Re: [RFC 0/8] iio: add support for hardware fifo

[Lars-Peter Clausen <lars@metafoo.de>]

On 11/18/2014 04:03 PM, Octavian Purdila wrote:
> On Tue, Nov 18, 2014 at 3:24 PM, <jic23@kernel.org> wrote:
[...]
>> Hence we started with something that at least superficially (I haven't
>> had a chance to go through the implementation in detail yet)
>> looks similar to what you have but ended up changing the method of
>> signalling to and from userspace.
>> Hardware buffer -> Software buffer -> user space.
>> Userspace watershed control -> Software buffer watershed control
>> -> Hardware buffer watershed control.
>> If userspace sets the watershed to say 16 then, as well as setting
>> that level in the software buffer it should be passed on to the
>> device driver allowing the watershed there to be set appropriately.
>> Now things get interesting if userspace sets the watershed to a value
>> that makes no sense for the hardware (say 17 on a device that does
>> power of 2 values only) as then it will have to fall back to
>> grabbing every one (Watershed of 1).  Perhaps we can provide 'hints'
>> for this?
>
> As far as I understood, the proposed watermark implementation only
> affects the device buffer and as I mentioned above that will not help
> with reducing the interrupt rate.

By setting the watershed level the userspace application tells you that it 
is OK with getting data with a higher latency than one sample. This allows 
the driver to configure the FIFO level and hence reduce the interrupt rate.

- Lars