linux-kernel - Re: Question: Linux changes required for off control board RTC

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Date:   Tue, 21 Mar 2017 19:37:14 +0100
From:   Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:     Tracy Smith <tlsmith3777@...il.com>
Cc:     Lee Jones <lee.jones@...aro.org>,
        lkml <linux-kernel@...r.kernel.org>, rtc-linux@...glegroups.com
Subject: Re: Question: Linux changes required for off control board RTC

Hi,

On 21/03/2017 at 13:08:54 -0500, Tracy Smith wrote:
> Hi, not sure if this is the correct venue to post a question, so
> please forgive and direct me to the correct board or list if not.
> 

Lee is probably not the one to ask :)

> A custom board implementation using a ARM-8 Cortex A53 NXP LS1043ardb
> is considering moving the RTC from the control board to an FPGA.
> Reason is for accuracy of time/date and efficiency for their FPGA/DSP
> application.
> 
> 1) What kernel changes would be required to support such an
> implementation and these would need to be pushed to the community
> under the GPL?
> 

You'll have to write a proper driver. It will have to be licensed under
the GPLv2 but you don't necessarily have to push it anywhere (that is
not how the GPL is working).

> 2) Is it a trivial task to make such changes in the kernel to support
> an RTC not located on the control board/SoC itself?
> 

RTC drivers are not so complicated. I'd say that the difficulty mainly
depends on how the SoC can access the FPGA.


> 3) What is the expected behavior of the kernel with no changes added
> to accommodate such an architecture?
> 

The kernel will work, the RTC is not required.

> 4) Is there a configurable kernel option to allow for such a change,
> or have these or similar changes already been added to the kernel?
> 

More details are needed to answer that but basically if your FPGa
emulates an RTC that is already supported, you don't need to write
much.

-- 
Alexandre Belloni, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com