linux-kernel - Re: [PATCH] media: usb: ttusb-dec: reduce the number of memory reads in ttusb_dec_handle

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Message-ID: <20200507111526.GA1498671@kroah.com>
Date:   Thu, 7 May 2020 13:15:26 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
Cc:     mchehab@...nel.org, allison@...utok.net, tglx@...utronix.de,
        rfontana@...hat.com, tomasbortoli@...il.com, sean@...s.org,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: usb: ttusb-dec: reduce the number of memory reads
 in ttusb_dec_handle_irq()

On Thu, May 07, 2020 at 06:25:21PM +0800, Jia-Ju Bai wrote:
> In ttusb_dec_handle_irq(), buffer[4] is continuously read from memory 
> three times, without being modified.
> To reduce the number of memory reads, buffer[4] is first assigned to a
> local variable index, and then index is used to replace buffer[4].
> 
> Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
> ---
>  drivers/media/usb/ttusb-dec/ttusb_dec.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/media/usb/ttusb-dec/ttusb_dec.c b/drivers/media/usb/ttusb-dec/ttusb_dec.c
> index 3198f9624b7c..8543c552515b 100644
> --- a/drivers/media/usb/ttusb-dec/ttusb_dec.c
> +++ b/drivers/media/usb/ttusb-dec/ttusb_dec.c
> @@ -250,6 +250,7 @@ static void ttusb_dec_handle_irq( struct urb *urb)
>  	struct ttusb_dec *dec = urb->context;
>  	char *buffer = dec->irq_buffer;
>  	int retval;
> +	int index = buffer[4];
>  
>  	switch(urb->status) {
>  		case 0: /*success*/
> @@ -281,11 +282,11 @@ static void ttusb_dec_handle_irq( struct urb *urb)
>  		 * this should/could be added later ...
>  		 * for now lets report each signal as a key down and up
>  		 */
> -		if (buffer[4] - 1 < ARRAY_SIZE(rc_keys)) {
> -			dprintk("%s:rc signal:%d\n", __func__, buffer[4]);
> -			input_report_key(dec->rc_input_dev, rc_keys[buffer[4] - 1], 1);
> +		if (index - 1 < ARRAY_SIZE(rc_keys)) {
> +			dprintk("%s:rc signal:%d\n", __func__, index);
> +			input_report_key(dec->rc_input_dev, rc_keys[index - 1], 1);
>  			input_sync(dec->rc_input_dev);
> -			input_report_key(dec->rc_input_dev, rc_keys[buffer[4] - 1], 0);
> +			input_report_key(dec->rc_input_dev, rc_keys[index - 1], 0);
>  			input_sync(dec->rc_input_dev);
>  		}
>  	}
> -- 
> 2.17.1
> 

Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>