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

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Message-ID: <1821148.enBAg0iGPe@aspire.rjw.lan>
Date:   Wed, 25 Apr 2018 11:00:47 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Johan Hovold <johan@...nel.org>,
        Shrirang Bagul <shrirang.bagul@...onical.com>
Cc:     Hans de Goede <hdegoede@...hat.com>, linux-serial@...r.kernel.org,
        gregkh@...uxfoundation.org, Rob Herring <robh@...nel.org>,
        Frédéric Danis 
        <frederic.danis.oss@...il.com>, Lukas Wunner <lukas@...ner.de>,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org
Subject: Re: [RFC][PATCH 0/1] serdev: Support HS-UART serdev slaves over tty

On Wednesday, April 25, 2018 9:47:36 AM CEST Johan Hovold wrote:
> 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.

OK, so let's have the ACPI discussion on linux-acpi pretty please.

Could this be resent with a CC to linux-acpi for some more complete context?