linux-kernel - Re: [PATCH] media: uvcvideo: Add boottime clock support

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Message-ID: <13883852.6N9L7C0n48@avalon>
Date:   Wed, 17 Oct 2018 11:02:40 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Heng-Ruey Hsu <henryhsu@...omium.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        jcliang@...omium.org, tfiga@...omium.org
Subject: Re: [PATCH] media: uvcvideo: Add boottime clock support

Hi Heng-Ruey,

Thank you for the patch.

On Wednesday, 17 October 2018 10:52:42 EEST Heng-Ruey Hsu wrote:
> Android requires camera timestamps to be reported with
> CLOCK_BOOTTIME to sync timestamp with other sensor sources.

What's the rationale behind this, why can't CLOCK_MONOTONIC work ? If the 
monotonic clock has shortcomings that make its use impossible for proper 
synchronization, then we should consider switching to CLOCK_BOOTTIME globally 
in V4L2, not in selected drivers only.

> Signed-off-by: Heng-Ruey Hsu <henryhsu@...omium.org>
> ---
>  drivers/media/usb/uvc/uvc_driver.c | 4 ++++
>  drivers/media/usb/uvc/uvc_video.c  | 2 ++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/drivers/media/usb/uvc/uvc_driver.c
> b/drivers/media/usb/uvc/uvc_driver.c index d46dc432456c..a9658f38c586
> 100644
> --- a/drivers/media/usb/uvc/uvc_driver.c
> +++ b/drivers/media/usb/uvc/uvc_driver.c
> @@ -2287,6 +2287,8 @@ static int uvc_clock_param_get(char *buffer, const
> struct kernel_param *kp) {
>  	if (uvc_clock_param == CLOCK_MONOTONIC)
>  		return sprintf(buffer, "CLOCK_MONOTONIC");
> +	else if (uvc_clock_param == CLOCK_BOOTTIME)
> +		return sprintf(buffer, "CLOCK_BOOTTIME");
>  	else
>  		return sprintf(buffer, "CLOCK_REALTIME");
>  }
> @@ -2298,6 +2300,8 @@ static int uvc_clock_param_set(const char *val, const
> struct kernel_param *kp)
> 
>  	if (strcasecmp(val, "monotonic") == 0)
>  		uvc_clock_param = CLOCK_MONOTONIC;
> +	else if (strcasecmp(val, "boottime") == 0)
> +		uvc_clock_param = CLOCK_BOOTTIME;
>  	else if (strcasecmp(val, "realtime") == 0)
>  		uvc_clock_param = CLOCK_REALTIME;
>  	else
> diff --git a/drivers/media/usb/uvc/uvc_video.c
> b/drivers/media/usb/uvc/uvc_video.c index 86a99f461fd8..d4248d5cd9cd 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -425,6 +425,8 @@ static inline ktime_t uvc_video_get_time(void)
>  {
>  	if (uvc_clock_param == CLOCK_MONOTONIC)
>  		return ktime_get();
> +	else if (uvc_clock_param == CLOCK_BOOTTIME)
> +		return ktime_get_boottime();
>  	else
>  		return ktime_get_real();
>  }

-- 
Regards,

Laurent Pinchart