Re: [PATCH] dmaengine: dw-edma: reduce stack usage after debugfs rework

[Serge Semin]

Hi Arnd

On Mon, Jan 30, 2023 at 02:08:10PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> After the dw_edma_debugfs_entry arrays are no longer compile-time
> constant, they take up space on the stack, which exceeds the
> warning limit after inlining:
> 
> drivers/dma/dw-edma/dw-edma-v0-debugfs.c:280:6: error: stack frame size (1784) exceeds limit (1400) in 'dw_edma_v0_debugfs_on' [-Werror,-Wframe-larger-than]
> void dw_edma_v0_debugfs_on(struct dw_edma *dw)
> 

> Work around this by marking the functions with the largest arrays
> as 'noinline_for_stack' to make them not use up space on the same
> stack together.

You mean the same stack !frame!, right?

> 
> Fixes: 5c0373eafd83 ("dmaengine: dw-edma: Move eDMA data pointer to debugfs node descriptor")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  drivers/dma/dw-edma/dw-edma-v0-debugfs.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/dma/dw-edma/dw-edma-v0-debugfs.c b/drivers/dma/dw-edma/dw-edma-v0-debugfs.c
> index 7be23c26ac88..9dfcbbdfb27b 100644
> --- a/drivers/dma/dw-edma/dw-edma-v0-debugfs.c
> +++ b/drivers/dma/dw-edma/dw-edma-v0-debugfs.c
> @@ -116,7 +116,8 @@ static void dw_edma_debugfs_create_x32(struct dw_edma *dw,
>  	}
>  }
>  

> -static void dw_edma_debugfs_regs_ch(struct dw_edma *dw, enum dw_edma_dir dir,
> +static noinline_for_stack void
> +dw_edma_debugfs_regs_ch(struct dw_edma *dw, enum dw_edma_dir dir,
>  				    u16 ch, struct dentry *dent)

This doesn't seem like required. Does it? This method is called from
two functions: dw_edma_debugfs_regs_wr() and dw_edma_debugfs_regs_rd()
and if I am not mistaken will unlikely be inlined. Even if compiler will
inline it the stack memory consumption won't change much since the
functions aren't called from each other.

>  {
>  	struct dw_edma_debugfs_entry debugfs_regs[] = {
> @@ -136,9 +137,10 @@ static void dw_edma_debugfs_regs_ch(struct dw_edma *dw, enum dw_edma_dir dir,
>  	dw_edma_debugfs_create_x32(dw, debugfs_regs, nr_entries, dent);
>  }
>  
> -static void dw_edma_debugfs_regs_wr(struct dw_edma *dw, struct dentry *dent)
> +static noinline_for_stack void
> +dw_edma_debugfs_regs_wr(struct dw_edma *dw, struct dentry *dent)
>  {

> -	static const struct dw_edma_debugfs_entry debugfs_regs[] = {
> +	const struct dw_edma_debugfs_entry debugfs_regs[] = {
>  		/* eDMA global registers */
>  		WR_REGISTER(dw, engine_en),
>  		WR_REGISTER(dw, doorbell),
> @@ -159,7 +161,7 @@ static void dw_edma_debugfs_regs_wr(struct dw_edma *dw, struct dentry *dent)
>  		WR_REGISTER(dw, ch67_imwr_data),
>  		WR_REGISTER(dw, linked_list_err_en),
>  	};
> -	static const struct dw_edma_debugfs_entry debugfs_unroll_regs[] = {
> +	const struct dw_edma_debugfs_entry debugfs_unroll_regs[] = {

I am confused how come these locals've turned to be static const?
Neither my patches nor the Bjorn repo provide such changes:
https://git.kernel.org/pub/scm/linux/kernel/git/helgaas/pci.git/tree/drivers/dma/dw-edma/dw-edma-v0-debugfs.c?h=next#n139
they were supposed to be just const.

What repo is this patch based on?

>  		/* eDMA channel context grouping */
>  		WR_REGISTER_UNROLL(dw, engine_chgroup),
>  		WR_REGISTER_UNROLL(dw, engine_hshake_cnt.lsb),
> @@ -197,7 +199,8 @@ static void dw_edma_debugfs_regs_wr(struct dw_edma *dw, struct dentry *dent)
>  	}
>  }
>  

> -static void dw_edma_debugfs_regs_rd(struct dw_edma *dw, struct dentry *dent)
> +static noinline void

noinline_for_stack, right?

-Serge(y)

> +dw_edma_debugfs_regs_rd(struct dw_edma *dw, struct dentry *dent)
>  {
>  	const struct dw_edma_debugfs_entry debugfs_regs[] = {
>  		/* eDMA global registers */
> -- 
> 2.39.0
>