Question: Linux changes required for off control board RTC

Question: Linux changes required for off control board RTC

[Tracy Smith]

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.

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?

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?

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

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?

thx,
Tracy

Re: Question: Linux changes required for off control board RTC

[Alexandre Belloni]

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