linux-kernel - Re: [PATCH] pps: Add support for read operations and return a useful value in poll

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Message-ID: <CABkLObqCWn3WZzs-eMYaBAOYuM6VKxL1Cg1j9Ft05JAjENJXnw@mail.gmail.com>
Date:	Wed, 1 Apr 2015 14:01:36 +0200
From:	Christian Riesch <christian.riesch@...cron.at>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rodolfo Giometti <giometti@...eenne.com>
Subject: Re: [PATCH] pps: Add support for read operations and return a useful
 value in poll

[sent again with Rodolfo in the list of recipients, something went
wrong, sorry!]

Hi Rodolfo,

Thanks for your reply!

On Wed, Apr 1, 2015 at 10:11 AM, Rodolfo Giometti <giometti@...eenne.com> wrote:
> On Tue, Mar 31, 2015 at 11:31:22PM +0200, Christian Riesch wrote:
>> The PPS_FETCH ioctl in drivers/pps/pps.c blocks until a new PPS event
>> occurs, then returns the time stamp data. While this is fine for
>> lots of applications, sometimes it would be nice if the poll system
>> call and a subsequent read could be used to obtain the pps data.
>
> Nak. The read syscall can't be forced to return fix amount of
> data.

I just copied/modified the behavior of ptp_read() in drivers/ptp/ptp_chardev.c:

if (cnt % sizeof(struct ptp_extts_event) != 0)
        return -EINVAL;

There the amount of data is forced to be a multiple of sizeof(struct
ptp_extts_event). But if you prefer an ioctl, I can change that of
course.

> Use a dedicated ioctl instead.

Is it ok to pair POLLIN | POLLRDNORM in the poll function with an
ioctl? Or should returning POLLIN | POLLRDNORM mean that a read() will
not block? Should I use POLLPRI instead or something else?

Regards, Christian
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