Re: [RFC PATCH 0/2] raw read performance improvement

[Jonathan Cameron <jic23@kernel.org>]

On 28/06/14 14:16, Srinivas Pandruvada wrote:
>
> On 06/28/2014 01:49 AM, Jonathan Cameron wrote:
>> On 27/06/14 01:40, Srinivas Pandruvada wrote:
>>> We see several performance issues reading individual axis, when
>>> raw read is the only option. Since most of the time user space reads x, y
>>> and z, one after other, this results in three sysfs reads, and based on
>>> chipset we have to power up or set up measurement mode and wait for
>>> response.
>> That's probably a case for a change in the driver to schedule the power
>> down for a little period after the read rather than immediately...
> Agree, if we can do that. But some chips have a single measurement mode,
> after measurement is done they go to sleep mode, like AK8975 (they
> do have other modes with power penalty).
>>> For example reading x,y and z takes 160ms for ak8975, it is true for
>>> other too. But reading together takes only 80ms.
>>>
>>> IIO types already defined a modifier for X_AND_Y_AND_Z, which is used
>>> by one driver to send event code. This modifier has no value assigned
>>> so (null) appears in sysfs. If this is not correct then we may need
>>> another modifier.
>>>
>>> Since we have now raw_read with capability to read multiple values,
>>> we can use this callback to return values to iio core.
>>>
>> I've actually argued fairly strongly against this sort of multiple
>> channel simply because it means there are multiple ways of presenting the
>> same data in the ABI. The big difference with the Quaternion case is that
>> the elements of that have no meaning whatsoever if you don't have them
>> all.
>>
>> Previously any indication that people wanted higher performance reads
>> would mean that I'd just tell them that is what the chardev interface
>> is for.  Here a simple sysfs trigger and a couple of extra lines in the
>> driver and you'd have what you want with considerably less overhead.
> But using these I/F, you loose IIO ABI advantage for user space. This was
> the model followed and still followed by, but requires special user space changes
> for every sensor driver. With IIO model, we let vendors pick their components
> as long as they have an IIO driver, with no change in user space.
I'm unclear by why this needs special purpose userspace.
Generally it's trivial to establish if a part supplies it's own
trigger (data ready for example). If not then a sysfs trigger
or similar is needed and can be instantiated just fine by
generic userspace.  
>>
>> Hence my initial thought to this patch is that I'm not keen. However,
>> lets let it sit for a while and see what other comments people have.
>> Perhaps if there is enough demand I might rethink my position.
>>
> Let's see, I feel that it will help for phone/tablet like power sensitive devices but demands performance.
Sure - though you are never going to get significant performance through
sysfs interfaces...  They just aren't intended for that purpose.  Last
thing we want to do is convert back and forwards from a string for starters!

J
>
> Thanks,
> Srinivas
>
>> Jonathan
>>> Srinivas Pandruvada (2):
>>>    iio: core: add xyz modifier value
>>>    iio: magnetometer: ak8975: Improve performance of raw reads
>>>
>>>   drivers/iio/industrialio-core.c   |  1 +
>>>   drivers/iio/magnetometer/ak8975.c | 58 ++++++++++++++++++++++++++-------------
>>>   2 files changed, 40 insertions(+), 19 deletions(-)
>>>
>>
>>
>