Re: [PATCH v2 1/8] media: v4l: ctrls: add a control for flash/strobe duration

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hi Richard,

On Fri, Mar 14, 2025 at 11:25:09AM +0100, Richard Leitner wrote:
> On Fri, Mar 14, 2025 at 09:20:23AM +0000, Sakari Ailus wrote:
> > Hi Richard,
> > 
> > Thanks for the set.
> 
> Hi Sakari,
> thanks for the quick response!
> 
> > 
> > On Fri, Mar 14, 2025 at 09:49:55AM +0100, Richard Leitner wrote:
> > > Add a control V4L2_CID_FLASH_DURATION to set the duration of a
> > > flash/strobe pulse. This is different to the V4L2_CID_FLASH_TIMEOUT
> > > control, as the timeout defines a limit after which the flash is
> > > "forcefully" turned off again.
> > > 
> > > On the other hand the new V4L2_CID_FLASH_DURATION is the desired length
> > > of the flash/strobe pulse
> > 
> > What's the actual difference between the two? To me they appear the same,
> > just expressed in a different way.
> 
> According to FLASH_TIMEOUT documentation:
> 
> 	Hardware timeout for flash. The flash strobe is stopped after this
> 	period of time has passed from the start of the strobe. [1]
> 
> This is a little bit unspecific, but as also discussed with Dave [2]
> according to the documentation of V4L2_FLASH_FAULT_TIMEOUT it seems to
> be targeted at providing a "real timeout" control, not settings the
> desired duration:
> 
> 	The flash strobe was still on when the timeout set by the user
> 	--- V4L2_CID_FLASH_TIMEOUT control --- has expired. Not all flash
> 	controllers may set this in all such conditions. [1]
> 
> If I understood that wrong, I'm also happy to use FLASH_TIMEOUT for this
> use-case. But tbh I think FLASH_DURATION would be more specific.
> 
> As this still seems unclear: Should the documentation be
> changed/rewritten if we stick with the FLASH_DURATION approach?
> 
> [1] https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/ext-ctrls-flash.html
> [2] https://lore.kernel.org/lkml/CAPY8ntB8i4OyUWAL8k899yUd5QsRifJXiOfWXKceGQ7TNZ4OUw@mail.gmail.com/

Right, I think I can see what you're after.

How does the sensor determine when to start the strobe, i.e. on which frame
and which part of the exposure of that frame?

> 
> > 
> > > 
> > > Signed-off-by: Richard Leitner <richard.leitner@linux.dev>
> > > ---
> > >  drivers/media/v4l2-core/v4l2-ctrls-defs.c | 1 +
> > >  include/uapi/linux/v4l2-controls.h        | 1 +
> > >  2 files changed, 2 insertions(+)
> > > 
> > > diff --git a/drivers/media/v4l2-core/v4l2-ctrls-defs.c b/drivers/media/v4l2-core/v4l2-ctrls-defs.c
> > > index 1ea52011247accc51d0261f56eab1cf13c0624a0..f9ed7273a9f3eafe01c31b638e1c8d9fcf5424af 100644
> > > --- a/drivers/media/v4l2-core/v4l2-ctrls-defs.c
> > > +++ b/drivers/media/v4l2-core/v4l2-ctrls-defs.c
> > > @@ -1135,6 +1135,7 @@ const char *v4l2_ctrl_get_name(u32 id)
> > >  	case V4L2_CID_FLASH_FAULT:		return "Faults";
> > >  	case V4L2_CID_FLASH_CHARGE:		return "Charge";
> > >  	case V4L2_CID_FLASH_READY:		return "Ready to Strobe";
> > > +	case V4L2_CID_FLASH_DURATION:		return "Strobe Duration";
> > >  
> > >  	/* JPEG encoder controls */
> > >  	/* Keep the order of the 'case's the same as in v4l2-controls.h! */
> > > diff --git a/include/uapi/linux/v4l2-controls.h b/include/uapi/linux/v4l2-controls.h
> > > index 974fd254e57309e6def95b4a4f8e4de13a3972a7..80050cadb8377e3070ebbadc493fcd08b2c12c0b 100644
> > > --- a/include/uapi/linux/v4l2-controls.h
> > > +++ b/include/uapi/linux/v4l2-controls.h
> > > @@ -1173,6 +1173,7 @@ enum v4l2_flash_strobe_source {
> > >  
> > >  #define V4L2_CID_FLASH_CHARGE			(V4L2_CID_FLASH_CLASS_BASE + 11)
> > >  #define V4L2_CID_FLASH_READY			(V4L2_CID_FLASH_CLASS_BASE + 12)
> > > +#define V4L2_CID_FLASH_DURATION			(V4L2_CID_FLASH_CLASS_BASE + 13)
> > >  
> > >  
> > >  /* JPEG-class control IDs */
> > > 
> > 

-- 
Regards,

Sakari Ailus