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

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Date:   Wed, 14 Dec 2016 14:12:41 +0100
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
Cc:     mark.rutland@....com,
        Jaghathiswari Rankappagounder Natarajan <jaghu@...gle.com>,
        arnd@...db.de, devicetree@...r.kernel.org,
        openbmc@...ts.ozlabs.org, linux@...linux.org.uk,
        linux-kernel@...r.kernel.org, robh+dt@...nel.org, joel@....id.au,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH linux v1 0/4] Seven segment display support

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...
> 
> thanks,
> 
> greg k-h

Hi Greg,

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.

Neil