linux-kernel - Re: [PATCH] clk: <linux/clk.h>: correct clk_get

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Date:   Tue, 27 Apr 2021 19:03:54 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org
Subject: Re: [PATCH] clk: <linux/clk.h>: correct clk_get_parent()
 documentation

On Tue, Apr 27, 2021 at 10:07:04AM -0700, Randy Dunlap wrote:
> On 4/27/21 2:38 AM, Russell King - ARM Linux admin wrote:
> > On Mon, Apr 26, 2021 at 08:13:42PM -0700, Randy Dunlap wrote:
> >> Make the kernel-doc return value agree with both the stub implementation
> >> in <linux/clk.h> and the non-stub function in drivers/clk/clk.c.
> >>
> >> Signed-off-by: Randy Dunlap <rdunlap@...radead.org>
> >> Cc: Russell King <linux@...linux.org.uk>
> >> Cc: linux-clk@...r.kernel.org
> >> ---
> >> There are several drivers that call clk_get_parent() and use its
> >> return value as though it returns an ERR_PTR(). I am working on a
> >> patch series to fix those.
> > 
> > The whole premise of the CLK API is that:
> > 
> > - clk values that IS_ERR() returns true are errors
> > - clk values that IS_ERR() returns false are valid
> > 
> > and that is that - nice, simple, easy to understand. So the
> > documentation is correct. If clk_get_parent() needs to return an
> > error, it does so by returning an error-pointer, not by returning
> > NULL.
> 
> Hm, so the 6 drivers that do this are OK then, if
> clk_get_parent() returns NULL?
> 
> 
> 	parent = clk_get_parent(clk);
> 	if (IS_ERR(parent)) {
> 		dev_err(cpu_dev, "Cannot get parent clock for CPU0\n");
> 		clk_put(clk);
> 		return PTR_ERR(parent);
> 	}
> 
> 	/* Get parent CPU frequency */
> 	base_frequency =  clk_get_rate(parent);
> 
> boom?

As I say, the premise is that only clocks where IS_ERR(clk) is true
are an error. Everything else must be treated as an acceptable clock.

Sometimes the NULL clock is used for "I don't have a clock", and that
must not cause the API to go "boom".

In the case of the CCF implementation, clk_get_rate(NULL) will return
0.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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