Re: [PATCH linux v1 0/4] Seven segment display support

From: David Daney
Date: Wed Dec 14 2016 - 16:10:51 EST

On 12/14/2016 06:15 AM, Arnd Bergmann wrote:
On Wednesday, December 14, 2016 2:12:41 PM CET Neil Armstrong wrote:
On 12/14/2016 01:56 PM, Greg KH wrote:
On Wed, Dec 14, 2016 at 01:45:30PM +0100, Thomas Petazzoni wrote:
Hello,

On Tue, 13 Dec 2016 23:55:00 -0800, Jaghathiswari Rankappagounder
Natarajan wrote:

Documentation for the binding which provides an interface for adding clock,
data and clear signal GPIO lines to control seven segment display.

The platform device driver provides an API for displaying on two 7-segment
displays, and implements the required bit-banging. The hardware assumed is
74HC164 wired to two 7-segment displays.

The character device driver implements the user-space API for letting a user
write to two 7-segment displays including any conversion methods necessary
to map the user input to two 7-segment displays.

Adding clock, data and clear signal GPIO lines in the devicetree to control
seven segment display on zaius platform.

The platform driver matches on the device tree node; the platform driver also
initializes the character device.

Tested that the seven segment display works properly by writing to the
character device file on a EVB AST2500 board which also has 74HC164 wired
to two 7-segment displays.

FWIW, I proposed a driver for seven segment displays back in 2013:

http://lists.infradead.org/pipermail/linux-arm-kernel/2013-January/139986.html

And the feedback from Greg KH was: we don't need a driver for that, do
it from userspace. See:

http://lists.infradead.org/pipermail/linux-arm-kernel/2013-January/139992.html

So: good luck

Did anyone ever write a library for this type of thing?

Again, I don't want to see one-off drivers for random devices like this
that should be able to all be controlled from userspace in a common
manner. Much like we did for fingerprint readers a long long time
ago...


Actually, it's more than a random interface, a lot of SoCs and boards actually have such displays
and it's a pity to use UIO, sysfs gpio bitbanging and all sort of ugly stuff to only print a few
characters a simple and clean driver could achieve.
Some very well known SoCs even have integrated registers to lower the BOM and bypass the need for
a 74HC164 like component and avoid gpio bit banging.

My personal concern is that you could also need to drive more segments, thus 7-segments
is too restrictive.

But this driver is well structured, the gpio-bitbanging sub-driver is welcome.

Maybe we can find a way to fit this into the existing drivers/leds/ subsystem?

That already supports blinking, brightness and colour attributes of LEDs,
so could this be extended to support (one of) digit, number, character
or string with a common sysfs attribute and a way to hook up a led driver
to that?

We have a lot of boards with an 8-cell dot matrix LED. Each cell is programmed with an 8-bit value. The mapping of these values to the dots defaults to ASCII character rendering, but there is the facility to install other bitmaps as well.

Really I view these things not as part of the LED subsystem, but more as a very small frame buffer.

We like to display entire words, and the most useful interface from a user point of view is something that consumes entire strings rather than having to manage each cell independently.

You could imagine that if the text to be displayed were longer than the display, that the driver would make it continuously scroll. I would like to see a framework where a simple character device were exposed, and from userspace you could do: "echo message > /dev/small-display" and get something sensible.


Arnd

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