RE: [PATCH v3 4/4] USB: serial: cp210x: Switch to new register access functions for large registers

[Konstantin Shkolnyy]

> -----Original Message-----
> From: linux-usb-owner@xxxxxxxxxxxxxxx [mailto:linux-usb-
> owner@xxxxxxxxxxxxxxx] On Behalf Of Konstantin Shkolnyy
> Sent: Thursday, January 14, 2016 12:16
> To: Martyn Welch; Johan Hovold
> Cc: linux-usb@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: RE: [PATCH v3 4/4] USB: serial: cp210x: Switch to new register access
> functions for large registers
> 
> > -----Original Message-----
> > From: Martyn Welch [mailto:martyn.welch@xxxxxxxxxxxxxxx]
> > Sent: Thursday, January 14, 2016 11:52
> > To: Konstantin Shkolnyy; Johan Hovold
> > Cc: linux-usb@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > Subject: Re: [PATCH v3 4/4] USB: serial: cp210x: Switch to new register
> access
> > functions for large registers
> ...
> 
> > > @@ -1038,27 +941,35 @@ static void cp210x_set_termios(struct
> tty_struct
> > *tty,
> > >   	}
> > >
> > >   	if ((cflag & CRTSCTS) != (old_cflag & CRTSCTS)) {
> > > -		cp210x_get_config(port, CP210X_GET_FLOW, modem_ctl,
> > 16);
> > > -		dev_dbg(dev, "%s - read modem controls = 0x%.4x 0x%.4x
> > 0x%.4x 0x%.4x\n",
> > > -			__func__, modem_ctl[0], modem_ctl[1],
> > > -			modem_ctl[2], modem_ctl[3]);
> > > +
> > > +		/* Only bytes 0, 4 and 7 out of first 8 have functional bits */
> > > +
> > > +		cp210x_read_reg_block(port, CP210X_GET_FLOW,
> > modem_ctl,
> > > +				sizeof(modem_ctl));
> > > +		dev_dbg(dev, "%s - read modem controls = %02x .. .. .. %02x
> > .. .. %02x\n",
> > > +			__func__, modem_ctl[0], modem_ctl[4],
> > modem_ctl[7]);
> > >
> > >   		if (cflag & CRTSCTS) {
> > >   			modem_ctl[0] &= ~0x7B;
> > >   			modem_ctl[0] |= 0x09;
> > > -			modem_ctl[1] = 0x80;
> > > +			modem_ctl[4] = 0x80;
> > > +			/* FIXME - why clear reserved bits just read? */
> > > +			modem_ctl[5] = 0;
> > > +			modem_ctl[6] = 0;
> > > +			modem_ctl[7] = 0;
> > >   			dev_dbg(dev, "%s - flow control = CRTSCTS\n",
> > __func__);
> > >   		} else {
> > >   			modem_ctl[0] &= ~0x7B;
> > >   			modem_ctl[0] |= 0x01;
> > > -			modem_ctl[1] |= 0x40;
> > > +			/* FIXME - OR here istead of assignment looks wrong
> > */
> >
> > s/istead/instead/
> >
> > I'm a little unsure about FIXME comments being added rather than the
> > issue being addressed. If I'm reading this right, then this is the
> > control for the RTS/CTS lines, could the operation without these bits
> > being cleared/ORed be checked by using a serial cable (connected to
> > another serial port) and writing data with and without flow control
> > enabled through a terminal emulator?
> 
> The patching procedure enforced by maintainers dictates that each separate
> patch addresses 1 issue.
> It's much easier to review changes this way. So this particular patch just
> converts from one register access function to another.
> The bugfix patch will come later.
> 
> While doing this cleanup, I also noticed another bug - this function will always
> set the low 2 bits of byte 0 to  01b: "modem_ctl[0] |= 0x01".
> This field is called SERIAL_DTR_MASK. It's 0 by default. ("DTR is held
> inactive"). The function will only write it when CRTSCTS changes.
> So the device will start with 0, then, if CRTSCTS ever changes, it'll become 1
> and stay 1 forever. Looks wrong to me.
> I'm still researching the subject when and how it should be set.
> 
> 	 * Wikipedia: "DTR and DSR are usually on all the time and, per the
> 	 * RS-232 standard and its successors, are used to signal from each
> 	 * end that the other equipment is actually present and powered-
> up."
> 	 * So perhaps DTR should be turned ON in open() and OFF in close()?

And the same problem with SERIAL_RTS_MASK. Here is what this piece of code turned it into after fixing the FIXME you mentioned and assigning names to all the bits.

	if ((cflag & CRTSCTS) != (old_cflag & CRTSCTS)) {
		struct cp210x_flow_ctl flow_ctl;
	
		/* Only bytes 0, 4 and 7 out of first 8 have functional bits */

		cp210x_read_reg_block(port, CP210X_GET_FLOW, &flow_ctl,
				sizeof(flow_ctl));

		if (cflag & CRTSCTS) {
			/* "DTR is held active" */
			flow_ctl.SERIAL_DTR_MASK        = 1;
			/* "CTS is a handshake line" */
			flow_ctl.SERIAL_CTS_HANDSHAKE   = 1;
			flow_ctl.SERIAL_DSR_HANDSHAKE   = 0;
			flow_ctl.SERIAL_DCD_HANDSHAKE   = 0;
			flow_ctl.SERIAL_DSR_SENSITIVITY = 0;

			flow_ctl.SERIAL_AUTO_TRANSMIT   = 0;
			flow_ctl.SERIAL_AUTO_RECEIVE    = 0;
			flow_ctl.SERIAL_ERROR_CHAR      = 0;
			flow_ctl.SERIAL_NULL_STRIPPING  = 0;
			flow_ctl.SERIAL_BREAK_CHAR      = 0;
			/* "RTS is used for receive flow control" */
			flow_ctl.SERIAL_RTS_MASK        = 2;

			dev_dbg(dev, "%s - flow control = CRTSCTS\n", __func__);
		} else {
			/* "DTR is held active" */
			flow_ctl.SERIAL_DTR_MASK        = 1;
			/* "CTS is simply a status input" */
			flow_ctl.SERIAL_CTS_HANDSHAKE   = 0;
			flow_ctl.SERIAL_DSR_HANDSHAKE   = 0;
			flow_ctl.SERIAL_DCD_HANDSHAKE   = 0;
			flow_ctl.SERIAL_DSR_SENSITIVITY = 0;

			flow_ctl.SERIAL_AUTO_TRANSMIT   = 0;
			flow_ctl.SERIAL_AUTO_RECEIVE    = 0;
			flow_ctl.SERIAL_ERROR_CHAR      = 0;
			flow_ctl.SERIAL_NULL_STRIPPING  = 0;
			flow_ctl.SERIAL_BREAK_CHAR      = 0;
			/* "RTS is statically active" */
			flow_ctl.SERIAL_RTS_MASK        = 1;

			dev_dbg(dev, "%s - flow control = NONE\n", __func__);
		}
		flow_ctl.SERIAL_XOFF_CONTINUE = 0;

		cp210x_write_reg_block(port, CP210X_SET_FLOW, flow_ctl,
				sizeof(flow_ctl));
	}