linux-kernel - Re: [RFC PATCH 0/3] Introduce IIO interface for fingerprint sensors

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Date:	Tue, 27 Jan 2015 12:32:50 +0100
From:	Sylwester Nawrocki <s.nawrocki@...sung.com>
To:	"Baluta, Teodora" <teodora.baluta@...el.com>,
	Jonathan Cameron <jic23@...nel.org>
Cc:	Mauro Carvalho Chehab <m.chehab@...sung.com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-iio <linux-iio@...r.kernel.org>,
	LMML <linux-media@...r.kernel.org>,
	Hans Verkuil <hverkuil@...all.nl>
Subject: Re: [RFC PATCH 0/3] Introduce IIO interface for fingerprint sensors

Hi,

On 23/01/15 14:05, Baluta, Teodora wrote:
...
>>>>>>> So why not v4l?  These are effectively image sensors..
>>>>>>
>>>>>> Well, here's why I don't think v4l would be the best option:
>>>>>>
>>>>>> - an image scanner could be implemented in the v4l subsystem, but
>>>>>> it seems far more complicated for a simple fingerprint scanner - it
>>>>>> usually has drivers for webcams, TVs or video streaming devices.
>>>>>> The v4l subsystem (with all its support for colorspace, decoders,
>>>>>> image compression, frame control) seems a bit of an overkill for a
>>>>>> very straightforward fingerprint imaging sensor.
>>>
>>>> Whilst those are there, I would doubt the irrelevant bits would put
>>>> much burden on a fingerprint scanning driver.  Been a while since I
>>>> did anything in that area though so I could be wrong!
>>
>> IMO V4L is much better fit for this kind of devices than IIO. You can use just a
>> subset of the API, it shouldn't take much effort to write a simple
>> v4l2 capture driver, supporting fixed (probably vendor/chip specific) image
>> format.  I'm not sure if it's better to use the v4l2 controls [1], define a new
>> v4l2 controls class for the fingerprint scanner processing features, rather than
>> trying to pass raw data to user space and interpret it then in some library.  I
>> know there has been resistance to allowing passing unknown binary blobs to
>> user space, due to possible abuses.
>>
>> [1] Documentation/video4linux/v4l2-controls.txt
>                                                                                                                
> The fingerprint sensor acts more like a scanner device, so the closest type 
> is the V4L2_CAP_VIDEO_CAPTURE. However, this is not a perfect match because
> the driver only sends an image, once, when triggered. Would it be a better
> alternative to define a new capability type? Or it would be acceptable to
> simply have a video device with no frame buffer or frame rate and the user
> space application to read from the character device /dev/videoX?

I don't think a new capability is needed for just one buffer capture.
The capture driver could just support read() and signal it by setting the
V4L2_CAP_READWRITE capability flag [2], [3].

[2]
http://linuxtv.org/downloads/v4l-dvb-apis/vidioc-querycap.html#device-capabilities
[3] http://linuxtv.org/downloads/v4l-dvb-apis/io.html#rw

--
Regards,
Sylwester
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