RE: [PATCH v4 1/5] iwlwifi: mei: add the driver to allow cooperation with CSME

quoted

CSME in two words
-----------------
CSME stands for Converged Security and Management Engine. It is a CPU
on the chipset and runs a dedicated firmware.
AMT (Active Management Technology) is one of the applications that run
on that CPU. AMT allows to control the platform remotely.
Here is a partial list of the use cases:
* View the screen of the plaform, with keyboard and mouse (KVM)
* Attach a remote IDE device
* Have a serial console to the device
* Query the state of the platform
* Reset / shut down / boot the platform

Networking in CSME
------------------
For those uses cases, CSME's firmware has an embedded network stack
and is able to use the network devices of the system: LAN and WLAN.
This is thanks to the CSME's firmware WLAN driver.

One can add a profile (SSID / key / certificate) to the CSME's OS and
CSME will connect to that profile. Then, one can use the WLAN link to
access the applications that run on CSME (AMT is one of them). Note
that CSME is active during power state and power state transitions.
For example, it is possible to have a KVM session open to the system
while the system is rebooting and actually configure the BIOS remotely
over WLAN thanks to AMT.

How all this is related to Linux
--------------------------------
In Linux, there is a driver that allows the OS to talk to the CSME
firmware, this driver is drivers/misc/mei. This driver advertises a
bus that allows other kernel drivers or even user
space) to talk to components inside the CSME firmware.
In practice, the system advertises a PCI device that allows to send /
receive data to / from the CSME firmware. The mei bus drivers in
drivers/misc/mei is an abstration on top of this PCI device.
The driver being added here is called iwlmei and talks to the WLAN
driver inside the CSME firmware through the mei bus driver.
Note that the mei bus driver only gives bus services, it doesn't
define the content of the communication.

Why do we need this driver?
--------------------------
CSME uses the same WLAN device that the OS is expecting to see hence
we need an arbitration mechanism. This is what iwlmei is in charge of.
iwlmei maintains the communication with the CSME firmware's WLAN
driver. The language / protocol that is used between the CSME's
firmware WLAN driver and iwlmei is OS agnostic and is called SAP which
stands for Software Abritration Protocol.
With SAP, iwlmei will be able to tell the CSME firmware's WLAN
driver:
1) Please give me the device.
2) Please note that the SW/HW rfkill state change.
3) Please note that I am now associated to X.
4) Please note that I received this packet.
etc...

There are messages that go the opposite direction as well:
1) Please note that AMT is en/disable.
2) Please note that I believe the OS is broken and hence I'll take
   the device *now*, whether you like it or not, to make sure that
   connectivity is preserved.
3) Please note that I am willing to give the device if the OS
   needs it.
4) Please give me any packet that is sent on UDP / TCP on IP address
   XX.XX.XX.XX and an port ZZ.
5) Please send this packet.
etc...

Please check drivers/net/wireless/intel/iwlwifi/mei/sap.h for the full
protocol specification.

Arbitration is not the only purpose of iwlmei and SAP. SAP also allows
to maintain the AMT's functionality even when the OS owns the device.
To connect to AMT, one needs to initiate an HTTP connection to port
16992. iwlmei will listen to the Rx path and forward (through SAP) to
the CSME firmware the data it got. Then, the embedded HTTP server in
the chipset will reply to the request and send a SAP notification to
ask iwlmei to send the reply.
This way, AMT running on the CSME can still work.

In practice this means that all the use cases quoted above (KVM,
remote IDE device, etc...) will work even when the OS uses the WLAN
device.

How to disable all this?
---------------------------
iwlmei won't be able to do anything if the CSME's networking stack is
not enabled. By default, CSME's networking stack is disabled (this is
a BIOS setting).
In case the CSME's networking stack is disabled, iwlwifi will just get
access to the device because there is no contention with any other
actor and, hence, no arbitration is needed.

In this patch, I only add the iwlmei driver. Integration with iwlwifi
will be implemented in the next one.

Co-Developed-by: Ayala Beker <redacted>
Signed-off-by: Emmanuel Grumbach <redacted>
---
v2: fix a few warnings raised by the different bots
v3: rewrite the commit message
v4: put the debugfs content in a different patch
---

I think I did all you wanted, I can understand if this series doesn't get merged
for 5.14, but I really would like to make sure that we are ready to apply this
after the merge window.
If you're happy with this, great. If not, let me know so that I can fix what you
want to see fixed.
FYI - there is also the patch for mei that I don't send for review because it's
already ACK'ed by Greg, and there is another patch that I haven't sent yet
because it relies on a cfg80211 patch that is now in
net-next: 358ae88881adc3ac1544104272eb7e9408f80b39
If you merge net-next, I'll be able to rebase and send this one as well.

Merge window is now closed, can we make progress here?
Thanks