linux-kernel - Re: [PATCH 3/3] pwm: axi-pwmgen: add support for external clock

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Message-ID: <1cc3232e94318f22ff30345326151e8db5381084.camel@gmail.com>
Date: Wed, 21 May 2025 16:40:35 +0100
From: Nuno Sá <noname.nuno@...il.com>
To: David Lechner <dlechner@...libre.com>, Uwe
 Kleine-König
	 <ukleinek@...nel.org>
Cc: Michael Hennerich <michael.hennerich@...log.com>, Nuno
 Sá	 <nuno.sa@...log.com>, Trevor Gamblin
 <tgamblin@...libre.com>, Rob Herring	 <robh@...nel.org>, Krzysztof
 Kozlowski <krzk+dt@...nel.org>, Conor Dooley	 <conor+dt@...nel.org>,
 linux-pwm@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] pwm: axi-pwmgen: add support for external clock

On Wed, 2025-05-21 at 10:05 -0500, David Lechner wrote:
> On 5/21/25 9:22 AM, Nuno Sá wrote:
> > On Wed, 2025-05-21 at 15:54 +0200, Uwe Kleine-König wrote:
> > > Hello David,
> > > 
> > > On Wed, May 21, 2025 at 08:19:51AM -0500, David Lechner wrote:
> > > > On 5/21/25 4:22 AM, Uwe Kleine-König wrote:
> > > > > Can you achieve the same effect with the (IMHO slightly nicer but
> > > > > hand-crafted) following patch:
> > > > > 
> > > > >  	ddata = pwmchip_get_drvdata(chip);
> > > > >  	ddata->regmap = regmap;
> > > > >  
> > > > > -	clk = devm_clk_get_enabled(dev, NULL);
> > > > > -	if (IS_ERR(clk))
> > > > > -		return dev_err_probe(dev, PTR_ERR(clk), "failed to
> > > > > get
> > > > > clock\n");
> > > > > +	axi_clk = devm_clk_get_enabled(dev, "axi");
> > > > > +	if (IS_ERR(axi_clk))
> > > > > +		return dev_err_probe(dev, PTR_ERR(axi_clk), "failed
> > > > > to
> > > > > get axi clock\n");
> > > > > 
> > > > > +	clk = devm_clk_get_enabled_optional(dev, "ext");
> > > > > +	if (IS_ERR(clk))
> > > > > +		return dev_err_probe(dev, PTR_ERR(clk), "failed to
> > > > > get
> > > > > ext clock\n");
> > > > > +	}
> > > > 
> > > > The trouble with this is that it would not work with existing .dtbs
> > > > that don't have clock-names set. I think it would need to be more like
> > > > this:
> > > > 
> > > > 
> > > > 	axi_clk = devm_clk_get_enabled(dev, NULL);
> > > > 	if (IS_ERR(axi_clk))
> > > > 		return dev_err_probe(dev, PTR_ERR(axi_clk), "failed to
> > > > get
> > > > axi clock\n");
> > > > 
> > > > 	clk = devm_clk_get_enabled_optional(dev, "ext");
> > > > 	if (IS_ERR(clk))
> > > > 		return dev_err_probe(dev, PTR_ERR(clk), "failed to get
> > > > ext
> > > > clock\n");
> > > > 
> > > > 	if (!clk)
> > > > 		clk = axi_clk
> > > > 
> > > 
> > > If there are no clock-names, the parameter is ignored. (I didn't test,
> > > only quickly checked the code.) So passing "axi" instead of NULL should
> > > work and yield a more robust solution.
> > > 
> > > 
> > 
> > Are you sure? If there are no clock-names and you pass an id, you should get
> > an
> > error back:
> > 
> > https://elixir.bootlin.com/linux/v6.14.7/source/drivers/clk/clk.c#L5198
> > 
> > 
> > I know it's not exactly the same we're discussing but
> > of_property_match_string()
> > return -EINVAL if the property is not found which leads to an index < 0 and
> > thus
> > of_parse_phandle_with_args() also returns an error back.
> > 
> > I think I'm not missing anything but it's always a possibility.
> > 
> > - Nuno Sá
> 
> Testing agrees:
> 
> Given:
> 
> 	clocks = <&some_clock>;
> 	/delete-property/clock-names;
> 
> And:
> 
> 	axi_clk = devm_clk_get_enabled(dev, "axi");
> 
> We get:
> 
> [    1.190040] axi-pwmgen 44b00000.pwm: error -ENOENT: failed to get axi clock

Hmm, so it seems I have no bug (in the other link I pasted in the other reply).
Looking at the code I would expect -EINVAL to be returned...

I guess it's because we still try  __clk_get_sys().

- Nuno Sá