RE: [PATCH] Support for write-only controls

["Martin Rubli" <linux1@rubli.info>]

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Hi Brandon,

> -----Original Message-----
> From: Brandon Philips [mailto:brandon@ifup.org]
> Sent: Tuesday, April 15, 2008 08:44
> To: Martin Rubli; linux1@rubli.info; Laurent Pinchart
> Cc: Linux and Kernel Video
> Subject: Re: [PATCH] Support for write-only controls
> 
> On 15:12 Wed 13 Feb 2008, Brandon Philips wrote:
> > On 01:01 Tue 18 Dec 2007, Martin Rubli wrote:
> > > Thanks a lot for all your feedback and the constructive discussion and
> sorry
> > > for the delay while I was without Internet on the weekend. I'll try to
> > > summarize what we have so far:
> > >
> > > Write-only controls:
> > >
> > > It seems, everybody likes EACCES. Michael, maybe we could get some
> feedback
> > > from you on this? It would be nice to change the spec, so that EACCES
> also
> > > becomes the error for writing read-only controls--it seems
appropriate.
> But
> > > if for some reason we can't change that we should probably make the
> > > write-only controls consistent and return EINVAL as well.
> > >
> > > Unusable controls due to device communication error:
> > >
> > > The easiest solution seems to be to set the V4L2_CTRL_FLAG_DISABLED
> flag as
> > > was suggested. The documentation currently says "permanently disabled
> and
> > > should be ignored by the application" which I think is exactly what
> applies
> > > to the situation. The V4L2_CTRL_FLAG_NEXT_CTRL would still be
> respected by
> > > drivers supporting the extended control enumeration, so no need to the
> spec
> > > is required. But I would still add a short paragraph about the first
part as
> > > a guide for future implementations and a witness of this thread. ;-)
> > >
> > > As soon as everyone agrees on this, I will propose new patches. Let me
> know
> > > what you think ...
> >
> 
> Ping.  I never saw patches come across for this.

Attached the patch for videodev2.h and the documentation sources.

Signed-off-by: Martin Rubli <linux1 rubli info>

Cheers,
Martin

[-- Attachment #2: v4l2_ctrl_flag_write_only.patch --]
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diff -Naur v4l-dvb-5de66b04c598/linux/include/linux/videodev2.h v4l-dvb-new/linux/include/linux/videodev2.h
--- v4l-dvb-5de66b04c598/linux/include/linux/videodev2.h	2008-04-16 06:22:11.000000000 +0800
+++ v4l-dvb-new/linux/include/linux/videodev2.h	2008-04-16 17:37:38.000000000 +0800
@@ -831,6 +831,7 @@
 #define V4L2_CTRL_FLAG_UPDATE 		0x0008
 #define V4L2_CTRL_FLAG_INACTIVE 	0x0010
 #define V4L2_CTRL_FLAG_SLIDER 		0x0020
+#define V4L2_CTRL_FLAG_WRITE_ONLY 	0x0040
 
 /*  Query flag, to be ORed with the control ID */
 #define V4L2_CTRL_FLAG_NEXT_CTRL	0x80000000

[-- Attachment #3: v4l2_ctrl_flag_write_only-doc.patch --]
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diff -Naur v4l2spec-0.24/vidioc-queryctrl.sgml v4l2spec-0.24a/vidioc-queryctrl.sgml
--- v4l2spec-0.24/vidioc-queryctrl.sgml	2008-03-06 23:42:13.000000000 +0800
+++ v4l2spec-0.24a/vidioc-queryctrl.sgml	2008-04-16 17:27:36.000000000 +0800
@@ -361,6 +361,15 @@
             <entry>A hint that this control is best represented as a
 slider-like element in a user interface.</entry>
           </row>
+          <row>
+            <entry><constant>V4L2_CTRL_FLAG_WRITE_ONLY</constant></entry>
+            <entry>0x0040</entry>
+            <entry>This control is permanently writable only. Any
+attempt to read the control will result in an EACCES error code. This
+flag is typically present for relative controls or action controls where
+writing a value will cause the device to carry out a given action
+(e. g. motor control) but no meaningful value can be returned.</entry>
+          </row>
         </tbody>
       </tgroup>
     </table>

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