Driver for MaxLinear/Exar USB (UART) Serial adapters. - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Patong Yang <patong.mxl@gmail.com>
Cc: johan@kernel.org, pyang@maxlinear.com, linux-usb@vger.kernel.org
Subject: Driver for MaxLinear/Exar USB (UART) Serial adapters.
Date: Thu, 5 Apr 2018 08:26:54 +0200	[thread overview]
Message-ID: <20180405062654.GA5078@kroah.com> (raw)

On Wed, Apr 04, 2018 at 12:03:51PM -0700, Patong Yang wrote:
> Thanks for the quick reply and feedback.  This needs to be a different
> driver because although the USB UARTs can work with the CDC-ACM driver,
> there are limitations in the CDC-ACM spec and therefore the driver that
> prevents it from fully functioning as a standard serial port (ie. status of
> CTS pin, ability to enable/disable flow control).  Therefore, the USB UARTs
> also support custom USB Vendor requests so that it can behave as a standard
> serial port and support other device features such as GPIO control and the
> automatic direction control feature typically used in RS-485 and RS-422
> designs.

The GPIO stuff needs to be a separate interface, using the kernel's GPIO
api, not custom ioctls.

> The USB device can describe itself properly.  The SMBIOS function is a
> requirement from our customer who designed a board using our device where
> their CPU reads from a specific BIOS location and initializes GPIOs based
> on the settings.  These GPIOs set the mode of the transceiver (LOOPBACK,
> RS232, RS485, or RS422).  Therefore, the driver also reads the same
> settings from the BIOS, so that it can enable the appropriate mode.

That logic can be done in userspace, no need to do that within the
kernel, right?

> This same customer is designing a new board to support the same transceiver
> configurations.  Instead of using the BIOS settings, they would like to use
> a user-space application to configure the additional GPIOs in a newer USB
> UART to set the transceiver modes, however, there are no standard APIs to
> support setting different modes, hence the custom IOCTLs.

What is wrong with the current GPIO Linux api?  Doesn't that support
everything you need here?

thanks,

greg k-h
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