Re: [PATCH] staging: ade7754: Replace mlock with buf_lock and refactor code

[Jonathan Cameron <jic23@kernel.org>]

On 21/03/17 19:52, Gargi Sharma wrote:
> On Wed, Mar 22, 2017 at 1:15 AM, Jonathan Cameron <jic23@kernel.org> wrote:
>> On 20/03/17 07:39, Gargi Sharma 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 buf_lock in the devices global data.
>>>
>>> As buf_lock already protects operations in ade7754_write_frequency,
>>> there isn't a need to acquire the lock inside ade7754_spi_write_reg_8
>>> when writing to the register.
>>>
>>> Signed-off-by: Gargi Sharma <gs051095@gmail.com>
>> Question inline.
>>> ---
>>>  drivers/staging/iio/meter/ade7754.c | 13 +++++++++++--
>>>  1 file changed, 11 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/staging/iio/meter/ade7754.c b/drivers/staging/iio/meter/ade7754.c
>>> index 024463a..eb03469 100644
>>> --- a/drivers/staging/iio/meter/ade7754.c
>>> +++ b/drivers/staging/iio/meter/ade7754.c
>>> @@ -29,6 +29,15 @@ static int ade7754_spi_write_reg_8(struct device *dev, u8 reg_address, u8 val)
>>>       struct iio_dev *indio_dev = dev_to_iio_dev(dev);
>>>       struct ade7754_state *st = iio_priv(indio_dev);
>>>
>>> +     if (reg_address == ADE7754_WAVMODE) {
>>> +             st->tx[0] = ADE7754_WRITE_REG(reg_address);
>>> +             st->tx[1] = val;
>>> +
>>> +             ret = spi_write(st->us, st->tx, 2);
>>> +
>>> +             return ret;
>>> +     }
> 
>> This block doesn't fit with the description.  What's going on here?
> 
> When the function ade_spi_write_reg_8() is called inside
> ade7754_write_frequency(), we are writing to this( ADE7754_WAVMODE)
> register. When writing to this register we don’t need to hold the
> buf_lock since ade7754_write_frequency() already takes care of that.
> This block of code does not use the buf_lock since that is taken care
> of by ade7754_write_frequency().
This strikes me as a very fragile way of coding this.

Also, unless I'm missing something, the lock taken there is still mlock...
> 
> Gargi
>>> +
>>>       mutex_lock(&st->buf_lock);
>>>       st->tx[0] = ADE7754_WRITE_REG(reg_address);
>>>       st->tx[1] = val;
>>> @@ -430,7 +439,7 @@ static ssize_t ade7754_write_frequency(struct device *dev,
>>>       if (!val)
>>>               return -EINVAL;
>>>
>>> -     mutex_lock(&indio_dev->mlock);
>>> +     mutex_lock(&st->buf_lock);
>>>
>>>       t = 26000 / val;
>>>       if (t > 0)
>>> @@ -451,7 +460,7 @@ static ssize_t ade7754_write_frequency(struct device *dev,
>>>       ret = ade7754_spi_write_reg_8(dev, ADE7754_WAVMODE, reg);
>>>
>>>  out:
>>> -     mutex_unlock(&indio_dev->mlock);
>>> +     mutex_unlock(&st->buf_lock);
>>>
>>>       return ret ? ret : len;
>>>  }
>>>
>>
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