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

From: [email-masked] (Jonathan Richardson)
Date: Fri, 8 May 2015 13:02:17 -0700

On 15-05-01 12:40 PM, Arnd Bergmann wrote:
> On Friday 01 May 2015 20:30:04 One Thousand Gnomes wrote:
>>
>>> channel control, unless I'm missing something. If people are flexible
>>> with extending that I could propose something. Let me know which way you
>>> prefer to go. Thanks.
>>
>> I would strongly favour fixing PTP to do this right. Otherwise we will
>> have 2 or 3 adhoc drivers, then end up moving them to PTP and then end up
>> dealing with the compat mess.
> 
> Agreed. I was hoping that there was already a subsystem in the kernel that
> could be extended to work with this driver. If all that is needed is to
> pass more fields of the existing timex to ptp drivers, that should be
> fairly easy to do.
> 
> We also have some padding bytes available in struct timex in case some
> extra data needs to be passed, or we could add another clock_* system call
> if it's absolutely required to have another entry into the driver, and
> connect that through struct k_clock and posix_clock_operations(). I would
> hope we don't even need that.
> 
> 	Arnd
> 

Thanks Arnd/Alan for the feedback. I looked into PTP further to see what
exactly would need changing.

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.

The other client events (set IRQ interval, enable client, set client
divider, get client timestamp) could be handled as follows:

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.

Enable Client: The external timestamping channel handling in PTP allows
channels to be enabled so we can re-use this (PTP_EXTTS_REQUEST ioctl).

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.

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. 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. 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?

I've tried to minimize the changes to PTP.

Alan, what is IIO? I'm not familiar.

Thanks.