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

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Date:   Fri, 20 Oct 2017 15:32:20 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     srinivas.kandagatla@...aro.org
Cc:     linux-kernel@...r.kernel.org,
        Masahiro Yamada <yamada.masahiro@...ionext.com>
Subject: Re: [PATCH 01/12] nvmem: imx-iim: use stack for nvmem_config instead
 of malloc'ing it

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?

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.)

thanks,

greg k-h