linux-kernel - Re: [PATCH v6 03/11] Documentation: uAPI: media: add V4L2_CID_FLASH

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Message-ID: <rcgche43hzctpxbe2xt2cfksjbtyntc6ef26wptgkygcdlg5ga@sdhkkcpdfaek>
Date: Mon, 1 Sep 2025 12:44:07 +0200
From: Richard Leitner <richard.leitner@...ux.dev>
To: Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc: Dave Stevenson <dave.stevenson@...pberrypi.com>, 
	Mauro Carvalho Chehab <mchehab@...nel.org>, Lee Jones <lee@...nel.org>, Pavel Machek <pavel@...nel.org>, 
	Laurent Pinchart <laurent.pinchart@...asonboard.com>, Hans Verkuil <hverkuil@...all.nl>, linux-media@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-leds@...r.kernel.org
Subject: Re: [PATCH v6 03/11] Documentation: uAPI: media: add
 V4L2_CID_FLASH_DURATION

On Mon, Aug 25, 2025 at 08:42:17AM +0300, Sakari Ailus wrote:
> Hi Richard,
> 
> Thanks for the update (and for the ping!).
> 
> On Wed, Jul 16, 2025 at 11:06:53AM +0200, Richard Leitner wrote:
> > Add the new strobe_duration control to v4l uAPI documentation.
> > 
> > Signed-off-by: Richard Leitner <richard.leitner@...ux.dev>
> > ---
> >  Documentation/userspace-api/media/v4l/ext-ctrls-flash.rst | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-flash.rst b/Documentation/userspace-api/media/v4l/ext-ctrls-flash.rst
> > index d22c5efb806a183a3ad67ec3e6550b002a51659a..03a58ef94be7c870f55d5a9bb09503995dbfb402 100644
> > --- a/Documentation/userspace-api/media/v4l/ext-ctrls-flash.rst
> > +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-flash.rst
> > @@ -186,3 +186,8 @@ Flash Control IDs
> >      charged before strobing. LED flashes often require a cooldown period
> >      after strobe during which another strobe will not be possible. This
> >      is a read-only control.
> > +
> > +``V4L2_CID_FLASH_DURATION (integer)``
> > +    Duration the flash should be on when the flash LED is in flash mode
> > +    (V4L2_FLASH_LED_MODE_FLASH). The unit should be microseconds (µs)
> > +    if possible.
> > 
> 
> I think we should add this is related to the hardware strobe.
> 
> How about:
> 
> ``V4L2_CID_FLASH_DURATION (integer)``
> 
>     Duration of the flash strobe from the strobe source, typically a camera
>     sensor. Controlling the flash LED strobe length this way requires that the
>     flash LED driver's :ref:`flash LED mode <v4l2-cid-flash-led-mode>` is set
>     to ``V4L2_FLASH_LED_MODE_FLASH`` and :ref:`strobe source
>     <v4l2-cid-strobe-source>` is set to ``V4L2_FLASH_STROBE_SOURCE_EXTERNAL``.
>     The unit should be microseconds (µs) if possible.
> 
> This requires of course the appropriate labels.
> 
> I also wonder whether we should use 0,1 µs or even ns for the unit.

Thanks for the pointer.

I did a quick internet search and the shortest flash duration of
speedlights/studio strobes I found was Profoto D2 with 1/63000 sec ~> 15.9µs.
So I IMHO we are fine with 1µs resolution here.

regards;rl

> 
> -- 
> Kind regards,
> 
> Sakari Ailus