Re: non-standard baud rates with Prolific 2303 USB-serial

From: "Michael G. Katzmann" <michaelk@IEEE.org>
To: Johan Hovold <johan@kernel.org>
Cc: charles-yeh@prolific.com.tw, linux-serial@vger.kernel.org,
	linux-usb@vger.kernel.org, Charles Yeh <charlesyeh522@gmail.com>,
	Joe Abbott <jabbott@rollanet.org>
Subject: Re: non-standard baud rates with Prolific 2303 USB-serial
Date: Wed, 24 Feb 2021 13:13:39 -0500	[thread overview]
Message-ID: <2162ce18-32e9-e54c-f266-47febdea11f1@IEEE.org> (raw)
In-Reply-To: <YDaHEtQCGkiM/pad@hovoldconsulting.com>

On 2/24/21 12:04 PM, Johan Hovold wrote:
> Perhaps you can even figure out how to poll for an empty TX FIFO from
> it, unless Charles is able to provide some details on that separate
> matter?
>
I presume from the code below, that when the device is closed, all data waiting to send is clobbered (if so, so the problem is the driver and not the device)

I would have thought that the driver should drain the buffers. I can see that this might be a problem if there is flow control (it may never drain) but the current method seems pretty brutal.

119 void usb_serial_generic_close(struct usb_serial_port *port)
120 {
121         unsigned long flags;
122         int i;
123
124         if (port->bulk_out_size) {
125                 for (i = 0; i < ARRAY_SIZE(port->write_urbs); ++i)
126                         usb_kill_urb(port->write_urbs[i]);
127
128                 spin_lock_irqsave(&port->lock, flags);
129                 kfifo_reset_out(&port->write_fifo);
130                 spin_unlock_irqrestore(&port->lock, flags);
131         }
132         if (port->bulk_in_size) {
133                 for (i = 0; i < ARRAY_SIZE(port->read_urbs); ++i)
134                         usb_kill_urb(port->read_urbs[i]);
135         }
136 }