Re: non-standard baud rates with Prolific 2303 USB-serial

From: "Michael G. Katzmann" <michaelk@IEEE.org>
To: Johan Hovold <johan@kernel.org>
Cc: "Charles Yeh" <charlesyeh522@gmail.com>,
	"Yeh.Charles [葉榮鑫]" <charles-yeh@prolific.com.tw>,
	linux-serial@vger.kernel.org, linux-usb@vger.kernel.org,
	"Joe Abbott" <jabbott@rollanet.org>
Subject: Re: non-standard baud rates with Prolific 2303 USB-serial
Date: Fri, 12 Mar 2021 20:28:04 -0500	[thread overview]
Message-ID: <a475110e-2f44-eeca-3cd2-dd946e5abfe7@IEEE.org> (raw)
In-Reply-To: <YEtwNzhCmvyKhRto@hovoldconsulting.com>

On 3/12/21 8:44 AM, Johan Hovold wrote:


Let me try that patch in the right format 8-)

--- a/drivers/usb/serial/pl2303.c    2021-03-12 09:30:22.963992109 -0500
+++ b/drivers/usb/serial/pl2303.c    2021-03-12 20:00:20.003526891 -0500
@@ -188,6 +188,7 @@
     unsigned long quirks;
     unsigned int no_autoxonxoff:1;
     unsigned int no_divisors:1;
+    unsigned int alt_divisors:1;
 };
 
 struct pl2303_serial_private {
@@ -217,10 +218,12 @@
     [TYPE_TA] = {
         .name            = "TA",
         .max_baud_rate        = 6000000,
+        .alt_divisors        = true,
     },
     [TYPE_TB] = {
         .name            = "TB",
         .max_baud_rate        = 12000000,
+        .alt_divisors        = true,
     },
     [TYPE_HXD] = {
         .name            = "HXD",
@@ -618,6 +621,46 @@
     return baud;
 }
 
+static speed_t pl2303_encode_baud_rate_divisor_alt(unsigned char buf[4],
+                                                                speed_t baud)
+{
+        unsigned int baseline, mantissa, exponent;
+
+        /*
+         * Apparently, for the TA version the formula is:
+         *   baudrate = 12M * 32 / (mantissa * 2^exponent)
+         * where
+         *   mantissa = buf[10:0]
+         *   exponent = buf[15:13 16]
+         */
+        baseline = 12000000 * 32;
+        mantissa = baseline / baud;
+        if (mantissa == 0)
+                mantissa = 1;   /* Avoid dividing by zero if baud > 32*12M. */
+        exponent = 0;
+        while (mantissa >= 2048) {
+                if (exponent < 15) {
+                        mantissa >>= 1; /* divide by 2 */
+                        exponent++;
+                } else {
+                        /* Exponent is maxed. Trim mantissa and leave. */
+                        mantissa = 2047;
+                        break;
+                }
+        }
+
+        buf[3] = 0x80;
+        buf[2] = exponent & 0x01; // LS bit of exponent
+        buf[1] = (exponent & ~0x01) << 4 | mantissa >> 8; // 3 bits of the exponent and MS 3 bits of the mantissa
+        buf[0] = mantissa & 0xff; // LS 8 bits of the mantissa
+
+        /* Calculate and return the exact baud rate. */
+        baud = (baseline / mantissa) >> exponent;
+
+        return baud;
+}
+
+
 static void pl2303_encode_baud_rate(struct tty_struct *tty,
                     struct usb_serial_port *port,
                     u8 buf[4])
@@ -645,6 +688,8 @@
 
     if (baud == baud_sup)
         baud = pl2303_encode_baud_rate_direct(buf, baud);
+    else if (spriv->type->alt_divisors)
+                baud = pl2303_encode_baud_rate_divisor_alt(buf, baud);
     else
         baud = pl2303_encode_baud_rate_divisor(buf, baud);

