linux-kernel - Re: [PATCH v2 1/3] media: mt9m111: add V4L2_CID_LINK

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Message-ID: <20220920085617.7cfflloegh7en4mj@pengutronix.de>
Date:   Tue, 20 Sep 2022 10:56:17 +0200
From:   Marco Felsch <m.felsch@...gutronix.de>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     mchehab@...nel.org, laurent.pinchart+renesas@...asonboard.com,
        akinobu.mita@...il.com, jacopo+renesas@...ndi.org,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] media: mt9m111: add V4L2_CID_LINK_FREQ support

Hi Sakari,

On 22-09-19, Sakari Ailus wrote:

...

> > > > +	ret = clk_prepare_enable(mt9m111->clk);
> > > > +	if (ret < 0)
> > > > +		return ret;
> > > > +
> > > > +	extclk_rate = clk_get_rate(mt9m111->clk);
> > > > +	clk_disable_unprepare(mt9m111->clk);
> > > 
> > > I don't think you'll need to enable a clock to just get its frequency.
> > 
> > The official API states that you need to turn on the clk before
> > requesting it and it makes sense. Also there is a new helper
> > devm_clk_get_enabled() which addresses simple clk usage since most of
> > drivers don't enable it before requesting the rate.
> 
> I guess the rate could change in the meantime, unless exclusive access is
> requested. 

Not only that, there are a bunch of clk provider hw around which may
need to turned on first. Anyway, I really don't care on this topic. As
I said I wanted to fullfil the API and if drop clk_prepare_enable() I
don't. So if this okay for you I will go that way.

> The clock framework currently doesn't offer a way to set the assigned
> rate and prevent changing it. But above, couldn't the clock frequency
> be changed again once the clock has been disabled?

Yes it could.

Regards,
  Marco