On Mon, Nov 01, 2021 at 12:53:03PM +0800, Charles Yeh wrote:

This issue can be closed.

Sorry, it took so long to reply...
Finally, our company(Prolific) decided to change the new chip from
"Class FFh (Vendor Specific)" to "Class 02h (Communications and CDC Control)"
In this way, the chip's default support can be backward compatible
with the lower version of the Linux kernel (such as 3.X / 4.X,...)

But is the device really compliant with the CDC specification?  If so,
wonderful, but if not, then:

If there are customers who need RTS/CTS and other flow control in the future.
We(Prolific) will provide patch file to support

That is very odd, we can take such a patch now in the kernel source tree
so that you will not have to deal with this for the thousands of
different companies wanting this.  Please submit the needed patch here
and we will be glad to review it.

When a person has to run a patched kernel, they loose the support of the
vendor they are getting their kernel from, so you would require anyone
who runs your changed driver to invalidate their vendor support
contract.  Not something that I am sure anyone would really want to do
"just" for a usb to serial device.

thanks,

greg k-h