Re: [PATCH] media: rc: reduce size of struct ir_raw_event

Sean Young
2016-03-16T22:28:29Z

On Wed, Mar 16, 2016 at 10:18:38PM +0100, Heiner Kallweit wrote:
> struct ir_raw_event currently has a size of 12 bytes on most (all?)
> architectures. This can be reduced to 8 bytes whilst maintaining
> full backwards compatibility.
> This saves 2KB in size of struct ir_raw_event_ctrl (as element
> kfifo is reduced by 512 * 4 bytes) and it allows to copy the
> full struct ir_raw_event with a single 64 bit operation.
>
> Successfully tested with the Nuvoton driver and successfully
> compile-tested with the ene_ir driver (as it uses the carrier /
> duty_cycle elements).
>
> Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
> ---
> include/media/rc-core.h | 26 ++++++++------------------
> 1 file changed, 8 insertions(+), 18 deletions(-)
>
> diff --git a/include/media/rc-core.h b/include/media/rc-core.h
> index 0f77b3d..b8f27c9 100644
> --- a/include/media/rc-core.h
> +++ b/include/media/rc-core.h
> @@ -214,27 +214,17 @@ enum raw_event_type {
>
> struct ir_raw_event {
> union {
> - u32 duration;
> -
> - struct {
> - u32 carrier;
> - u8 duty_cycle;
> - };
> + u32 duration;
> + u32 carrier;
> };
> -
> - unsigned pulse:1;
> - unsigned reset:1;
> - unsigned timeout:1;
> - unsigned carrier_report:1;
> + u8 duty_cycle;

Moving duty_cycle does indeed reduce the structure size from 12 to 8.

> + u8 pulse:1;
> + u8 reset:1;
> + u8 timeout:1;
> + u8 carrier_report:1;

Why are you changing the type of the bitfields?

> };
>
> -#define DEFINE_IR_RAW_EVENT(event) \
> - struct ir_raw_event event = { \
> - { .duration = 0 } , \
> - .pulse = 0, \
> - .reset = 0, \
> - .timeout = 0, \
> - .carrier_report = 0 }
> +#define DEFINE_IR_RAW_EVENT(event) struct ir_raw_event event = {}

Seems fine. I've always kinda wondered why this macro is needed.

Sean