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Bluetooth: hci_bcsp: Use the shared CRC-CCITT byte helper

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bcsp_crc_update() processes each byte as two nibbles, requiring two
dependent table lookups for every header and payload byte when CRC is
enabled.

The existing crc_ccitt_byte() helper implements the same reflected
polynomial with one lookup per byte. Use it instead of the private
nibble-based implementation and select CRC_CCITT for BCSP-only UART
configurations as well. The initial CRC value and final bit reversal
remain unchanged.

This replaces the private 16-entry table with the shared 256-entry table,
trading table size for fewer dependent lookups. An exhaustive comparison
of all 65536 CRC states and 256 input bytes matches both the old code and
a bitwise reference implementation.

Signed-off-by: Xuhua Zhang <zhangxuhua@kylinsec.com.cn>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
This commit is contained in:
Xuhua Zhang 2026-09-07 23:10:15 +08:00 committed by Luiz Augusto von Dentz
parent 1430308fc9
commit 266e44d71b
2 changed files with 4 additions and 23 deletions
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1
drivers/bluetooth/Kconfig
View File

@ -151,6 +151,7 @@ config BT_HCIUART_BCSP
bool "BCSP protocol support"
depends on BT_HCIUART
select BITREVERSE
select CRC_CCITT
help
BCSP (BlueCore Serial Protocol) is serial protocol for communication
between Bluetooth device and host. This protocol is required for non

26
drivers/bluetooth/hci_bcsp.c
View File

@ -25,6 +25,7 @@
#include <linux/ioctl.h>
#include <linux/skbuff.h>
#include <linux/bitrev.h>
#include <linux/crc-ccitt.h>
#include <linux/unaligned.h>
#include <net/bluetooth/bluetooth.h>
@ -75,34 +76,13 @@ struct bcsp_struct {
/* ---- BCSP CRC calculation ---- */
/* Table for calculating CRC for polynomial 0x1021, LSB processed first,
* initial value 0xffff, bits shifted in reverse order.
*/
static const u16 crc_table[] = {
0x0000, 0x1081, 0x2102, 0x3183,
0x4204, 0x5285, 0x6306, 0x7387,
0x8408, 0x9489, 0xa50a, 0xb58b,
0xc60c, 0xd68d, 0xe70e, 0xf78f
};
/* Initialise the crc calculator */
#define BCSP_CRC_INIT(x) x = 0xffff
/* Update crc with next data byte
*
* Implementation note
* The data byte is treated as two nibbles. The crc is generated
* in reverse, i.e., bits are fed into the register from the top.
*/
/* Update crc with next data byte */
static void bcsp_crc_update(u16 *crc, u8 d)
{
u16 reg = *crc;
reg = (reg >> 4) ^ crc_table[(reg ^ d) & 0x000f];
reg = (reg >> 4) ^ crc_table[(reg ^ (d >> 4)) & 0x000f];
*crc = reg;
*crc = crc_ccitt_byte(*crc, d);
}
/* ---- BCSP core ---- */