linux-kernel - Re: [PATCH v2 1/2] clk: Warn and add workaround on misuse of .parent

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Message-ID: <63ed6ddb.050a0220.beb77.bf66@mx.google.com>
Date:   Thu, 16 Feb 2023 00:33:29 +0100
From:   Christian Marangi <ansuelsmth@...il.com>
To:     Stephen Boyd <sboyd@...nel.org>
Cc:     Michael Turquette <mturquette@...libre.com>,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] clk: Warn and add workaround on misuse of
 .parent_data with .name only

On Wed, Feb 15, 2023 at 10:54:56AM -0800, Stephen Boyd wrote:
> Quoting Christian Marangi (2023-02-10 10:34:11)
> > On Fri, Feb 10, 2023 at 04:40:29PM -0800, Stephen Boyd wrote:
> > > Quoting Christian Marangi (2023-01-31 08:08:28)
> > > > By a simple mistake in a .parent_names to .parent_data conversion it was
> > > > found that clk core assume fw_name is always provided with a parent_data
> > > > struct for each parent and never fallback to .name to get parent name even
> > > > if declared.
> > > 
> > > It sounds like you have clk_parent_data and the .index member is 0? Can
> > > you show an example structure? I'm guessing it is like this:
> > > 
> > >       struct clk_parent_data pdata = { .name = "global_name" };
> > >
> > 
> > An example of this problem and the relative fix is here
> > 35dc8e101a8e08f69f4725839b98ec0f11a8e2d3
> > 
> > You example is also ok and this patch wants to handle just a case like
> > that.
> 
> Ok, so you have a firmware .index of 0. The .name is a fallback. I
> suppose you want the .name to be a fallback if there isn't a clocks
> property in the registering device node? I thought that should already
> work but maybe there is a bug somewhere. Presumably you have a gcc node
> that doesn't have a clocks property
> 
>                 gcc: gcc@...0000 {
>                         compatible = "qcom,gcc-ipq8074";
>                         reg = <0x01800000 0x80000>;
>                         #clock-cells = <0x1>;
>                         #power-domain-cells = <1>;
>                         #reset-cells = <0x1>;
>                 };	
> 
> Looking at clk_core_get() we'll call of_parse_clkspec() and that should fail
> 
> 	struct clk_hw *hw = ERR_PTR(-ENOENT);
> 
> 	...
> 
>         if (np && (name || index >= 0) &&
>             !of_parse_clkspec(np, index, name, &clkspec)) {
> 		...
> 	} else if (name) {
> 		...
> 	}
> 
>         if (IS_ERR(hw))
>                 return ERR_CAST(hw);
> 
> so we should have a -ENOENT clk_hw pointer in
> clk_core_fill_parent_index(). That should land in this if condition in
> clk_core_fill_parent_index()
> 
>                 parent = clk_core_get(core, index);
>                 if (PTR_ERR(parent) == -ENOENT && entry->name)
>                         parent = clk_core_lookup(entry->name);
> 
> and then entry->name should be used. 
>

Hi, thanks for making me give this an extra check... I think I found
the real cause.
I send a patch that should suppress this and give an extensive
explaination of the problem.
This is the ID: 20230215232712.17072-1-ansuelsmth@...il.com

The hint that made me get what was wrong was a problem with index and
the fact that it should have returned -ENOENT... Fun to discover a clock
was actually returned and the function never returned an error.

> > 
> > > > 
> > > > This is caused by clk_core_get that only checks for parent .fw_name and
> > > > doesn't handle .name.
> > > 
> > > clk_core_get() is not supposed to operate on the .name member. It is a
> > > firmware based lookup with clkdev as a fallback because clkdev is a
> > > psudeo-firmware interface to assign a name to a clk when some device
> > > pointer is used in conjunction with it.
> > > 
> > 
> > And the problem is just that. We currently permit to have a
> > configuration with .name but no .fw_name. In a case like that a dev may
> > think that this configuration is valid but in reality the clk is
> > silently ignored/not found and cause clk problem with selecting a
> > parent.
> 
> It is valid though.
> 
> > 
> > Took some good hours to discover this and to me it seems an error that
> > everybody can do since nowhere is specificed that the following
> > parent_data configuration is illegal. 
> > 
> 
> I'll look at adding a test. Seems to be the best way to solve this.

Eh probably a test may have made this more clear. The main problem  here
was that the function never returned an error but under the hood the
parent was pointing to another clock.

-- 
	Ansuel