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

[Mark Cave-Ayland]

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:
> > Mark Cave-Ayland, le lun. 26 oct. 2020 10:58:43 +0000, a ecrit:
> > > On 26/10/2020 09:54, Samuel Thibault wrote:
> > > > 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;
> > [...]
> > > 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.
> >
> > 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 */
> >           break;
> >       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;
> >           break;
> >
> > do you know exactly what it is for?
>
> Hi,
>
> 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:
> https://elixir.bootlin.com/linux/latest/source/drivers/usb/serial/ftdi_sio.h#L541
>
> 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: 
https://elixir.bootlin.com/linux/latest/source/drivers/usb/serial/ftdi_sio.h#L423.

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


ATB,

Mark.