Re: [PATCH 1/1] USB: serial: pl2303: TA & TB alternate divider

[Johan Hovold <johan@kernel.org>]

On Tue, Mar 16, 2021 at 08:06:20AM -0400, michaelk@IEEE.org wrote:
> From: Michael Katzmann <michaelk@IEEE.org>
> 
> Use an alternate clock divider algorithm and bit ordering for the TA and
> TB versions of the pl2303. It was discovered that these variants do not
> produce the correct baud rates with the existing scheme.
> 
> see https://lore.kernel.org/r/3aee5708-7961-f464-8c5f-6685d96920d6@IEEE.org
> 
> Signed-off-by: Michael G. Katzmann <michaelk@IEEE.org>
> ---
>  drivers/usb/serial/pl2303.c | 45 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
 
> +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;
> +        buf[1] = (exponent & ~0x01) << 4 | mantissa >> 8;
> +        buf[0] = mantissa & 0xff;
> +
> +        /* Calculate and return the exact baud rate. */
> +        baud = (baseline / mantissa) >> exponent;
> +
> +        return baud;
> +}

The code above is still indented using spaces rather than tabs.

> +
> +
>  static void pl2303_encode_baud_rate(struct tty_struct *tty,
>  					struct usb_serial_port *port,
>  					u8 buf[4])
> @@ -645,6 +688,8 @@ static void pl2303_encode_baud_rate(struct tty_struct *tty,
>  
>  	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);

Same here.

>  	else
>  		baud = pl2303_encode_baud_rate_divisor(buf, baud);

Please include a revision in the subject prefix when resending (e.g.
this would be "[PATCH v2]: USB: ...", generally there should be a
matching short changelog below the '---' cut-off line as well).

Johan