linux-kernel - Re: [PATCH] pps: add epoll support

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Message-ID: <a0f5aff2-3437-4b70-baa0-fe1555ec81e7@enneenne.com>
Date: Tue, 25 Feb 2025 15:24:10 +0100
From: Rodolfo Giometti <giometti@...eenne.com>
To: Denis OSTERLAND-HEIM <denis.osterland@...hl.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pps: add epoll support

On 25/02/25 14:39, Denis OSTERLAND-HEIM wrote:
> Hi,
> 
> I fixed-up s/pps->fetched_ev/pps->last_fetched_ev/ and tested it.
> 
> With one process it works well.
> 
> ```
> # cat /sys/class/pps/pps1/assert; PpsPollTest /dev/pps1; cat /sys/class/pps/pps1/assert
> 1520599383.268749168#29
> assert: 29
> time: 1520599383.268749168
> assert: 30
> time: 1520599384.292728352
> assert: 31
> time: 1520599385.316788416
> 1520599385.316788416#31
> ```

OK.

> If I start multiple user space programs data races are visible.
> 
> ```
> # for i in 0 1 2 3 4 5 6; do PpsPollTest /dev/pps1 > log$i & done
> # sleep 6
> # tail log*
> ==> log0 <==
> timeout
> assert: 196
> time: 1520599554.276752928
> assert: 197
> time: 1520599555.300692704

This is the same behavior we have when working with a serial port: if more than 
one process gets access to it, data is stolen.

>>>From my point of view it would be great to fix this bug without such an limitation.

I disagree, it is not a limitation! It is like a normal char device work.

What we have to test now is if your initial goal has been addressed! That is, in 
an application that has more to do than just dealing with one PPS device, we can 
use poll()/select() in order to avoid using threads.

Ciao,

Rodolfo

-- 
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