linux-kernel - Re: [PATCH 01/12] nvmem: imx-iim: use stack for nvmem

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Date:   Fri, 20 Oct 2017 23:26:16 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/12] nvmem: imx-iim: use stack for nvmem_config instead
 of malloc'ing it

2017-10-20 22:54 GMT+09:00 Greg KH <gregkh@...uxfoundation.org>:
> On Fri, Oct 20, 2017 at 10:47:16PM +0900, Masahiro Yamada wrote:
>> Hi Greg,
>>
>> 2017-10-20 22:32 GMT+09:00 Greg KH <gregkh@...uxfoundation.org>:
>> > On Mon, Oct 09, 2017 at 03:26:30PM +0200, srinivas.kandagatla@...aro.org wrote:
>> >> From: Masahiro Yamada <yamada.masahiro@...ionext.com>
>> >>
>> >> nvmem_register() copies all the members of nvmem_config to
>> >> nvmem_device.  So, nvmem_config is one-time use data during
>> >> probing.  There is no point to keep it until the driver detach.
>> >> Using stack should be no problem because nvmem_config is pretty
>> >> small.
>> >>
>> >> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
>> >> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
>> >> ---
>> >>  drivers/nvmem/imx-iim.c | 27 ++++++++++++---------------
>> >>  1 file changed, 12 insertions(+), 15 deletions(-)
>> >>
>> >> diff --git a/drivers/nvmem/imx-iim.c b/drivers/nvmem/imx-iim.c
>> >> index 52ff65e0673f..a5992602709a 100644
>> >> --- a/drivers/nvmem/imx-iim.c
>> >> +++ b/drivers/nvmem/imx-iim.c
>> >> @@ -34,7 +34,6 @@ struct imx_iim_drvdata {
>> >>  struct iim_priv {
>> >>       void __iomem *base;
>> >>       struct clk *clk;
>> >> -     struct nvmem_config nvmem;
>> >>  };
>> >>
>> >>  static int imx_iim_read(void *context, unsigned int offset,
>> >> @@ -108,7 +107,7 @@ static int imx_iim_probe(struct platform_device *pdev)
>> >>       struct resource *res;
>> >>       struct iim_priv *iim;
>> >>       struct nvmem_device *nvmem;
>> >> -     struct nvmem_config *cfg;
>> >> +     struct nvmem_config cfg = {};
>> >
>> > You do realize you are now not zeroing out this structure, and have to
>> > explicitly initialize all of the fields, right?
>>
>> Why?
>>
>> I am surely zeroing out the structure.
>>
>> Did you miss "= {};" in my code?
>
> Are you sure that does zero it out?  I know we have had issues with this
> in the past...

Do you have a reference for that?

All members that are not specified in the initializer
are set to 0 (or NULL).

"git show c7836d1593b87cb813c58cf64e08b052ebbe2a78"
and do you agree that this is correct?


>> > What is the real problem with doing a dynamic allocation for this?
>> > Putting structures on the stack is a "bad idea" for all of the obvious
>> > reasons (small stack in the kernel, initialized data, lower layers
>> > expect it to be dma-able, etc.)
>>
>>
>> Why is this a problem?
>>
>> Did you really understand this patch?
>>
>>  - This structure is very small.
>>    struct uart_8250_port is five times bigger
>>    and it is allocated in the stack and it is fine.
>>
>>  - All data are initialized.
>>
>>  - Why DMA?
>>    Please do not exaggerate things by introducing unrelated topic.
>
> I just want you to realize the change, the initialized is the big thing.
>
> And keeping structures off of the stack is a good thing, if this is not
> a performance issue, I suggest keeping it as-is, right?
>

I do not see logical explanation in your comment.

The structure is initialized.
Other subsystem use stack for such a small structure.
Why is (devm_)kzalloc necessary?




-- 
Best Regards
Masahiro Yamada