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

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Date:	Mon, 26 Jan 2015 10:24:44 +0000
From:	"Baluta, Teodora" <teodora.baluta@...el.com>
To:	"floe@...terbrot.org" <floe@...terbrot.org>
CC:	"s.nawrocki@...sung.com" <s.nawrocki@...sung.com>,
	"lars@...afoo.de" <lars@...afoo.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jic23@...nel.org" <jic23@...nel.org>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
	"m.chehab@...sung.com" <m.chehab@...sung.com>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
	"hverkuil@...all.nl" <hverkuil@...all.nl>
Subject: Re: [RFC PATCH 0/3] Introduce IIO interface for fingerprint sensors

Hi Florian,

Thanks for the reply!

On Vi, 2015-01-23 at 17:46 +0100, Florian Echtler wrote:
> Hello Teodora,
> 
> On 23.01.2015 14:05, Baluta, Teodora wrote:
> > 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?
> Sorry if I jump in here right in the middle of this discussion, but some
> time ago, I wrote a fingerprint sensor driver for the Siemens ID Mouse
> (still part of the kernel AFAICT) which acts as a misc device and just
> creates a character device node that can be used to directly read a PGM
> file.

I agree that it would be the simplest approach if you want a USB
fingerprint driver in kernel space, but I was looking for a way to
support fingerprint devices regardless of bus (there's also fingerprint
devices that use SPI) and provide a common interface for user space
applications.

The actual driver for UPEK's TouchChip sensor is just a proof of concept
on how the interface would work, rather than the final goal. There's
already a library that enables USB-only fingerprint devices (libfprint).
> 
> Maybe this would be a slightly simpler approach than pulling in all the
> streaming-optimized features of V4L2?

I agree that for a fingerprint device, the V4L2 subsystem offers too
much (the initial RFC is for IIO).

Thanks,
Teodora