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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: Tue, 27 Oct 2020 13:18:44 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.4.0

On 26/10/2020 15:04, Samuel Thibault wrote:

Mark Cave-Ayland, le lun. 26 oct. 2020 13:40:05 +0000, a ecrit:
On 26/10/2020 13:00, Jason Andryuk wrote:
On Mon, Oct 26, 2020 at 7:21 AM Samuel Thibault <samuel.thibault@gnu.org> wrote:
Aurelien, you introduced the "| 1" in

commit abb8a13918ecc1e8160aa78582de9d5224ea70df
Author: Aurelien Jarno <aurelien@aurel32.net>
Date:   Wed Aug 13 04:23:17 2008 +0000

      usb-serial: add support for modem lines

@@ -357,9 +393,9 @@ static int usb_serial_handle_control(USBDevice *dev, int 
request, int value,
           /* TODO: TX ON/OFF */
       case DeviceInVendor | FTDI_GET_MDM_ST:
-        /* TODO: return modem status */
-        data[0] = 0;
-        ret = 1;
+        data[0] = usb_get_modem_lines(s) | 1;
+        data[1] = 0;
+        ret = 2;

I'm not particularly familiar with the FTDI USB serial devices.  I
found setting FTDI_THRE | FTDI_TEMT by comparing with real hardware.

A little searching found this:

That shows "B0   Reserved - must be 1", so maybe that is why "| 1" was added?

Right - that's for the modem status returned as part of the first 2 status
bytes for incoming data which is slightly different from modem status
returned directly from FTDI_SIO_GET_MODEM_STATUS: 

It is the latter which this patch changes and appears to match what I see on
my Chipi-X hardware here.

Aurelien, do you remember the reason for the addition of 1 here? It does
look like the confusion between the incoming data bytes and the modem
status bytes.

I spent a bit of time this morning doing some further tests on Linux using 2 machines and a test program to check CTS and usbmon:

usbmon when adapter unplugged:
ffff95a4bf2dd300 2366831506 S Ci:4:004:0 s c0 05 0000 0000 0002 2 <
ffff95a4bf2dd300 2366831607 C Ci:4:004:0 0 2 = 0160

usbmon when adapter plugged in and remote connected to minicom:
ffff95a4452a79c0 2457273895 S Ci:4:004:0 s c0 05 0000 0000 0002 2 <
ffff95a4452a79c0 2457274057 C Ci:4:004:0 0 2 = 3160

It seems I made a mistake here since the response is interpreted as 2 bytes rather than a single little-endian word: with CTS asserted on the remote the first byte is 0x31 == FTDI_CTS | FTDI_DSR | 1, whilst the 2nd byte is 0x60 == FTDI_THRE | FTDI_TEMT which matches the existing QEMU code rather than the comments in ftdi_sio.h.

So sorry for the noise: I'll drop this patch from the series and post a v2 



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