linux-kernel - Re: [Patch 1/3] media: ov5640: add PIXEL

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Date:   Thu, 3 Oct 2019 08:55:06 +0200
From:   Jacopo Mondi <jacopo@...ndi.org>
To:     Benoit Parrot <bparrot@...com>
Cc:     Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Hans Verkuil <hverkuil@...all.nl>, linux-media@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        hugues.fruchet@...com
Subject: Re: [Patch 1/3] media: ov5640: add PIXEL_RATE control

Hi Benoit,

On Wed, Oct 02, 2019 at 10:06:15AM -0500, Benoit Parrot wrote:
> Jacopo Mondi <jacopo@...ndi.org> wrote on Wed [2019-Oct-02 16:32:26 +0200]:
> > Hi Benoit,
> >
> > On Wed, Oct 02, 2019 at 07:14:38AM -0500, Benoit Parrot wrote:
> > > Hi Jacopo,
> > >
> > > Maybe, I miss spoke when I mentioned a helper I did not intent a framework
> > > level generic function. Just a function to help in this case :)
> >
> > Yes indeed, the discussion thread I linked here was mostly interesting
> > because Hugues tried to do the same for LINK_FREQ iirc, and there
> > where some usefult pointers.
> >
> > >
> > > That being said, I re-read the thread you mentioned. And as Hughes pointed
> > > out dynamically generating a "working" link frequency value which can be
> > > used by a CSI2 receiver to properly configure its PHY is not trivial.
> > >
> > > When I created this patch, I also had another to add V4L2_CID_LINK_FREQ
> > > support. I am testing this against the TI CAL CSI2 receiver, which already
> > > uses the V4L2_CID_PIXEL_RATE value for that purpose, so I also had a patch
> > > to add support for V4L2_CID_LINK_FREQ to that driver as well.
> > >
> > > Unfortunately, similar to Hughes' findings I was not able to make it "work"
> > > with all supported resolution/framerate.
> >
> > As reported by Hugues findings, the PLL calculation procedure might be
> > faulty, and the actuall frequencies on the bus are different from the
> > calculated ones.
> >
> > I wish I had more time to re-look at that, as they worked for my and
> > Sam's use case, but deserve some rework.
> >
> > >
> > > Unlike my V4L2_CID_PIXEL_RATE solution which now works in all mode with the
> > > same receiver.
> > >
> >
> > It seems to me you're reporting a fixed rate. It might make your
> > receiver happy, but does not report what is acutally put on the bus.
> > Am I missing something ?
>
> No it is not fixed, the only fixed value was the initial value (which
> representative of the initial/default resolution and framerate), I
> fixed this in v2. The reported PIXEL_RATE is re-calculated every time there
> is a s_fmt and/or framerate change and the V4L2_CID_PIXEL_RATE control
> value is updated accordingly.

Oh I missed v2! I only seen this one.
I'll reply there.

Thanks
  j

>
> >
> > > So long story short I dropped the V4L2_CID_LINK_FREQ patch and focused on
> > > V4L2_CID_PIXEL_RATE instead.
> > >
> >
> > As Sakari pointed out, going from one to the other is trivial and
> > could be done on top.
>
> As you said it could be done on top. :)
>
> Benoit
>
> >
> > Thanks
> >    j
> >
> > > Regard,
> > > Benoit
> > >
> > > Jacopo Mondi <jacopo@...ndi.org> wrote on Wed [2019-Oct-02 09:59:51 +0200]:
> > > > Hi Benoit,
> > > >   +Hugues
> > > >
> > > > If you're considering an helper, this thread might be useful to you:
> > > > https://patchwork.kernel.org/patch/11019673/
> > > >
> > > > Thanks
> > > >    j
> > > >
> > >
> > >
>
>

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