Merge tag 'char-misc-6.18-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc

Pull Char/Misc/IIO/Binder updates from Greg KH:
"Here is the big set of char/misc/iio and other driv

Merge tag 'char-misc-6.18-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc

Pull Char/Misc/IIO/Binder updates from Greg KH:
"Here is the big set of char/misc/iio and other driver subsystem
changes for 6.18-rc1.

Loads of different stuff in here, it was a busy development cycle in
lots of different subsystems, with over 27k new lines added to the
tree.

Included in here are:

- IIO updates including new drivers, reworking of existing apis, and
other goodness in the sensor subsystems

- MEI driver updates and additions

- NVMEM driver updates

- slimbus removal for an unused driver and some other minor updates

- coresight driver updates and additions

- MHI driver updates

- comedi driver updates and fixes

- extcon driver updates

- interconnect driver additions

- eeprom driver updates and fixes

- minor UIO driver updates

- tiny W1 driver updates

But the majority of new code is in the rust bindings and additions,
which includes:

- misc driver rust binding updates for read/write support, we can now
write "normal" misc drivers in rust fully, and the sample driver
shows how this can be done.

- Initial framework for USB driver rust bindings, which are disabled
for now in the build, due to limited support, but coming in through
this tree due to dependencies on other rust binding changes that
were in here. I'll be enabling these back on in the build in the
usb.git tree after -rc1 is out so that developers can continue to
work on these in linux-next over the next development cycle.

- Android Binder driver implemented in Rust.

This is the big one, and was driving a huge majority of the rust
binding work over the past years. Right now there are two binder
drivers in the kernel, selected only at build time as to which one
to use as binder wants to be included in the system at boot time.

The binder C maintainers all agreed on this, as eventually, they
want the C code to be removed from the tree, but it will take a few
releases to get there while both are maintained to ensure that the
rust implementation is fully stable and compliant with the existing
userspace apis.

All of these have been in linux-next for a while"

* tag 'char-misc-6.18-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc: (320 commits)
rust: usb: keep usb::Device private for now
rust: usb: don't retain device context for the interface parent
USB: disable rust bindings from the build for now
samples: rust: add a USB driver sample
rust: usb: add basic USB abstractions
coresight: Add label sysfs node support
dt-bindings: arm: Add label in the coresight components
coresight: tnoc: add new AMBA ID to support Trace Noc V2
coresight: Fix incorrect handling for return value of devm_kzalloc
coresight: tpda: fix the logic to setup the element size
coresight: trbe: Return NULL pointer for allocation failures
coresight: Refactor runtime PM
coresight: Make clock sequence consistent
coresight: Refactor driver data allocation
coresight: Consolidate clock enabling
coresight: Avoid enable programming clock duplicately
coresight: Appropriately disable trace bus clocks
coresight: Appropriately disable programming clocks
coresight: etm4x: Support atclk
coresight: catu: Support atclk
...

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rust_binder: add Rust Binder driver

We're generally not proponents of rewrites (nasty uncomfortable things
that make you late for dinner!). So why rewrite Binder?

Binder has been evolving over the

rust_binder: add Rust Binder driver

We're generally not proponents of rewrites (nasty uncomfortable things
that make you late for dinner!). So why rewrite Binder?

Binder has been evolving over the past 15+ years to meet the evolving
needs of Android. Its responsibilities, expectations, and complexity
have grown considerably during that time. While we expect Binder to
continue to evolve along with Android, there are a number of factors
that currently constrain our ability to develop/maintain it. Briefly
those are:

1. Complexity: Binder is at the intersection of everything in Android and
fulfills many responsibilities beyond IPC. It has become many things
to many people, and due to its many features and their interactions
with each other, its complexity is quite high. In just 6kLOC it must
deliver transactions to the right threads. It must correctly parse
and translate the contents of transactions, which can contain several
objects of different types (e.g., pointers, fds) that can interact
with each other. It controls the size of thread pools in userspace,
and ensures that transactions are assigned to threads in ways that
avoid deadlocks where the threadpool has run out of threads. It must
track refcounts of objects that are shared by several processes by
forwarding refcount changes between the processes correctly. It must
handle numerous error scenarios and it combines/nests 13 different
locks, 7 reference counters, and atomic variables. Finally, It must
do all of this as fast and efficiently as possible. Minor performance
regressions can cause a noticeably degraded user experience.

2. Things to improve: Thousand-line functions [1], error-prone error
handling [2], and confusing structure can occur as a code base grows
organically. After more than a decade of development, this codebase
could use an overhaul.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/android/binder.c?h=v6.5#n2896
[2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/android/binder.c?h=v6.5#n3658

3. Security critical: Binder is a critical part of Android's sandboxing
strategy. Even Android's most de-privileged sandboxes (e.g. the
Chrome renderer, or SW Codec) have direct access to Binder. More than
just about any other component, it's important that Binder provide
robust security, and itself be robust against security
vulnerabilities.

It's #1 (high complexity) that has made continuing to evolve Binder and
resolving #2 (tech debt) exceptionally difficult without causing #3
(security issues). For Binder to continue to meet Android's needs, we
need better ways to manage (and reduce!) complexity without increasing
the risk.

The biggest change is obviously the choice of programming language. We
decided to use Rust because it directly addresses a number of the
challenges within Binder that we have faced during the last years. It
prevents mistakes with ref counting, locking, bounds checking, and also
does a lot to reduce the complexity of error handling. Additionally,
we've been able to use the more expressive type system to encode the
ownership semantics of the various structs and pointers, which takes the
complexity of managing object lifetimes out of the hands of the
programmer, reducing the risk of use-after-frees and similar problems.

Rust has many different pointer types that it uses to encode ownership
semantics into the type system, and this is probably one of the most
important aspects of how it helps in Binder. The Binder driver has a lot
of different objects that have complex ownership semantics; some
pointers own a refcount, some pointers have exclusive ownership, and
some pointers just reference the object and it is kept alive in some
other manner. With Rust, we can use a different pointer type for each
kind of pointer, which enables the compiler to enforce that the
ownership semantics are implemented correctly.

Another useful feature is Rust's error handling. Rust allows for more
simplified error handling with features such as destructors, and you get
compilation failures if errors are not properly handled. This means that
even though Rust requires you to spend more lines of code than C on
things such as writing down invariants that are left implicit in C, the
Rust driver is still slightly smaller than C binder: Rust is 5.5kLOC and
C is 5.8kLOC. (These numbers are excluding blank lines, comments,
binderfs, and any debugging facilities in C that are not yet implemented
in the Rust driver. The numbers include abstractions in rust/kernel/
that are unlikely to be used by other drivers than Binder.)

Although this rewrite completely rethinks how the code is structured and
how assumptions are enforced, we do not fundamentally change *how* the
driver does the things it does. A lot of careful thought has gone into
the existing design. The rewrite is aimed rather at improving code
health, structure, readability, robustness, security, maintainability
and extensibility. We also include more inline documentation, and
improve how assumptions in the code are enforced. Furthermore, all
unsafe code is annotated with a SAFETY comment that explains why it is
correct.

We have left the binderfs filesystem component in C. Rewriting it in
Rust would be a large amount of work and requires a lot of bindings to
the file system interfaces. Binderfs has not historically had the same
challenges with security and complexity, so rewriting binderfs seems to
have lower value than the rest of Binder.

Correctness and feature parity
------------------------------

Rust binder passes all tests that validate the correctness of Binder in
the Android Open Source Project. We can boot a device, and run a variety
of apps and functionality without issues. We have performed this both on
the Cuttlefish Android emulator device, and on a Pixel 6 Pro.

As for feature parity, Rust binder currently implements all features
that C binder supports, with the exception of some debugging facilities.
The missing debugging facilities will be added before we submit the Rust
implementation upstream.

Tracepoints
-----------

I did not include all of the tracepoints as I felt that the mechansim
for making C access fields of Rust structs should be discussed on list
separately. I also did not include the support for building Rust Binder
as a module since that requires exporting a bunch of additional symbols
on the C side.

Original RFC Link with old benchmark numbers:
https://lore.kernel.org/r/20231101-rust-binder-v1-0-08ba9197f637@google.com

Co-developed-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Co-developed-by: Matt Gilbride <mattgilbride@google.com>
Signed-off-by: Matt Gilbride <mattgilbride@google.com>
Acked-by: Carlos Llamas <cmllamas@google.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250919-rust-binder-v2-1-a384b09f28dd@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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