Subject: Re: [PATCH linux v1 0/4] Seven segment display support
To: Greg KH <gregkh@linuxfoundation.org>,
        Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
References: <1481702104-8617-1-git-send-email-jaghu@google.com>
 <20161214134530.2bd54a4e@free-electrons.com>
 <20161214125641.GA5379@kroah.com>
Cc: mark.rutland@arm.com,
        Jaghathiswari Rankappagounder Natarajan <jaghu@google.com>,
        arnd@arndb.de, devicetree@vger.kernel.org, openbmc@lists.ozlabs.org,
        linux@armlinux.org.uk, linux-kernel@vger.kernel.org,
        robh+dt@kernel.org, joel@jms.id.au,
        linux-arm-kernel@lists.infradead.org
From: Neil Armstrong <narmstrong@baylibre.com>
Organization: Baylibre
Message-ID: <ac324946-41da-c090-a0ca-78155611bb7e@baylibre.com>
Date: Wed, 14 Dec 2016 14:12:41 +0100
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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