linux-kernel - Re: [PATCH 3/4] pps: Use lookup list to reduce ldisc coupling

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Message-ID: <20130206221905.15297.qmail@science.horizon.com>
Date:	6 Feb 2013 17:19:05 -0500
From:	"George Spelvin" <linux@...izon.com>
To:	linux@...izon.com, peter@...leysoftware.com
Cc:	giometti@...eenne.com, gregkh@...uxfoundation.org, jslaby@...e.cz,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH 3/4] pps: Use lookup list to reduce ldisc coupling

> I did this first and it's a mess -- the patch basically ends up looking
> like a rewrite. But feel free to use these patches as a base for a
> version you do like and submit those instead for review. I just wanted
> to show the way.

I wouldn't think so, but I'll give it a try and see myself.  Thanks!

> (Well, actually that was the second version. When I reviewed the
> uart_handle_dcd_change() and saw the separate timestamp, I thought that
> maybe the latency was going to be a problem. So the first version used
> the same approach but with an rcu 'lockless' list instead -- then I went
> back and audited the IRQ path and realized there were 5 bus locks and an
> i/o port read already. So total overkill.)

Er... but you went and captured the timestamp *before* doing the list
lookup.  It was only moved one jsr later.

Really, what I'd *like* to do is to unconditionally capture a *raw*
timestamp (rdtsc or equivalent) very early in the interrupt handling,
for use by random seeding, pps, network timestamps, and so on.  But the
conversion to a "struct timespec" would be deferred until when and if
it was actually needed.

This is complicated because the conversion has to happen "soon" after
the capture, soon enough that the low-level clock that was read has not
wrapped and become ambiguous.

But that's a lot more complicated.

>> A more ambitious cleanup would use the existing pps_device list
>> (maintained to allocate minor device numbers) and add an "owner" field
>> that can be looked up on, without creating a new data structure and
>> allocation.

> Didn't see where that was (unless you mean the IDR allocation).

Exactly the IDR.  You can just do idr_for_each() until you find
the right one.

> Probably best to keep it separate in the event that relative lifetimes
> change at some point in the future.

I don't see how that could plausibly happen.  Currently, a device
is registered in the IDR immediately after allocation, and is freed
immediately before deallocation.  There is no time that it's permitted to
call any kernel PPS API function with a pps_device * that *not* in
the IDR.

Information with a longer life is segregated in the struct
pps_source_info.  (Which is where I was thinging of adding the
parent_dev field.)

> Please let us know if you plan to respin the patches, so these patches
> don't get pushed.

I do, in the next few hours.  Can you give mt until 0600 UTC,
or should I try for faster?
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