linux-kernel - Re: [PATCH v2 4/4] media: i2c: imx214: Remove hard-coded external clock frequency

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Message-ID: <aCZWV6xj4vap5PFZ@svinhufvud>
Date: Fri, 16 May 2025 00:02:15 +0300
From: Sakari Ailus <sakari.ailus@...ux.intel.com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: git@...tzsch.eu, Ricardo Ribalda <ribalda@...nel.org>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	~postmarketos/upstreaming@...ts.sr.ht, phone-devel@...r.kernel.org,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/4] media: i2c: imx214: Remove hard-coded external
 clock frequency

Hi Laurent,

On Thu, May 15, 2025 at 03:03:40PM +0200, Laurent Pinchart wrote:
> On Thu, May 15, 2025 at 02:54:54PM +0300, Sakari Ailus wrote:
> > On Thu, May 15, 2025 at 10:58:46AM +0200, Laurent Pinchart wrote:
> > > On Tue, May 06, 2025 at 08:24:03AM +0000, Sakari Ailus wrote:
> > > > On Mon, May 05, 2025 at 11:05:56PM +0200, André Apitzsch via B4 Relay wrote:
> > > > > From: André Apitzsch <git@...tzsch.eu>
> > > > > 
> > > > > Instead rely on the rate set on the clock (using assigned-clock-rates
> > > > > etc.)
> > > > > 
> > > > > Signed-off-by: André Apitzsch <git@...tzsch.eu>
> > > > > ---
> > > > >  drivers/media/i2c/imx214.c | 6 ------
> > > > >  1 file changed, 6 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/media/i2c/imx214.c b/drivers/media/i2c/imx214.c
> > > > > index 9e9be47394ec768a5b34d44b06b5bbb0988da5a1..c12996e294dccebb18c608254f1e0d14dc064423 100644
> > > > > --- a/drivers/media/i2c/imx214.c
> > > > > +++ b/drivers/media/i2c/imx214.c
> > > > > @@ -32,7 +32,6 @@
> > > > >  
> > > > >  #define IMX214_REG_FAST_STANDBY_CTRL	CCI_REG8(0x0106)
> > > > >  
> > > > > -#define IMX214_DEFAULT_CLK_FREQ	24000000
> > > > >  #define IMX214_DEFAULT_LINK_FREQ	600000000
> > > > >  /* Keep wrong link frequency for backward compatibility */
> > > > >  #define IMX214_DEFAULT_LINK_FREQ_LEGACY	480000000
> > > > > @@ -1405,11 +1404,6 @@ static int imx214_probe(struct i2c_client *client)
> > > > >  		return dev_err_probe(dev, PTR_ERR(imx214->xclk),
> > > > >  				     "failed to get xclk\n");
> > > > >  
> > > > > -	ret = clk_set_rate(imx214->xclk, IMX214_DEFAULT_CLK_FREQ);
> > > > > -	if (ret)
> > > > > -		return dev_err_probe(dev, ret,
> > > > > -				     "failed to set xclk frequency\n");
> > > > > -
> > > > 
> > > > Oops. I missed this is what the driver was doing already. Indeed, this is
> > > > one of the historic sensor drivers that do set the frequency in DT systems.
> > > > 
> > > > The driver never used the clock-frequency property and instead used a fixed
> > > > frequency. Changing the behaviour now could be problematic.
> > > > 
> > > > There are options here that I think we could do:
> > > > 
> > > > 1) use your v1 patch (4) which uses "clock-frequency" if it exists and
> > > > otherwise uses the default, fixed frequency or
> > > > 
> > > > 2) set the frequency only if the "clock-frequency" property exists. The DT
> > > > currently requires clock-frequency and the YAML conversion was done in 2020
> > > > whereas the driver is from 2018. If we do this, the clock-frequency should
> > > > be deprecated (or even removed from bingings).
> > > > 
> > > > I wonder what others think. Cc'd Laurent in any case.
> > > 
> > > Maybe I'm missing something, but I don't really see the issue here. The
> > > clock-frequency DT property is currently ignored, and this patch doesn't
> > > change that situation, does it ?
> > > 
> > > The change of behaviour here is related to the assigned-clock-rates
> > > property. If that property is specified today, it will set the clock
> > > rate, and the driver will override it to 24MHz right after. With this
> > > patch, the clock rate won't be overridden. I think the risk of
> > > regression is very low here, as I don't expect systems to set
> > > assigned-clock-rates in DT to a value different than 24MHz and expect
> > > the driver to override it.
> > 
> > If the DTS had assigned-clock-rates set correctly, then yes. How much can
> > we trust the older DTS did have that?
> 
> I am relatively confident that DT-based systems wouldn't have an
> assigned-clock-rates property with a frequency that doesn't match
> IMX214_DEFAULT_CLK_FREQ. The real question is whether or not I'm
> over-confident :-)

The assigned-clock stuff wasn't always there. But nowadays I guess a lot
of things in practice depends on it.

So I guess doing this should be fine then.

-- 
Sakari Ailus