Re: [PATCH v2 7/7] USB: serial: ftdi_sio: Fill c_*speed fields with real baud rate

[Johan Hovold <johan@kernel.org>]

On Tue, Jul 12, 2022 at 01:53:06PM +0200, Marek Behún wrote:
> From: Pali Rohár <pali@kernel.org>
> 
> Calculate baud rate value in c_*speed fields to the real values which were
> set on hardware. For this operation, add a new set of methods
> *_divisor_to_baud() for each chip and use them for calculating the real
> baud rate values.
> 
> Each *_divisor_to_baud() method is constructed as an inverse function of
> its corresponding *_baud_to_divisor() method.
> 
> Signed-off-by: Pali Rohár <pali@kernel.org>
> Tested-by: Marek Behún <kabel@kernel.org>
> Signed-off-by: Marek Behún <kabel@kernel.org>
> ---
>  drivers/usb/serial/ftdi_sio.c | 83 +++++++++++++++++++++++++++++++++++
>  1 file changed, 83 insertions(+)
> 
> diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c
> index 39e8c5d06157..838acce53e69 100644
> --- a/drivers/usb/serial/ftdi_sio.c
> +++ b/drivers/usb/serial/ftdi_sio.c
> @@ -1167,6 +1167,23 @@ static int ftdi_sio_baud_to_divisor(int baud)
>  	}
>  }
>  
> +static int ftdi_sdio_divisor_to_baud(u32 divisor)
> +{
> +	switch (divisor) {
> +	case ftdi_sio_b300: return 300;
> +	case ftdi_sio_b600: return 600;
> +	case ftdi_sio_b1200: return 1200;
> +	case ftdi_sio_b2400: return 2400;
> +	case ftdi_sio_b4800: return 4800;
> +	case ftdi_sio_b9600: return 9600;
> +	case ftdi_sio_b19200: return 19200;
> +	case ftdi_sio_b38400: return 38400;
> +	case ftdi_sio_b57600: return 57600;
> +	case ftdi_sio_b115200: return 115200;
> +	default: return 9600;
> +	}
> +}

This one should not be needed as sio only supports this discrete set of
values in the first place.

>  static unsigned short int ftdi_232am_baud_base_to_divisor(int baud, int base)
>  {
>  	unsigned short int divisor;
> @@ -1189,15 +1206,33 @@ static unsigned short int ftdi_232am_baud_base_to_divisor(int baud, int base)
>  	return divisor;
>  }
>  
> +static int ftdi_232am_divisor_base_to_baud(unsigned short int divisor, int base)

I believe "base" was used as a function name suffix in the inverse
function (due to the additional base argument).

> +{
> +	static const unsigned char divfrac_inv[4] = { 0, 4, 2, 1 };
> +	unsigned int divisor3;
> +
> +	if (divisor == 0)
> +		divisor = 1;
> +	divisor3 = (GENMASK(13, 0) & divisor) << 3;
> +	divisor3 |= divfrac_inv[(divisor >> 14) & 0x3];
> +	return DIV_ROUND_CLOSEST(base, 2 * divisor3);
> +}

I don't have the motivation to try to review these inverses right now.

Let's get the rest of the series in order first.

Johan