linux-kernel - Re: [PATCH v2 0/9] Add support for Microsoft Surface System Aggregator Module

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Message-ID: <8c6f7234-fc7e-66c2-948c-1232eb5ff813@redhat.com>
Date:   Sun, 6 Dec 2020 11:43:13 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Maximilian Luz <luzmaximilian@...il.com>,
        Leon Romanovsky <leon@...nel.org>,
        Blaž Hrastnik <blaz@...n.io>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Mark Gross <mgross@...ux.intel.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Barnabás Pőcze <pobrn@...tonmail.com>,
        Arnd Bergmann <arnd@...db.de>, Rob Herring <robh@...nel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Jonathan Corbet <corbet@....net>,
        Dorian Stoll <dorian.stoll@...p.io>,
        platform-driver-x86@...r.kernel.org, linux-serial@...r.kernel.org,
        linux-acpi@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v2 0/9] Add support for Microsoft Surface System
 Aggregator Module

Hi,

On 12/6/20 11:33 AM, Maximilian Luz wrote:
> On 12/6/20 10:06 AM, Leon Romanovsky wrote:> On Sun, Dec 06, 2020 at 05:58:32PM +0900, Blaž Hrastnik wrote:
>>>>
>>>>> More on that, the whole purpose of proposed interface is to debug and
>>>>> not intended to be used by any user space code.
>>>>
>>>> The purpose is to provide raw access to the Surface Serial Hub protocol,
>>>> just like we provide raw access to USB devices and have hidraw devices.
>>>>
>>>> So this goes a litle beyond just debugging; and eventually the choice
>>>> may be made to implement some functionality with userspace drivers,
>>>> just like we do for some HID and USB devices.
>>>>
>>>> Still I agree with you that adding new userspace API is something which
>>>> needs to be considered carefully. So I will look at this closely when
>>>> reviewing this set.
>>>
>>> To add to that: this was previously a debugfs interface but was moved to misc after review on the initial RFC:
>>> https://lkml.org/lkml/2020/9/24/96
>>
>> There is a huge difference between the suggestion and final implementation.
>>
>> Greg suggested to add new debug module to the drivers/misc that will
>> open char device explicitly after user loaded that module to debug this
>> hub. However, the author added full blown char device as a first citizen
>> that has all not-break-user constrains.
> 
> This module still needs to be loaded explicitly.

Good then I really do not see a problem with this.

> And (I might be wrong
> about this) the "not-break-user constraints" hold as soon as I register
> a misc device at all, no?

Correct.

> So I don't see how this is a) any different
> than previously discussed with Greg and b) how the uapi header now
> introduces any not-break-user constraints that would not be there
> without it.
> 
> This interface is intended as a stable interface. That's something that
> I committed to as soon as I decided to implement this via a misc-device.
> 
> Sure, I can move the definitions in the uapi header to the module
> itself, but I don't see any benefit in that.

Right, if we are going to use a misc chardev for this, then the
correct thing to do is to put the API bits for that chardev under
include/uapi.

It would still be good if you can provide a pointer to some userspace
tools using this new API; and for the next version maybe add that
pointer to the commit message

Regards,

Hans