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Message-ID: <0fb552f0-b069-4641-a5c1-48529b56cdbf@bluematt.me> Date: Thu, 16 Feb 2023 09:54:56 -0800 From: Matt Corallo <ntp-lists@...tcorallo.com> To: chrony-dev@...ony.tuxfamily.org, Miroslav Lichvar <mlichvar@...hat.com> Cc: Richard Cochran <richardcochran@...il.com>, "netdev@...r.kernel.org" <netdev@...r.kernel.org> Subject: Re: [chrony-dev] Support for Multiple PPS Inputs on single PHC On 2/16/23 12:19 AM, Miroslav Lichvar wrote: > On Wed, Feb 15, 2023 at 10:27:15PM -0800, Matt Corallo wrote: >> My naive solution from skimming the code would be to shove >> formerly-discarded samples into a global limited queue and check for >> available timestamps in `phc_poll`. However, I have no idea if the time >> difference between when the sample was taken by the hardware and when the >> `HCL_CookTime` call is done would impact accuracy (or maybe the opposite, >> since we'd then be cooking time with the hardware clock right after taking >> the HCL sample rather than when the PHC timestamp happens), or if such a >> patch would simply be rejected as a dirty, dirty hack rather than unifying >> the PHC read sockets across the devices into one socket (via some global >> tracking the device -> socket mapping?) and passing the samples out >> appropriately. Let me know what makes the most sense here. > > My first thought is that this should be addressed in the kernel, so > even different processes having open the PHC device can receive all > extts samples. If it turns out it's too difficult to do for the > character device (I'm not very familiar with that subsystem), maybe it > could be done at least in sysfs (/sys/class/ptp/ptp*/fifo or a new > file showing the last event like the PPS assert and clear). I mean my first thought seeing an ioctl on a socket that gives an explicit channel and then receives crap from other channels on the same socket was "wtf" so I went and read the kernel to figure out why first to see if its a driver bug. I can't seem to find *any* documentation for how these ioctls are supposed to work, but it seems the "request" here is kinda misnomer, its really a "configure hardware" request, and is unrelated to future reads on the socket, or really the specific socket at all. As for duplicating the output across sockets, ptp_chardev.c's `ptp_read` is pretty trivial - just pop the next sample off the queue and return it. Tweaking that to copy the sample into every reader is probably above my paygrade (and has a whole host of leak risk I'd probably screw up). `extts_fifo_show` appears to be functionally identical. I've CC'd the MAINTAINERs for ptp to see what they think about this, though it won't let chrony support this without a kernel upgrade - not sure if that's an issue for chrony or not. Matt
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