Re: [PATCH 8/9] dev-serial: fix FTDI_GET_MDM_ST response

[Samuel Thibault]

Hello,

(Cc-ing Aurelien who introduced the support for modem control, and Jason
who added the missing THRE and TEMT flags).

Mark Cave-Ayland, le lun. 26 oct. 2020 08:34:00 +0000, a ecrit:
> The FTDI_GET_MDM_ST response should only return a single byte indicating the
> modem status with bit 0 cleared (as documented in the Linux ftdi_sio.h header
> file).
> 
> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> ---
>  hw/usb/dev-serial.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/usb/dev-serial.c b/hw/usb/dev-serial.c
> index 4c374d0790..fa734bcf54 100644
> --- a/hw/usb/dev-serial.c
> +++ b/hw/usb/dev-serial.c
> @@ -360,9 +360,8 @@ static void usb_serial_handle_control(USBDevice *dev, 
> USBPacket *p,
>          /* TODO: TX ON/OFF */
>          break;
>      case VendorDeviceRequest | FTDI_GET_MDM_ST:
> -        data[0] = usb_get_modem_lines(s) | 1;
> -        data[1] = FTDI_THRE | FTDI_TEMT;
> -        p->actual_length = 2;
> +        data[0] = usb_get_modem_lines(s);
> +        p->actual_length = 1;

Err, but Linux' drivers/usb/serial/ftdi_sio.c:ftdi_process_packet()
contradicts this: 

        if (len < 2) {
                dev_dbg(&port->dev, "malformed packet\n");
                return 0;
        }

        status = buf[0] & FTDI_STATUS_B0_MASK;
        if (status != priv->prev_status) {
                char diff_status = status ^ priv->prev_status;

                if (diff_status & FTDI_RS0_CTS)
                        port->icount.cts++;

[...]

        /* save if the transmitter is empty or not */
        if (buf[1] & FTDI_RS_TEMT)
                priv->transmit_empty = 1;
        else
                priv->transmit_empty = 0;

Did you actually get an issue with seeing this packet contain two bytes?

Samuel