Re: [Ksummit-discuss] [TECH TOPIC] Sensors and similar - subsystem interactions, divisions, bindings etc.

[Hans Verkuil <hverkuil@xs4all.nl>]

On 07/28/2016 06:42 PM, Laurent Pinchart wrote:
> On Thursday 28 Jul 2016 08:58:46 Mauro Carvalho Chehab wrote:
>> Em Wed, 20 Jul 2016 22:18:11 +0100 Jonathan Cameron escreveu:
>>> Hi All,
>>>
>>> This topic would be around the way the various subsystems interact, in the
>>> rough area of 'sensors' (I haven't yet had much of an issue with subsystem
>>> crossing with output devices but maybe that's just over the next hill!)
>>
>> Yeah, there is a gray area here as devices become more complex.
>> So, I'm interested on such topic.
>>
>>> Scope may well be wider but includes:
>>> * input (some of it)
>>> * hwmon
>>> * iio
>>> * comedi(?)
>>> * thermal
>>> * power/battery
>>> * gpio - the blur between gpios and beaglescope / PLC type I/O.
>>> * v4l - when does a device jump from being a multi pixel thermopile
>>> to a thermal camera? Smart fingerprint scanners etc.  Gesture recognition
>>> sensors (effectively 9ish pixel cameras)
>>
>> For devices that provide images somehow, I'd say that the best would be
>> to implement them as via the V4L2 API.
> 
> This requires defining what an image is. Furthermore, we very low image 
> resolution devices, we will need to deal with high frame rates (1k-10k is a 
> range we need to consider). The V4L2 API will likely show performance issues.

The main problems there are the limit to the number of buffers (32) in v4l2 and
that the number of qbuf/dqbuf ioctls is large for these high framerates. You would
probably want to introduce mqbuf/mdqbuf ioctls to queue/dequeue multiple buffers.
The core internals of v4l2 (vb2) are all pretty low overhead.

Regards,

	Hans

> 
>> We'll likely need to discuss it case by case, specially for early cross-
>> subsystem drivers, though, as it is not trivial to identify the subsystem
>> boundaries, and sometimes multiple APIs from different subsystems is needed.
>> Also, it is not clear where such drivers would fit at the Kernel tree.
>>
>> One interesting case is an input device driver with multi-finger
>> support (drivers/input/touchscreen/sur40.c). I suspect we'll have
>> more cases like that over time.
>>
>>> * Lots of random things we haven't seen yet.
>