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

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

On 11/18/2014 06:04 PM, Octavian Purdila wrote:
> On Tue, Nov 18, 2014 at 6:44 PM, Lars-Peter Clausen <lars@metafoo.de> wrote:
>> 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.
>>
>
> Hi Lars,
>
> The implementation (as proposed in the patch by Josselin and Yannick)
> does not inform the driver of changes to watermark, that is only
> visible to core iio / buffer logic.

That should be trivial to add though.

>
> Also, the watermark alone is not enough to properly support hardware
> fifos: the fifo can operate in multiple modes, we need to read data
> from the hardware fifo even when the watermark interrupt is not issued
> (the flush operation in the current patch set).

What kind of modes are there?

- Lars