Driver for MaxLinear/Exar USB (UART) Serial Adapters

[Patong Yang <patong.mxl@gmail.com>]

On Thu, Aug 16, 2018 at 08:34:47AM +0200, Greg KH wrote:
> On Wed, Aug 15, 2018 at 10:56:47PM -0700, Patong Yang wrote:
> > Greg,
> > 
> > Please see my response inline below.
> > 
> > Patong
> > 
> > > But there is a bigger problem here:
> > > 
> > > > +	xrusb_tty_driver = alloc_tty_driver(XRUSB_TTY_MINORS);
> > > > +	if (!xrusb_tty_driver)
> > > > +		return -ENOMEM;
> > > 
> > > Why are you not using the usb serial core here?  You need to do that,
> > > not try to provide your own custom tty driver.  That way userspace
> > > programs will "just work" with your new device, no changes needed as
> > > your major/minor number and device name would be custom only for your
> > > device, which is not acceptable.
> > 
> > The MaxLinear/USB serial devices support the CDC-ACM commands.
> > Therefore, we used the cdc-acm driver instead of the usb serial driver
> > as the starting point for developing the driver.  We replaced "ACM" 
> > with "XRUSB" throughout the driver.  Would it be better if we just used 
> > the same major/minor number as the CDC-ACM driver since it was based on
> > the cdc-acm driver?  
> 
> No, just use the cdc-acm driver itself and add your product/device id to
> it and it should work just fine.  Why do you need to write a whole new
> driver at all?

The basic TX/RX functionality works fine now with the cdc-acm driver.
That's the driver that is loaded because it's advertised as a cdc-acm
compatible device in the device descriptors.    

However, the cdc-acm driver (and spec) does not have support all of the 
features in the MaxLinear/Exar USB UARTs.  Hence the reason for a separate
and new driver.  

Features not supported by the cdc-acm driver are 
enabling/disabling flow control, enabling/disabling RS-485 mode, 
GPIOs and GPIO modes, etc. Support for these features had to be added in this
new driver.  

Please let me know if there are any futher questions and advise on how
to proceed.

Thanks,
Patong