linux-kernel - Re: [PATCH v2 1/5] media: i2c: imx335: Drop setting of 0x3a00 register

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Message-ID: <Zc4IgUFFsjA-LC6T@kekkonen.localdomain>
Date: Thu, 15 Feb 2024 12:50:09 +0000
From: Sakari Ailus <sakari.ailus@...ux.intel.com>
To: Umang Jain <umang.jain@...asonboard.com>
Cc: Matthias Fend <matthias.fend@...end.at>,
	Kieran Bingham <kieran.bingham@...asonboard.com>,
	linux-media@...r.kernel.org,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/5] media: i2c: imx335: Drop setting of 0x3a00
 register

Hi folks,

On Wed, Feb 07, 2024 at 09:51:32AM +0530, Umang Jain wrote:
> Hi,
> 
> On 1/31/24 4:02 PM, Matthias Fend wrote:
> > Hi Kieran,
> > 
> > Am 31.01.2024 um 10:52 schrieb Kieran Bingham:
> > > Hi Umang,
> > > 
> > > + Cc: Matthias
> > > 
> > > Quoting Umang Jain (2024-01-31 05:52:04)
> > > > Register 0x3a00 is a reserved field as per the IMX335 datasheet,
> > > > hence shouldn't be set by the driver.
> > > 
> > > We still need to explain more about why we're dropping this register
> > > write, and what effects it causes.
> > > 
> > > Matthias - I believe this stemmed from the work you did, and I think I
> > > recall that you stated this register write broke the CSI2 configuration?
> > > 
> > > Can you clarify anything here for us please?
> > 
> > yes, that's correct.
> > 
> > Since this driver originally did not work in my setup, I came across
> > this register while searching for differences to my working reference
> > configuration.
> > With the default value of this register (0x00), the driver works
> > perfectly. With the value previously written to it by the driver (0x01),
> > I cannot receive any frames.
> > The problem may depend on the link frequency used.
> > I can only use and test a frequency of 445.5MHz on my hardware. Since
> > only link frequencies of 594MHz were supported so far, this may not have
> > been a problem.
> > 
> > Unfortunately I do not have a description of this register, so I can
> > only speculate about the exact cause.
> 
> Is it worth to frame the commit message around this speculation ?
> 
> My setup has no effect with this register being set or not.

Very interesting case indeed.

Please elaborate this a little in the commit message, but the message
shouldn't be that because the register is marked rewerved, it shouldn't be
written (it's a commonplace when it comes to sensors :().

-- 
Regards,

Sakari Ailus