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

[Hans de Goede]

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).

Regards,

Hans


> > 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