[PATCH] dmaengine: dw-edma: fix unnecessary stack usage

[PATCH] dmaengine: dw-edma: fix unnecessary stack usage

[Arnd Bergmann]

Putting large constant data on the stack causes unnecessary overhead
and stack usage:

drivers/dma/dw-edma/dw-edma-v0-debugfs.c:285:6: error: stack frame size of 1376 bytes in function 'dw_edma_v0_debugfs_on' [-Werror,-Wframe-larger-than=]

Mark the variable 'static const' in order for the compiler to move it
into the .rodata section where it does no such harm.

Fixes: 305aebeff879 ("dmaengine: Add Synopsys eDMA IP version 0 debugfs support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/dma/dw-edma/dw-edma-v0-debugfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/dma/dw-edma/dw-edma-v0-debugfs.c b/drivers/dma/dw-edma/dw-edma-v0-debugfs.c
index affa1ccaf7a0..42739508c0d8 100644
--- a/drivers/dma/dw-edma/dw-edma-v0-debugfs.c
+++ b/drivers/dma/dw-edma/dw-edma-v0-debugfs.c
@@ -48,7 +48,7 @@ static struct {
 } lim[2][EDMA_V0_MAX_NR_CH];
 
 struct debugfs_entries {
-	char					name[24];
+	const char				*name;
 	dma_addr_t				*reg;
 };
 
-- 
2.20.0

RE: [PATCH] dmaengine: dw-edma: fix unnecessary stack usage

[Gustavo Pimentel]

Hi,

On Mon, Jun 17, 2019 at 14:16:45, Arnd Bergmann <arnd@arndb.de> wrote:

> Putting large constant data on the stack causes unnecessary overhead
> and stack usage:
> 
> drivers/dma/dw-edma/dw-edma-v0-debugfs.c:285:6: error: stack frame size of 1376 bytes in function 'dw_edma_v0_debugfs_on' [-Werror,-Wframe-larger-than=]

I had that warning at the beginning of the development, that's why I 
divided the debugfs entries into several subfunctions. Perhaps my 
configuration has configured a bigger stack frame size than your 
configuration.

> 
> Mark the variable 'static const' in order for the compiler to move it
> into the .rodata section where it does no such harm.

Ok, sounds good.
Thanks.

> 
> Fixes: 305aebeff879 ("dmaengine: Add Synopsys eDMA IP version 0 debugfs support")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/dma/dw-edma/dw-edma-v0-debugfs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/dma/dw-edma/dw-edma-v0-debugfs.c b/drivers/dma/dw-edma/dw-edma-v0-debugfs.c
> index affa1ccaf7a0..42739508c0d8 100644
> --- a/drivers/dma/dw-edma/dw-edma-v0-debugfs.c
> +++ b/drivers/dma/dw-edma/dw-edma-v0-debugfs.c
> @@ -48,7 +48,7 @@ static struct {
>  } lim[2][EDMA_V0_MAX_NR_CH];
>  
>  struct debugfs_entries {
> -	char					name[24];
> +	const char				*name;
>  	dma_addr_t				*reg;
>  };
>  
> -- 
> 2.20.0

Re: [PATCH] dmaengine: dw-edma: fix unnecessary stack usage

[Arnd Bergmann]

On Fri, Jun 21, 2019 at 10:21 AM Gustavo Pimentel
<Gustavo.Pimentel@synopsys.com> wrote:

> On Mon, Jun 17, 2019 at 14:16:45, Arnd Bergmann <arnd@arndb.de> wrote:
>
> > Putting large constant data on the stack causes unnecessary overhead
> > and stack usage:
> >
> > drivers/dma/dw-edma/dw-edma-v0-debugfs.c:285:6: error: stack frame size of 1376 bytes in function 'dw_edma_v0_debugfs_on' [-Werror,-Wframe-larger-than=]
>
> I had that warning at the beginning of the development, that's why I
> divided the debugfs entries into several subfunctions. Perhaps my
> configuration has configured a bigger stack frame size than your
> configuration.

I suspect the problem is CONFIG_GCC_PLUGIN_STRUCTLEAK_VERBOSE,
which is a farily new configuration option that has the unintended side-effect
of sometimes preventing the compiler combining local variables from functions
inlined into the caller. Adding 'noinline_for_stack' to the sub-functions would
have also prevented this, but I think just not having the strings on the
stack is better anyway.

        Arnd