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

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Date:   Thu, 18 Mar 2021 12:48:06 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Stephen Boyd <sboyd@...nel.org>,
        Michael Tretter <m.tretter@...gutronix.de>
Cc:     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 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);
+       if (!mux)
                return;

-       clk_hw_unregister_divider(divider);
+       clk_hw_unregister_divider(mux);