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

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Date:   Tue, 24 Apr 2018 23:16:38 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Johan Hovold <johan@...nel.org>,
        Shrirang Bagul <shrirang.bagul@...onical.com>
Cc:     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

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
>