linux-kernel - Re: [RFC][PATCH 0/1] serdev: Support HS-UART serdev slaves over tty

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Date:   Tue, 24 Apr 2018 19:18:46 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Shrirang Bagul <shrirang.bagul@...onical.com>
Cc:     johan@...nel.org, linux-serial@...r.kernel.org,
        gregkh@...uxfoundation.org, Rob Herring <robh@...nel.org>,
        Frédéric Danis 
        <frederic.danis.oss@...il.com>,
        Hans de Goede <hdegoede@...hat.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Lukas Wunner <lukas@...ner.de>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 0/1] serdev: Support HS-UART serdev slaves over tty

[ Adding some more people on CC. ]

On Tue, Apr 24, 2018 at 04:29:53PM +0800, Shrirang Bagul wrote:
> On systems using Intel Atom (Baytrail-I) SoC's, slave devices connected on
> HSUART1/2 ports are described by the ACPI BIOS as virtual hardware using
> HID's INT3511/INT3512 [1].
> 
> As a consequence, HW manufacturers have complete freedom to install any
> devices on-board as long as they can be accessed over serial tty
> interface. Once such device is Dell Edge 3002 IoT Gateway which sports
> ZigBee & GPS devices on the HS-UART ports 1 & 2 respectively.
> 
> In kernels before the introduction of 'Serial Device Bus (serdev)'
> subsystem, these devices were accessible using /dev/ttySx nodes. But,
> kernels since 4.15 can no longer do so.
> 
> Post 4.15, with CONFIG_SERIAL_DEV_BUS=y, serdev port controller driver
> handles the enumeration for the slaves connected on these ports. Also,
> /dev/ttySx device nodes for these ports are no longer exposed to the
> userspace.
> 
> This patch implements a new driver which binds to the ACPI serdev slaves
> enumerated by the serdev port controller and exposes /dev/ttyHSx device
> nodes which the userspace applications can use. Otherwise, upgrades to 4.15
> or higher kernels would certainly render these devices unusable.
> 
> Considering serdev is new and evolving, this is one approach to solving
> the problem at hand. An obvious drawback is the change in the tty device
> node name from ttySx => ttyHSx, which means userspace applications have to
> be modified (I know that this is strongly discouraged). For the same
> reason, I am submitting these patches as RFC.
> 
> If there are other/better ways of solving this or improving on the
> proposed solution, that will be most helpful.

Yeah, I don't think this is the right solution to this problem. It seems
we need to blacklist (or maybe even use whitelists) ACPI-ids until there
are drivers for the slave devices that would otherwise be claimed by
serdev.

> This patch is based on:
> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git v4.17-rc2
> 
> [1] Enabling Multi-COM Port for Microsoft Windows OS 8.1 & 10 / IoT Core [Sec. 4.1]
> (https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/enabling-multi-com-port-white-paper.pdf)

Thanks,
Johan