linux-kernel - Re: [PATCH 4/6] iio: accel: bmg160: optimize transfers in trigger handler

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Date:	Mon, 28 Mar 2016 15:14:43 +0100
From:	Jonathan Cameron <jic23@...nel.org>
To:	Peter Meerwald-Stadler <pmeerw@...erw.net>
Cc:	Irina Tirdea <irina.tirdea@...el.com>, linux-iio@...r.kernel.org,
	linux-kernel@...r.kernel.org, Hartmut Knaack <knaack.h@....de>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Octavian Purdila <octavian.purdila@...el.com>,
	Markus Pargmann <mpa@...gutronix.de>,
	Srinivas Pandruvada <srinivas.pandruvada@...el.com>
Subject: Re: [PATCH 4/6] iio: accel: bmg160: optimize transfers in trigger
 handler

On 28/03/16 11:09, Peter Meerwald-Stadler wrote:
> 
>>> Some i2c busses (e.g.: Synopsys DesignWare I2C adapter) need to
>>> enable/disable the bus at each i2c transfer and must wait for
>>> the enable/disable to happen before sending the data.
>>>
>>> When reading data in the trigger handler, the bmc150 accel driver does
> 
> should refer to bmg160
Good spot.  It's also a gyroscope, not an accelerometer... I just fixed this
up and repushed out testing.
> 
>>> one bus transfer for each axis. This has an impact on the frequency
>>> of the accelerometer at high sample rates due to additional delays
>>> introduced by the bus at each transfer.
>>>
>>> Reading all axis values in one bus transfer reduces the delays
>>> introduced by the bus.
>>>
>>> Signed-off-by: Irina Tirdea <irina.tirdea@...el.com>
>> I forgot to highlight on the earlier driver that there is also 'technically'
>> a bit of an ABI change here because we are now exporting as LE rather than CPU
>> order.  However, I 'hope' anyone actually accessing the buffered data is either
>> doing it through a nice library or hasn't hacked the endian unwinding out of
>> the generic_buffer example!
> 
> the patch takes away the possibility to do buffered reads on individual 
> channels (not sure if this is useful per se)
> 
> this optimizes for the common case, ok;
> 
> wondering if adding 
> .endianness = IIO_LE
> is actually an unrelated fix
Good point, when I first read the code I assumed we were moving from an i2c_word
read to a bulk read, thus necessitating this addition.  However, we aren't as it
was previously as an i2c_bulk read of 2 bytes... 

Irina, could you confirm if this was broken before your patches?

I'll leave this as is, perhaps we need an additional fix patch specifying LE to
put out as a fix.
> 
>> Again, fingers crossed this doesn't break anything significant.
>>
>> Applied,
>>
>> Thanks,
>>
>> Jonathan
>>> ---
>>>  drivers/iio/gyro/bmg160_core.c | 17 ++++++-----------
>>>  1 file changed, 6 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/iio/gyro/bmg160_core.c b/drivers/iio/gyro/bmg160_core.c
>>> index 8d6e5b1..43570b8 100644
>>> --- a/drivers/iio/gyro/bmg160_core.c
>>> +++ b/drivers/iio/gyro/bmg160_core.c
>>> @@ -734,6 +734,7 @@ static const struct iio_event_spec bmg160_event = {
>>>  		.sign = 's',						\
>>>  		.realbits = 16,					\
>>>  		.storagebits = 16,					\
>>> +		.endianness = IIO_LE,					\
>>>  	},								\
>>>  	.event_spec = &bmg160_event,					\
>>>  	.num_event_specs = 1						\
>>> @@ -773,20 +774,14 @@ static irqreturn_t bmg160_trigger_handler(int irq, void *p)
>>>  	struct iio_poll_func *pf = p;
>>>  	struct iio_dev *indio_dev = pf->indio_dev;
>>>  	struct bmg160_data *data = iio_priv(indio_dev);
>>> -	int bit, ret, i = 0;
>>> -	unsigned int val;
>>> +	int ret;
>>>  
>>>  	mutex_lock(&data->mutex);
>>> -	for (bit = 0; bit < AXIS_MAX; bit++) {
>>> -		ret = regmap_bulk_read(data->regmap, BMG160_AXIS_TO_REG(bit),
>>> -				       &val, 2);
>>> -		if (ret < 0) {
>>> -			mutex_unlock(&data->mutex);
>>> -			goto err;
>>> -		}
>>> -		data->buffer[i++] = ret;
>>> -	}
>>> +	ret = regmap_bulk_read(data->regmap, BMG160_REG_XOUT_L,
>>> +			       data->buffer, AXIS_MAX * 2);
>>>  	mutex_unlock(&data->mutex);
>>> +	if (ret < 0)
>>> +		goto err;
>>>  
>>>  	iio_push_to_buffers_with_timestamp(indio_dev, data->buffer,
>>>  					   pf->timestamp);
>>>
>>
>