linux-kernel - Re: [PATCH][next] soc: xilinx: vcu: remove deadcode on null divider check

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Date:   Thu, 18 Mar 2021 15:44:14 +0100
From:   Michael Tretter <m.tretter@...gutronix.de>
To:     Colin Ian King <colin.king@...onical.com>
Cc:     Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Michal Simek <michal.simek@...inx.com>,
        linux-arm-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] soc: xilinx: vcu: remove deadcode on null divider
 check

On Thu, 18 Mar 2021 12:48:06 +0000, Colin Ian King wrote:
> On 11/02/2021 19:05, Stephen Boyd wrote:
> > Quoting Michael Tretter (2021-02-10 23:39:06)
> >> On Wed, 10 Feb 2021 19:28:18 -0800, Stephen Boyd wrote:
> >>> Quoting Colin King (2021-02-10 10:49:38)
> >>>> From: Colin Ian King <colin.king@...onical.com>
> >>>>
> >>>> The pointer 'divider' has previously been null checked followed by
> >>>> a return, hence the subsequent null check is redundant deadcode
> >>>> that can be removed.  Clean up the code and remove it.
> >>>>
> >>>> Fixes: 9c789deea206 ("soc: xilinx: vcu: implement clock provider for output clocks")
> >>>> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> >>>> ---
> >>>>  drivers/clk/xilinx/xlnx_vcu.c | 3 ---
> >>>>  1 file changed, 3 deletions(-)
> >>>>
> >>>> diff --git a/drivers/clk/xilinx/xlnx_vcu.c b/drivers/clk/xilinx/xlnx_vcu.c
> >>>> index d66b1315114e..607936d7a413 100644
> >>>> --- a/drivers/clk/xilinx/xlnx_vcu.c
> >>>> +++ b/drivers/clk/xilinx/xlnx_vcu.c
> >>>> @@ -512,9 +512,6 @@ static void xvcu_clk_hw_unregister_leaf(struct clk_hw *hw)
> >>>>  
> >>>>         mux = clk_hw_get_parent(divider);
> >>>>         clk_hw_unregister_mux(mux);
> >>>> -       if (!divider)
> >>>> -               return;
> >>>> -
> >>>
> >>> This code is pretty confusing. Waiting for m.tretter@...gutronix.de to
> >>> reply
> >>
> >> Can you elaborate what you find confusing about this code. I would gladly try
> >> to clarify and improve the code.
> > 
> > The fact that pointers are being checked and then bailing out of the
> > function early, vs. doing something if the pointer is non-NULL.
> > 
> >>
> >> What happens here is that the driver registers a mux -> divider -> gate chain
> >> for each output clock, but only stores the gate clock. When unregistering the
> >> clocks, the driver starts at the gate and walks up to the mux while
> >> unregistering the clocks.
> >>
> 
> OK, so I think I understand this better, should the order of
> unregisteration be as follows:
> 
> diff --git a/drivers/clk/xilinx/xlnx_vcu.c b/drivers/clk/xilinx/xlnx_vcu.c
> index d66b1315114e..66bac8421460 100644
> --- a/drivers/clk/xilinx/xlnx_vcu.c
> +++ b/drivers/clk/xilinx/xlnx_vcu.c
> @@ -511,11 +511,11 @@ static void xvcu_clk_hw_unregister_leaf(struct
> clk_hw *hw)
>                 return;
> 
>         mux = clk_hw_get_parent(divider);
> -       clk_hw_unregister_mux(mux);
> -       if (!divider)
> +       clk_hw_unregister_mux(divider);

The order is correct, but this must be:

	clk_hw_unregister_divider(divider);

> +       if (!mux)
>                 return;
> 
> -       clk_hw_unregister_divider(divider);
> +       clk_hw_unregister_divider(mux);
> 

	clk_hw_unregister_mux(mux);

Taking the confusion expressed by Stephen into account, I rewrote the entire
function to clarify what is happening in this function. Please take a look
that patch [0] and tell me, if it is now easier to understand.

[0] 20210318144230.3438009-1-m.tretter@...gutronix.de

Michael