Re: [PATCH] mtd: lpddr: fix excessive stack usage with clang

[Nathan Chancellor]

On Tue, May 05, 2020 at 04:01:16PM +0200, Arnd Bergmann wrote:
> Building lpddr2_nvm with clang can result in a giant stack usage
> in one function:
> 
> drivers/mtd/lpddr/lpddr2_nvm.c:399:12: error: stack frame size of 1144 bytes in function 'lpddr2_nvm_probe' [-Werror,-Wframe-larger-than=]
> 
> The problem is that clang decides to build a copy of the mtd_info
> structure on the stack and then do a memcpy() into the actual version. It
> shouldn't really do it that way, but it's not strictly a bug either.
> 
> As a workaround, use a static const version of the structure to assign
> most of the members upfront and then only set the few members that
> require runtime knowledge at probe time.
> 
> Fixes: 96ba9dd65788 ("mtd: lpddr: add driver for LPDDR2-NVM PCM memories")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx>

> ---
>  drivers/mtd/lpddr/lpddr2_nvm.c | 35 ++++++++++++++++++----------------
>  1 file changed, 19 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/mtd/lpddr/lpddr2_nvm.c b/drivers/mtd/lpddr/lpddr2_nvm.c
> index 0f1547f09d08..72f5c7b30079 100644
> --- a/drivers/mtd/lpddr/lpddr2_nvm.c
> +++ b/drivers/mtd/lpddr/lpddr2_nvm.c
> @@ -393,6 +393,17 @@ static int lpddr2_nvm_lock(struct mtd_info *mtd, loff_t start_add,
>  	return lpddr2_nvm_do_block_op(mtd, start_add, len, LPDDR2_NVM_LOCK);
>  }
>  
> +static const struct mtd_info lpddr2_nvm_mtd_info = {
> +	.type		= MTD_RAM,
> +	.writesize	= 1,
> +	.flags		= (MTD_CAP_NVRAM | MTD_POWERUP_LOCK),
> +	._read		= lpddr2_nvm_read,
> +	._write		= lpddr2_nvm_write,
> +	._erase		= lpddr2_nvm_erase,
> +	._unlock	= lpddr2_nvm_unlock,
> +	._lock		= lpddr2_nvm_lock,
> +};
> +
>  /*
>   * lpddr2_nvm driver probe method
>   */
> @@ -433,6 +444,7 @@ static int lpddr2_nvm_probe(struct platform_device *pdev)
>  		.pfow_base	= OW_BASE_ADDRESS,
>  		.fldrv_priv	= pcm_data,
>  	};
> +
>  	if (IS_ERR(map->virt))
>  		return PTR_ERR(map->virt);
>  
> @@ -444,22 +456,13 @@ static int lpddr2_nvm_probe(struct platform_device *pdev)
>  		return PTR_ERR(pcm_data->ctl_regs);
>  
>  	/* Populate mtd_info data structure */
> -	*mtd = (struct mtd_info) {
> -		.dev		= { .parent = &pdev->dev },
> -		.name		= pdev->dev.init_name,
> -		.type		= MTD_RAM,
> -		.priv		= map,
> -		.size		= resource_size(add_range),
> -		.erasesize	= ERASE_BLOCKSIZE * pcm_data->bus_width,
> -		.writesize	= 1,
> -		.writebufsize	= WRITE_BUFFSIZE * pcm_data->bus_width,
> -		.flags		= (MTD_CAP_NVRAM | MTD_POWERUP_LOCK),
> -		._read		= lpddr2_nvm_read,
> -		._write		= lpddr2_nvm_write,
> -		._erase		= lpddr2_nvm_erase,
> -		._unlock	= lpddr2_nvm_unlock,
> -		._lock		= lpddr2_nvm_lock,
> -	};
> +	*mtd = lpddr2_nvm_mtd_info;
> +	mtd->dev.parent		= &pdev->dev;
> +	mtd->name		= pdev->dev.init_name;
> +	mtd->priv		= map;
> +	mtd->size		= resource_size(add_range);
> +	mtd->erasesize		= ERASE_BLOCKSIZE * pcm_data->bus_width;
> +	mtd->writebufsize	= WRITE_BUFFSIZE * pcm_data->bus_width;
>  
>  	/* Verify the presence of the device looking for PFOW string */
>  	if (!lpddr2_nvm_pfow_present(map)) {
> -- 
> 2.26.0
>