linux-kernel - Re: [PATCH] USB: serial: cp210x: Workaround for cp2108 failure due to GET_LINE

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Date:	Mon, 19 Oct 2015 08:20:11 -0500
From:	Konstantin Shkolnyy <konstantin.shkolnyy@...il.com>
To:	Oliver Neukum <oneukum@...e.com>
Cc:	Johan Hovold <johan@...nel.org>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: serial: cp210x: Workaround for cp2108 failure due to
 GET_LINE_CTL bug

On Mon, Oct 19, 2015 at 4:14 AM, Oliver Neukum <oneukum@...e.com> wrote:
> On Thu, 2015-10-15 at 17:06 -0500, Konstantin Shkolnyy wrote:
>> cp2108 GET_LINE_CTL returns the 16-bit value with the 2 bytes swapped.
>> However, SET_LINE_CTL functions properly. When the driver tries to modify
>> the register, it reads it, modifies some bits and writes back. Because the
>> read bytes were swapped, this often results in an invalid value to be written.
>> In turn, this causes cp2108 respond with a stall. The stall sometimes doesn't
>> clear properly and cp2108 starts responding to following valid commands also
>> with stalls, effectively failing.
>
> Just a remark.
>>
>> @@ -865,18 +888,53 @@ static void cp210x_break_ctl(struct tty_struct *tty, int break_state)
>>
>>  static int cp210x_startup(struct usb_serial *serial)
>>  {
>> +     struct usb_serial_port *port;
>>       struct usb_host_interface *cur_altsetting;
>>       struct cp210x_serial_private *spriv;
>> +     unsigned int  line_ctl;
>> +     int           err;
>> +
>> +     /* We always expect a single port only */
>> +     if (serial->num_ports != 1) {
>> +             dev_err(&serial->dev->dev, "%s - expected 1 port, found %d\n",
>> +                     __func__, serial->num_ports);
>> +             return -EINVAL;
>> +     }
>
> Why do you introduce this new check? It has no obvious connect with the
> subject of this fix.

Because I later use the port[0] field that this function hasn't
touched before. But this all will go away in the new version of the
patch.
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