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path: root/drivers/usb/serial/pl2303.c
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Diffstat (limited to 'drivers/usb/serial/pl2303.c')
-rw-r--r--drivers/usb/serial/pl2303.c35
1 files changed, 26 insertions, 9 deletions
diff --git a/drivers/usb/serial/pl2303.c b/drivers/usb/serial/pl2303.c
index f5257af33ecf..ae682e4eeaef 100644
--- a/drivers/usb/serial/pl2303.c
+++ b/drivers/usb/serial/pl2303.c
@@ -362,21 +362,38 @@ static speed_t pl2303_encode_baud_rate_direct(unsigned char buf[4],
static speed_t pl2303_encode_baud_rate_divisor(unsigned char buf[4],
speed_t baud)
{
- unsigned int tmp;
+ unsigned int baseline, mantissa, exponent;
/*
* Apparently the formula is:
- * baudrate = 12M * 32 / (2^buf[1]) / buf[0]
+ * baudrate = 12M * 32 / (mantissa * 4^exponent)
+ * where
+ * mantissa = buf[8:0]
+ * exponent = buf[11:9]
*/
- tmp = 12000000 * 32 / baud;
+ baseline = 12000000 * 32;
+ mantissa = baseline / baud;
+ if (mantissa == 0)
+ mantissa = 1; /* Avoid dividing by zero if baud > 32*12M. */
+ exponent = 0;
+ while (mantissa >= 512) {
+ if (exponent < 7) {
+ mantissa >>= 2; /* divide by 4 */
+ exponent++;
+ } else {
+ /* Exponent is maxed. Trim mantissa and leave. */
+ mantissa = 511;
+ break;
+ }
+ }
+
buf[3] = 0x80;
buf[2] = 0;
- buf[1] = (tmp >= 256);
- while (tmp >= 256) {
- tmp >>= 2;
- buf[1] <<= 1;
- }
- buf[0] = tmp;
+ buf[1] = exponent << 1 | mantissa >> 8;
+ buf[0] = mantissa & 0xff;
+
+ /* Calculate and return the exact baud rate. */
+ baud = (baseline / mantissa) >> (exponent << 1);
return baud;
}