has also come up with a solution.
https://groups.google.com/forum/#!topic/outreachy-kernel/kzE9CrI5Bd8

Should I do like her as her's also seem correct or go ahead with this.

> alisons
>>
>> Signed-off-by: simran singhal <singhalsimran0@...il.com>
>> ---
>>
>>  v4:
>>    -Add mutex_init
>>
>>  drivers/staging/iio/meter/ade7753.c | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/staging/iio/meter/ade7753.c b/drivers/staging/iio/meter/ade7753.c
>> index b71fbd3..30aebaf 100644
>> --- a/drivers/staging/iio/meter/ade7753.c
>> +++ b/drivers/staging/iio/meter/ade7753.c
>> @@ -80,11 +80,13 @@
>>   * @us:         actual spi_device
>>   * @tx:         transmit buffer
>>   * @rx:         receive buffer
>> + * @lock:       protect sensor state
>>   * @buf_lock:       mutex to protect tx and rx
>>   **/
>>  struct ade7753_state {
>>       struct spi_device   *us;
>>       struct mutex        buf_lock;
>> +     struct mutex        lock;  /* protect sensor state */
>>       u8          tx[ADE7753_MAX_TX] ____cacheline_aligned;
>>       u8          rx[ADE7753_MAX_RX];
>>  };
>> @@ -484,7 +486,7 @@ static ssize_t ade7753_write_frequency(struct device *dev,
>>       if (!val)
>>               return -EINVAL;
>>
>> -     mutex_lock(&indio_dev->mlock);
>> +     mutex_lock(&st->lock);
>>
>>       t = 27900 / val;
>>       if (t > 0)
>> @@ -505,7 +507,7 @@ static ssize_t ade7753_write_frequency(struct device *dev,
>>       ret = ade7753_spi_write_reg_16(dev, ADE7753_MODE, reg);
>>
>>  out:
>> -     mutex_unlock(&indio_dev->mlock);
>> +     mutex_unlock(&st->lock);
>>
>>       return ret ? ret : len;
>>  }
>> @@ -581,6 +583,7 @@ static int ade7753_probe(struct spi_device *spi)
>>       st = iio_priv(indio_dev);
>>       st->us = spi;
>>       mutex_init(&st->buf_lock);
>> +     mutex_init(&st->lock);
>>
>>       indio_dev->name = spi->dev.driver->name;
>>       indio_dev->dev.parent = &spi->dev;
>> --
>> 2.7.4
>>
>> --
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