linux-kernel - Re: [PATCH] media: uvcvideo: Explicit alignment of uvc_frame and uvc

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Date: Fri, 22 Mar 2024 13:56:06 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: uvcvideo: Explicit alignment of uvc_frame and
 uvc_format

Hi Ricardo,

Thank you for the patch.

On Mon, May 01, 2023 at 04:49:31PM +0200, Ricardo Ribalda wrote:
> Struct uvc_frame and uvc_format are packaged together on
> streaming->formats on a sigle allocation.

s/sigle/single/

> 
> This is working fine because both structures have a field with a
> pointer, but it will stop working when the sizeof() of any of those
> structs is not a muliple of the sizeof(void*).
> 
> Make that aligment contract explicit.
> 
> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> ---
> This is better than 3 allocations, and do not have any performance
> penalty.
> ---
>  drivers/media/usb/uvc/uvcvideo.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
> index 9a596c8d894a..03e8a543c8e6 100644
> --- a/drivers/media/usb/uvc/uvcvideo.h
> +++ b/drivers/media/usb/uvc/uvcvideo.h
> @@ -252,7 +252,7 @@ struct uvc_frame {
>  	u8  bFrameIntervalType;
>  	u32 dwDefaultFrameInterval;
>  	u32 *dwFrameInterval;
> -};
> +} __aligned(sizeof(void *)); /* uvc_frame is packed on streaming->formats. */

Don't we need u32 alignment here, not void * alignment, given that
uvc_frame is followed by an array of u32 ?

>  
>  struct uvc_format {
>  	u8 type;
> @@ -266,7 +266,7 @@ struct uvc_format {
>  
>  	unsigned int nframes;
>  	struct uvc_frame *frame;
> -};
> +} __aligned(sizeof(void *)); /* uvc_format is packed on streaming->formats. */

Same here, technically we need to ensure that the following uvc_frame
will be aligned. void * alignment will give us that now, but that's not
the actual constraint.

Wouldn't it be better to handle the alignment constraints explicitly
when allocating the memory ? It's not that uvc_frame and uvc_format have
intrinsic alignment constraints, the constraints are only needed because
of the way memory is allocated.

>  
>  struct uvc_streaming_header {
>  	u8 bNumFormats;
> 
> ---
> base-commit: 58390c8ce1bddb6c623f62e7ed36383e7fa5c02f
> change-id: 20230501-uvc-align-6ff202b68dab

-- 
Regards,

Laurent Pinchart