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

From: Jonathan Richardson
Date: Wed May 13 2015 - 15:50:13 EST

On 15-05-13 08:35 AM, Richard Cochran wrote:
> 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".

ptp_clock_adjtime() casts it to an unsigned and returns an error:

if ((unsigned long) ts.tv_nsec >= NSEC_PER_SEC)
return -EINVAL;
>> 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.

See reply to previous email. We can use this ioctl or add a new one as
Arnd suggested. It doesn't matter to me.

>> 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.

See reply to previous email.

>> 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.

Yes that's why I wanted to re-use it. As it currently is, it's not going
to work for reasons I explained in previous follow up yesterday:

>> 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".
> Thanks,
> Richard

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