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Date: Sun, 19 Mar 2017 17:08:17 +0000
From: Jonathan Cameron <jic23@...nel.org>
To: Gargi Sharma <gs051095@...il.com>
Cc: Lars-Peter Clausen <lars@...afoo.de>,
simran singhal <singhalsimran0@...il.com>,
Michael Hennerich <Michael.Hennerich@...log.com>,
Hartmut Knaack <knaack.h@....de>,
Pete Meerwald-Stadler <pmeerw@...erw.net>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-iio@...r.kernel.org, devel@...verdev.osuosl.org,
linux-kernel@...r.kernel.org, outreachy-kernel@...glegroups.com
Subject: Re: [Outreachy kernel] Re: [PATCH] staging: iio: ade7753: replace
mlock with driver private lock
On 19/03/17 13:16, Gargi Sharma wrote:
> On Sun, Mar 19, 2017 at 4:01 PM, Jonathan Cameron <jic23@...nel.org> wrote:
>> On 17/03/17 09:32, Gargi Sharma wrote:
>>> On Mon, Mar 13, 2017 at 5:30 PM, Lars-Peter Clausen <lars@...afoo.de> wrote:
>>>>
>>>> On 03/12/2017 02:32 PM, 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.
>>>>>
>>>>> Fix some coding style issues related to white space also.
>>>>>
>>>>> Signed-off-by: simran singhal <singhalsimran0@...il.com>
>>>>> ---
>>>>> drivers/staging/iio/meter/ade7753.c | 14 ++++++++------
>>>>> 1 file changed, 8 insertions(+), 6 deletions(-)
>>>>>
>>>>> diff --git a/drivers/staging/iio/meter/ade7753.c b/drivers/staging/iio/meter/ade7753.c
>>>>> index dfd8b71..ca99d82 100644
>>>>> --- a/drivers/staging/iio/meter/ade7753.c
>>>>> +++ b/drivers/staging/iio/meter/ade7753.c
>>>>> @@ -81,12 +81,14 @@
>>>>> * @tx: transmit buffer
>>>>> * @rx: receive buffer
>>>>> * @buf_lock: mutex to protect tx and rx
>>>>> + * @lock: protect sensor state
>>>>
>>>> It might make sense to reuse the existing lock which currently protects the
>>>> read/write functions. You can do this by introducing a variant of
>>>> ade7753_spi_{read,write}_reg_16() that does not take a lock and use these to
>>>> implement the read-modify-write cycle in a protected section.
>>>
>>> There are other read/write functions for example,
>>> ade7753_spi_{read/write}_reg_8 that use the mutex as well. Should a
>>> variant of these functions be introduced as well? Also, how does one
>>> go about implementing RMW inside a protected section.
>> Hmm. Simran has also been progressing with patches for this.
>>
> I was trying to work through a patch for ade7754. So ran into the same
> problem :)
>
>> You raise a good question. There are other read/modify/write sequences in
>> the driver. They don't have the same issue with potentially deadlocking
>> against the buf lock as they are all using the spi subsystems provisions
>> for small write/read cycles where buffer protection is handled internally.
>>
>> So let us address the cases in turn:
>>
>> static int ade7753_reset(struct device *dev)
>> {
>> u16 val;
>> int ret;
>>
>> ret = ade7753_spi_read_reg_16(dev, ADE7753_MODE, &val);
>> if (ret)
>> return ret;
>>
>> val |= BIT(6); /* Software Chip Reset */
>>
>> return ade7753_spi_write_reg_16(dev, ADE7753_MODE, val);
>> }
>> This is only called in the device initialization. At that point
>> we should be fine in assuming no parallel calls. Crucial point
>> is it is before the call to iio_device_register which exposes
>> the userspace interfaces.
>>
>> static int ade7753_set_irq(struct device *dev, bool enable)
>> {
>> int ret;
>> u8 irqen;
>>
>> ret = ade7753_spi_read_reg_8(dev, ADE7753_IRQEN, &irqen);
>> if (ret)
>> goto error_ret;
>>
>> if (enable)
>> irqen |= BIT(3); /* Enables an interrupt when a data is
>> * present in the waveform register
>> */
>> else
>> irqen &= ~BIT(3);
>>
>> ret = ade7753_spi_write_reg_8(dev, ADE7753_IRQEN, irqen);
>>
>> error_ret:
>> return ret;
>> }
>>
>> This one is actually safe because it is the only function that
>> modifies that particular register.
>>
>> /* Power down the device */
>> static int ade7753_stop_device(struct device *dev)
>> {
>> u16 val;
>> int ret;
>>
>> ret = ade7753_spi_read_reg_16(dev, ADE7753_MODE, &val);
>> if (ret)
>> return ret;
>>
>> val |= BIT(4); /* AD converters can be turned off */
>>
>> return ade7753_spi_write_reg_16(dev, ADE7753_MODE, val);
>> }
>>
>> Only called in remove (after userspace interfaces have been
>> removed by the iio_device_unregister call so also should not
>> be running concurrently with much else.
>>
>
> The only nested lock here is ade7754_spi_write_reg_16, so as long as
> that is refactored, it'll be fine.
>
>> So I think all the other cases are safe. Perhaps it would have
>> been better to have had a lock around them, purely to make
>> the code more resilient against future changes though.
>> Probably a job to do as part of a larger scale pile of work
>> on that driver rather than as a one off patch.
>
> Another question that I have is why are we writing inside a read
> function(ade7754_spi_read_reg_24)?
>
It's a register read (sort of) hence the reg in the name.
It's telling it which register to read by first writing that.
> static int ade7754_spi_read_reg_24(struct device *dev,
> u8 reg_address, u32 *val)
> {
> struct iio_dev *indio_dev = dev_to_iio_dev(dev);
> struct ade7754_state *st = iio_priv(indio_dev);
> int ret;
> struct spi_transfer xfers[] = {
> {
> .tx_buf = st->tx,
> .rx_buf = st->rx,
> .bits_per_word = 8,
> .len = 4,
> },
> };
>
> mutex_lock(&st->buf_lock);
> st->tx[0] = ADE7754_READ_REG(reg_address);
> st->tx[1] = 0;
> st->tx[2] = 0;
> st->tx[3] = 0;
>
> ret = spi_sync_transfer(st->us, xfers, ARRAY_SIZE(xfers));
> if (ret) {
> dev_err(&st->us->dev, "problem when reading 24 bit
> register 0x%02X",
> reg_address);
> goto error_ret;
> }
> *val = (st->rx[1] << 16) | (st->rx[2] << 8) | st->rx[3];
>
> error_ret:
> mutex_unlock(&st->buf_lock);
> return ret;
> }
>
> Thanks!
> Gargi
>
>>
>> Jonathan
>>
>>
>>
>>
>>
>>
>>
>>>
>>>
>>>>
>>>> Looking through the driver there seem to be other places as well that do
>>>> read-modify-write that should be protected by a lock, but currently are not.
>>>> This might be a good task.
>>>
>>> Am I right in understanding that we want to introduce mutex lock for
>>> writes in other drivers as well?
>>>
>>> Thanks,
>>> Gargi
>>>>
>>>>> **/
>>>>> struct ade7753_state {
>>>>> - struct spi_device *us;
>>>>> - struct mutex buf_lock;
>>>>> - u8 tx[ADE7753_MAX_TX] ____cacheline_aligned;
>>>>> - u8 rx[ADE7753_MAX_RX];
>>>>> + 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];
>>>>> };
>>>>>
>>>>> static int ade7753_spi_write_reg_8(struct device *dev,
>>>>> @@ -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;
>>>>> }
>>>>>
>>>>
>>>> --
>>> --
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