Re: [PATCH 1/2] usb: serial: Add MaxLinear/Exar USB to Serial driver

[Manivannan Sadhasivam <mani@kernel.org>]

Hi Greg,

On Wed, Apr 29, 2020 at 09:20:36AM +0200, Greg KH wrote:
> On Wed, Apr 29, 2020 at 01:26:50AM +0530, mani@kernel.org wrote:
> > From: Manivannan Sadhasivam <mani@kernel.org>
> > 
> > Add support for MaxLinear/Exar USB to Serial converters. This driver
> > only supports XR21V141X series but provision has been made to support
> > other series in future.
> > 
> > This driver is inspired from the initial one submitted by Patong Yang:
> > 
> > https://patchwork.kernel.org/patch/10543261/
> > 
> > While the initial driver was a custom tty USB driver exposing whole
> > new serial interface ttyXRUSBn, this version is completely based on USB
> > serial core thus exposing the interfaces as ttyUSBn. This will avoid
> > the overhead of exposing a new USB serial interface which the userspace
> > tools are unaware of.
> 
> Nice work!
> 
> Some comments below:
> 
> > +// SPDX-License-Identifier: GPL-2.0+
> > +/*
> > + * MaxLinear/Exar USB to Serial driver
> > + *
> > + * Based on initial driver written by Patong Yang <patong.mxl@gmail.com>
> > + *
> > + * Copyright (c) 2020 Manivannan Sadhasivam <mani@kernel.org>
> > + */
> > +
> > +#include <linux/kernel.h>
> > +#include <linux/module.h>
> > +#include <linux/slab.h>
> > +#include <linux/tty.h>
> > +#include <linux/usb.h>
> > +#include <linux/usb/serial.h>
> > +
> > +#include "xr_serial.h"
> 
> No need for a .h file for a single .c file.
> 

Yeah but since this driver can support multiple series of XR chips (they
might have separate register definitions and such), I thought it is a good
idea to have a header file to keep the driver sane. But can club it to the
source file for now.

> > +static int xr_get_reg(struct usb_serial_port *port, u8 block, u16 reg,
> > +		      u16 *val)
> > +{
> > +	struct usb_serial *serial = port->serial;
> > +	struct xr_port_private *port_priv = usb_get_serial_port_data(port);
> > +	void *dmabuf;
> > +	int ret = -EINVAL;
> > +
> > +	dmabuf = kmalloc(sizeof(reg), GFP_KERNEL);
> 
> So that is 2 bytes?
> 

Explanation below...

> > +	if (!dmabuf)
> > +		return -ENOMEM;
> > +
> > +	if (port_priv->idProduct == XR21V141X_ID) {
> > +		/* XR21V141X uses custom command for reading UART registers */
> > +		ret = usb_control_msg(serial->dev,
> > +				      usb_rcvctrlpipe(serial->dev, 0),
> > +				      XR_GET_XR21V141X,
> > +				      USB_DIR_IN | USB_TYPE_VENDOR, 0,
> > +				      reg | (block << 8), dmabuf,
> > +				      port_priv->reg_width,
> > +				      USB_CTRL_SET_TIMEOUT);
> > +	}
> > +
> > +	if (ret == port_priv->reg_width) {
> > +		memcpy(val, dmabuf, port_priv->reg_width);
> 
> But here you copy ->reg_width bytes in?  How do you know val can hold
> that much?  It's only set to be 1, so you copy 1 byte to a 16bit value?
> What part of the 16bits did you just copy those 8 bits to (hint, think
> cpu endian issues...)
> 
> That feels really really odd and a bit broken.
> 

Right. The reason is, the other series which can be supported by this driver
have different register widths. For instance XR2280x. I haven't used them
personally but seen this in initial driver. So I just used the max u16 type
to make the reg_{set/get} routines work with those.

But agree, I should've used le16_to_cpu() cast to avoid endian issues.

If you think this hack is not required now, I can just use u8 and worry about
compatibility later.

> > --- /dev/null
> > +++ b/drivers/usb/serial/xr_serial.h
> > @@ -0,0 +1,103 @@
> > +/* SPDX-License-Identifier: GPL-2.0+ */
> 
> Are you sure about the "+"?  I have to ask :)
> 

I'm not a fan but since I've inherited the code from initial driver (which
was GPL-2.0+), I kept it.

> > +
> > +#ifndef __LINUX_USB_SERIAL_XR_SERIAL_H
> > +#define __LINUX_USB_SERIAL_XR_SERIAL_H
> 
> As you will drop this file, just a general statement, no need for
> __LINUX as this is all Linux :)
> 

Sure.

Thanks,
Mani

> thanks,
> 
> greg k-h