netdev - Re: [RFC PATCH 4/5] PTP: Add flag for non-periodic output

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Message-ID: <20190716163927.GA2125@localhost>
Date:   Tue, 16 Jul 2019 09:39:27 -0700
From:   Richard Cochran <richardcochran@...il.com>
To:     Felipe Balbi <felipe.balbi@...ux.intel.com>
Cc:     netdev@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        "Christopher S . Hall" <christopher.s.hall@...el.com>
Subject: Re: [RFC PATCH 4/5] PTP: Add flag for non-periodic output

On Tue, Jul 16, 2019 at 10:20:37AM +0300, Felipe Balbi wrote:
> When this new flag is set, we can use single-shot output.
> 
> Signed-off-by: Felipe Balbi <felipe.balbi@...ux.intel.com>
> ---
>  include/uapi/linux/ptp_clock.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/ptp_clock.h b/include/uapi/linux/ptp_clock.h
> index 674db7de64f3..439cbdfc3d9b 100644
> --- a/include/uapi/linux/ptp_clock.h
> +++ b/include/uapi/linux/ptp_clock.h
> @@ -67,7 +67,9 @@ struct ptp_perout_request {
>  	struct ptp_clock_time start;  /* Absolute start time. */
>  	struct ptp_clock_time period; /* Desired period, zero means disable. */
>  	unsigned int index;           /* Which channel to configure. */
> -	unsigned int flags;           /* Reserved for future use. */
> +
> +#define PTP_PEROUT_ONE_SHOT BIT(0)
> +	unsigned int flags;           /* Bit 0 -> oneshot output. */
>  	unsigned int rsv[4];          /* Reserved for future use. */

Unfortunately, the code never checked that .flags and .rsv are zero,
and so the de-facto ABI makes extending these fields impossible.  That
was my mistake from the beginning.

In order to actually support extensions, you will first have to
introduce a new ioctl.

Sorry,
Richard

>  };
>  
> -- 
> 2.22.0
>