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

From: Mark Cave-Ayland
Subject: Re: [PATCH 8/9] dev-serial: fix FTDI_GET_MDM_ST response
Date: Mon, 26 Oct 2020 10:58:43 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1

On 26/10/2020 09:54, Samuel Thibault wrote:


(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

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 */
      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)


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

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

Hi Samuel,

Thanks for the review! There are 2 different places where the status is read: the one above in ftdi_process_packet() relates to the control packet header for incoming data which is always 2 bytes, whereas this relates to FTDI_SIO_GET_MODEM_STATUS_REQUEST in https://elixir.bootlin.com/linux/latest/source/drivers/usb/serial/ftdi_sio.c#L2825.

I have a FTDI Chipi-X adapter to compare with and that returns 2 bytes, but it looks like I mis-read the code and it's the SIO chipsets that return 1 byte which are older than the chipset being emulated by QEMU. This came from reading https://elixir.bootlin.com/linux/latest/source/drivers/usb/serial/ftdi_sio.h#L415 which states that only 1 byte should be returned, but of course that comment could be out of date.

A quick test shows my Chipi-X returns 0x1 0x60 with nothing attached in response to FTDI_SIO_GET_MODEM_STATUS_REQUEST: assuming the reply length should be 2 bytes, the comment about B0-B3 being zero and the response from my Chip-X above suggests that the "| 1" should still be dropped from the response.



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