Re: [PATCH 2/2] misc: Add initial Digital Timing Engine (DTE) driver for cygnus

From: Richard Cochran
Date: Wed May 13 2015 - 11:35:57 EST

On Fri, May 08, 2015 at 01:02:17PM -0700, Jonathan Richardson wrote:
> For the clock functions I think we can use the existing framework
> unchanged with one exception: ptp_clock_adjtime() doesn't allow negative
> time adjustments and we would like to allow this.


* struct ptp_clock_info - decribes a PTP hardware clock

* @adjtime: Shifts the time of the hardware clock.
* parameter delta: Desired change in nanoseconds.

int (*adjtime)(struct ptp_clock_info *ptp, s64 delta);

That s64 is 's' as in "signed".

> IRQ interval: I mentioned before that we may be able to calculate the
> isochronous interrupt rate automatically but this isn't possible because
> the DTE doesn't know the frequency of the clients. One solution is to
> use the 'PTP_PEROUT_REQUEST' ioctl to set a periodic timer frequency.
> Not really a timer but good enough for our purposes.

As I said in my other reply, I don't understand what the problem is.

> Set divider: There is no ability to set a frequency or do anything to a
> channel. We could re-use the PTP_EXTTS_REQUEST ioctl but extend 'struct
> ptp_extts_request' by using the reserved field rsv to allow setting an
> integer value representing either a frequency or divider value in our
> case - some value to configure a channel. A new flag would have to be
> added to the existing PTP_ENABLE_FEATURE, RISING and FALLING EDGE.

I don't get this, either.

> Get timestamp: This is a bit more complicated. Currently the PTP driver
> does list management for timestamps from external timestamp channels.
> Timestamps from all channels go into the same list. In our driver we
> have a s/w FIFO for each client and it closely matches the h/w FIFO and
> handles any overflow. We would like to keep it this way because it also
> allows multiple user space processes to only fetch timestamps for the
> client it's handling.

But having many readers is less efficient and more complex.

Also, we can adjust the buffer if needed to prevent HW FIFO overflows.

> We could add a new ioctl to get a timestamp from
> the driver instead of doing it through ptp_read() but it would be nice
> if we could let ptp_read() allow the driver to do timestamp management
> instead of PTP. Maybe provide an option to obtain the timestamps from a
> container in the driver instead of the one managed by PTP. I like being
> able to use read/poll to obtain data instead of polling the kernel with
> ioctls as we are currently doing.

The PTP interface supports poll/read just fine already.

> Also, avoiding the kmalloc in ptp_read
> would be nice because this of the frequency it would be called at. Do
> you have any preference on how to handle this?

Originally I had the buffer on the stack, but DaveM didn't like it,
saying performance is no excuse for not doing it "the right way".

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