Date:   Wed, 25 Apr 2018 09:47:36 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Johan Hovold <johan@...nel.org>,
        Shrirang Bagul <shrirang.bagul@...onical.com>,
        linux-serial@...r.kernel.org, gregkh@...uxfoundation.org,
        Rob Herring <robh@...nel.org>,
        Frédéric Danis 
        <frederic.danis.oss@...il.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

On Tue, Apr 24, 2018 at 11:16:38PM +0200, Hans de Goede wrote:
> Hi,
> 
> On 24-04-18 19:18, Johan Hovold wrote:
> > [ 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.
> 
> FWIW I've been using this patch for a while for realtek UART attached bluetooth:
> https://github.com/jwrdegoede/linux-sunxi/commit/bc904e3703940600ca66c65fcdb0a8cb01dff55d
> which is a gross hack.
> 
> If we're going to do a whitelist for this, it better support some sort of
> wildcards as there are a LOT of BCM2E?? devices which need to be on the
> whitelist. I think a blacklist would actually be better though, this also
> documents which devices are lacking a proper kernel (where applicable).

Yeah, you guys know the ACPI space better than I do. I just fear that
if we go with the blacklist approach, we'll be playing a whack-a-mole
with this for a long time when people start upgrading there systems to
4.15 and discover that their serial ports are gone.

Since this would qualify as a severe regression, me may need to consider
adding a serdev whitelist for every ACPI id we add to a serdev driver
instead.

Thanks,
Johan

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