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

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Message-ID: <20191002123513.GI972@paasikivi.fi.intel.com>
Date:   Wed, 2 Oct 2019 15:35:13 +0300
From:   Sakari Ailus <sakari.ailus@...ux.intel.com>
To:     Benoit Parrot <bparrot@...com>
Cc:     Jacopo Mondi <jacopo@...ndi.org>,
        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 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 :)
> 
> 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.
> 
> Unlike my V4L2_CID_PIXEL_RATE solution which now works in all mode with the
> same receiver.
> 
> So long story short I dropped the V4L2_CID_LINK_FREQ patch and focused on
> V4L2_CID_PIXEL_RATE instead.

It shouldn't make a difference which one you use; if you know the bus type
(and if it's CSI-2 with D-PHY, number of lanes and how many bits per pixel
the media bus format has), you can convert fairly trivially between the
two.

-- 
Sakari Ailus
sakari.ailus@...ux.intel.com