Re: [PATCH v2 0/9] Add support for Microsoft Surface System Aggregator Module
From: Maximilian Luz
Date: Sun Dec 06 2020 - 05:52:49 EST
On 12/6/20 11:04 AM, Hans de Goede wrote:
Hi,
On 12/6/20 9:56 AM, Leon Romanovsky wrote:
On Sun, Dec 06, 2020 at 09:41:21AM +0100, Hans de Goede wrote:
Hi Leon,
On 12/6/20 8:07 AM, Leon Romanovsky wrote:
On Thu, Dec 03, 2020 at 10:26:31PM +0100, Maximilian Luz wrote:
Hello,
Here is version two of the Surface System Aggregator Module (SAM/SSAM)
driver series, adding initial support for the embedded controller on 5th
and later generation Microsoft Surface devices. Initial support includes
the ACPI interface to the controller, via which battery and thermal
information is provided on some of these devices.
The previous version and cover letter detailing what this series is
about can be found at
https://lore.kernel.org/platform-driver-x86/20201115192143.21571-1-luzmaximilian@xxxxxxxxx/
This patch-set can also be found at the following repository and
reference, if you prefer to look at a kernel tree instead of these
emails:
https://github.com/linux-surface/kernel tags/s/surface-aggregator/v2
Thank you all for the feedback to v1, I hope I have addressed all
comments.
I think that it is too far fetched to attempt and expose UAPI headers
for some obscure char device that we are all know won't be around in
a couple of years from now due to the nature of how this embedded world
works.
This is not for an embedded device, but for the popular line of
Microsoft Surface laptops / 2-in-1s...
It is the naming, we don't have char device for every "laptop" vendor.
Why is Microsoft different here?
Because their hardware department has invented a whole new way of dealing
with a bunch of things at the hardware level (for some reason).
Also almost all laptop vendors have a whole bunch of laptop vendor
specific userspace API in the form of sysfs files exported by
drivers/platform/x86/laptop-vendor.c drivers. E.g. do:
ls /sys/bus/platform/devices/thinkpad_acpi/
An any IBM/Lenovo Thinkpad (and only on a Thinkpad) to see a bunch
of laptop vendor specific UAPI.
Since I've become the pdx86 subsys maintainer I've actually been
pushing back against adding more of this, instead trying to
either use existing UAPIs, or defining new common UAPIs which can
be shared between vendors.
So I agree very much with you that we need to be careful about
needlessly introducing new UAPI.
But there is a difference between being careful and just nacking
it because no new UAPI may be added at all (also see GKH's response).
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.
USB devices implement standard protocol, this surface hub is nothing
even close to that.
The USB protocol just defines a transport layer, outside of the USB classes
there are plenty of proprietary protocols on top of that transport.
And this chardev just offers access to the Surface Serial Hub transport
protocol. And if you want something even closer the i2cdev module offers
raw I2C transfer access and I2C defines no protocol other then
how to read or write a number of bytes.
I do a lot of hw enablement work and being able to poke HID / USB / I2C
devices directly from userspace is very useful for this.
This is pretty much the whole reason for this module. Surface devices
vary from generation to generation, and I can't expect some random user
to write some kernel module (or repeatedly pull 10 different versions
of it)to test if some EC command does x or y. I can tell them to run a
script with some arguments though. The main reason for this is
development, debugging is secondary and IMHO part of development.
So, IOW this interface provides direct access to the EC that would,
without it, require you to write a kernel driver.
The whole "this is intended for development and debugging" shtick is to
deter people from using it to implement user-space based drivers. While
this may be possible at some point, at the moment there is no good way
to (reliably) detect which client devices are present from user-space.
So any attempts at that will likely be unstable and should be
implemented in the kernel anyway. It is a way of prototyping drivers
though.
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.
I don't know how it goes in device/platform area, but in other large
subsystems, UAPI should be presented with working user-space part.
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.
So this ^^^ still stands, I agree with you that adding new UAPI needs
to be considered carefully and when I get around to reviewing this
that is exactly what I will do.
Maximilian, can you perhaps explain a bit more of what you want / expect
to use the chardev for, and maybe provide pointers to the matching
userspace utilities (which I presume you have) ?
Sure. There's a bunch of scripts at
https://github.com/linux-surface/surface-aggregator-module/tree/master/scripts/ssam
As described above, the main idea behind this simplifying development
for devices that I can't test myself. See e.g. the "ctrl.py" script
which can be used to send a basic command to the EC. The "hid.py" script
is one that was successfully used to test commands for the Keyboard
driver on the Surface Laptop 1 and 2.
At some point my plan is to maybe split that out into its own repo and
improve usability for all that, but I haven't gotten to that yet.
[...]
Regards,
Max