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

[Jonathan Cameron]

On 28/03/17 19:37, Alison Schofield wrote:
> On Tue, Mar 28, 2017 at 10:55:17PM +0530, SIMRAN SINGHAL wrote:
>> On Fri, Mar 24, 2017 at 12:51 AM, Alison Schofield <amsfield22@xxxxxxxxx> 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.
> 
> I see it protecting this path from being re-entered.  My uncertainty
> is about other paths to read/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.
> 
> My suggestion would be to wait for feedback on Gargi's patch. 
> (See the Outreachy log about creating similar solutions.)
> 
> We will not be able to close on this set of patches during the
> Outreachy application window.  You can continue to push for closure
> beyond the March 30th date as your time allows :)
>
It is a close choice between the two approaches. In some ways
yours is easier to follow, but Gargi's is more elegant.

Lets go with that one for consistency across similar drivers,
but if you had been the original author and done it this way
I certainly wouldn't bother asking you to change it!

So in conclusion both patches are good.

Jonathan
 
> Thanks,
> alisons
> 
>>
>>> alisons
>>>>
>>>> Signed-off-by: simran singhal <singhalsimran0@xxxxxxxxx>
>>>> ---
>>>>
>>>>  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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