Re: [PATCH] pac_common: redesign function for finding Start Of Frame

[Hans de Goede <hdegoede@redhat.com>]

Hi,

Good one,

Acked-by: Hans de Goede <hdegoede@redhat.com>

Jean-Francois, can you please add this patch to your tree?

Thanks,

Hans


On 10/04/2009 10:55 PM, Németh Márton wrote:
> From: Márton Németh<nm127@freemail.hu>
>
> The original implementation of pac_find_sof() does not always find
> the Start Of Frame (SOF) marker. Replace it with a state machine
> based design.
>
> The change was tested with Labtec Webcam 2200.
>
> Signed-off-by: Márton Németh<nm127@freemail.hu>
> ---
> --- linux-2.6.32-rc1.orig/drivers/media/video/gspca/pac_common.h	2009-09-10 00:13:59.000000000 +0200
> +++ linux-2.6.32-rc1/drivers/media/video/gspca/pac_common.h	2009-10-04 21:49:19.000000000 +0200
> @@ -33,6 +33,45 @@
>   static const unsigned char pac_sof_marker[5] =
>   		{ 0xff, 0xff, 0x00, 0xff, 0x96 };
>
> +/*
> +   The following state machine finds the SOF marker sequence
> +   0xff, 0xff, 0x00, 0xff, 0x96 in a byte stream.
> +
> +	   +----------+
> +	   | 0: START |<---------------\
> +	   +----------+<-\             |
> +	     |       \---/otherwise    |
> +	     v 0xff                    |
> +	   +----------+ otherwise      |
> +	   |     1    |--------------->*
> +	   |          |                ^
> +	   +----------+                |
> +	     |                         |
> +	     v 0xff                    |
> +	   +----------+<-\0xff         |
> +	/->|          |--/             |
> +	|  |     2    |--------------->*
> +	|  |          | otherwise      ^
> +	|  +----------+                |
> +	|    |                         |
> +	|    v 0x00                    |
> +	|  +----------+                |
> +	|  |     3    |                |
> +	|  |          |--------------->*
> +	|  +----------+ otherwise      ^
> +	|    |                         |
> +   0xff |    v 0xff                    |
> +	|  +----------+                |
> +	\--|     4    |                |
> +	   |          |----------------/
> +	   +----------+ otherwise
> +	     |
> +	     v 0x96
> +	   +----------+
> +	   |  FOUND   |
> +	   +----------+
> +*/
> +
>   static unsigned char *pac_find_sof(struct gspca_dev *gspca_dev,
>   					unsigned char *m, int len)
>   {
> @@ -41,17 +80,54 @@ static unsigned char *pac_find_sof(struc
>
>   	/* Search for the SOF marker (fixed part) in the header */
>   	for (i = 0; i<  len; i++) {
> -		if (m[i] == pac_sof_marker[sd->sof_read]) {
> -			sd->sof_read++;
> -			if (sd->sof_read == sizeof(pac_sof_marker)) {
> +		switch (sd->sof_read) {
> +		case 0:
> +			if (m[i] == 0xff)
> +				sd->sof_read = 1;
> +			break;
> +		case 1:
> +			if (m[i] == 0xff)
> +				sd->sof_read = 2;
> +			else
> +				sd->sof_read = 0;
> +			break;
> +		case 2:
> +			switch (m[i]) {
> +			case 0x00:
> +				sd->sof_read = 3;
> +				break;
> +			case 0xff:
> +				/* stay in this state */
> +				break;
> +			default:
> +				sd->sof_read = 0;
> +			}
> +			break;
> +		case 3:
> +			if (m[i] == 0xff)
> +				sd->sof_read = 4;
> +			else
> +				sd->sof_read = 0;
> +			break;
> +		case 4:
> +			switch (m[i]) {
> +			case 0x96:
> +				/* Pattern found */
>   				PDEBUG(D_FRAM,
>   					"SOF found, bytes to analyze: %u."
>   					" Frame starts at byte #%u",
>   					len, i + 1);
>   				sd->sof_read = 0;
>   				return m + i + 1;
> +				break;
> +			case 0xff:
> +				sd->sof_read = 2;
> +				break;
> +			default:
> +				sd->sof_read = 0;
>   			}
> -		} else {
> +			break;
> +		default:
>   			sd->sof_read = 0;
>   		}
>   	}
> --
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