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...
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. 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. 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(-)
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