linux-kernel - Re: [PATCH] media: stkwebcam: move stk_camera_read_reg() scratch buffer to struct stk

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Message-ID: <11fa30cb-9b56-ab8c-1352-279052945432@gmail.com>
Date:   Sat, 12 Mar 2022 22:50:37 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     trix@...hat.com, mchehab@...nel.org, hverkuil-cisco@...all.nl,
        cai.huoqing@...ux.dev, xose.vazquez@...il.com
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: stkwebcam: move stk_camera_read_reg() scratch
 buffer to struct stk_camera

Hi Trix,

On 3/12/22 20:30, trix@...hat.com wrote:
> From: Tom Rix <trix@...hat.com>
> 
> In stk_camera_read_reg() a single byte buffer is alloc-ed and
> freed on every function call.  Since the size is known,
> move the buffer to the struct stk_camera where it will be alloc-ed
> and freed once.
> 
> Signed-off-by: Tom Rix <trix@...hat.com>
> ---
>   drivers/media/usb/stkwebcam/stk-webcam.c | 11 ++---------
>   drivers/media/usb/stkwebcam/stk-webcam.h |  2 ++
>   2 files changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/media/usb/stkwebcam/stk-webcam.c b/drivers/media/usb/stkwebcam/stk-webcam.c
> index 5b822214ccc5c..787edb3d47c23 100644
> --- a/drivers/media/usb/stkwebcam/stk-webcam.c
> +++ b/drivers/media/usb/stkwebcam/stk-webcam.c
> @@ -150,25 +150,18 @@ int stk_camera_write_reg(struct stk_camera *dev, u16 index, u8 value)
>   int stk_camera_read_reg(struct stk_camera *dev, u16 index, u8 *value)
>   {
>   	struct usb_device *udev = dev->udev;
> -	unsigned char *buf;
>   	int ret;
>   
> -	buf = kmalloc(sizeof(u8), GFP_KERNEL);
> -	if (!buf)
> -		return -ENOMEM;
> -
>   	ret = usb_control_msg(udev, usb_rcvctrlpipe(udev, 0),
>   			0x00,
>   			USB_DIR_IN | USB_TYPE_VENDOR | USB_RECIP_DEVICE,
>   			0x00,
>   			index,
> -			buf,
> +			&dev->read_reg_scratch,
>   			sizeof(u8),
>   			500);


Wouldn't it be better to move to modern USB API like usb_control_msg_recv()?

This API does not require buffer to be allocated via kmalloc(), so you 
will be able to use value directly.




With regards,
Pavel Skripkin