ANDROID: ashmem_rust: define Rust ashmem driver

For now, we just register a miscdevice with no operations defined on it.

Bug: 370906207
Change-Id: I9ea307095ffd66859239ec4926523222466d3602
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

drivers/staging/Makefile

@@ -14,7 +14,7 @@ obj-$(CONFIG_VME_BUS)		+= vme_user/
 obj-$(CONFIG_IIO)		+= iio/
 obj-$(CONFIG_FB_SM750)		+= sm750fb/
 obj-$(CONFIG_MFD_NVEC)		+= nvec/
-obj-$(CONFIG_ASHMEM)		+= android/
+obj-$(CONFIG_ANDROID_STAGING)	+= android/
 obj-$(CONFIG_LTE_GDM724X)	+= gdm724x/
 obj-$(CONFIG_FB_TFT)		+= fbtft/
 obj-$(CONFIG_MOST)		+= most/

drivers/staging/android/Kconfig

@@ -1,9 +1,25 @@
 # SPDX-License-Identifier: GPL-2.0
 menu "Android"
 
+config ANDROID_STAGING
+	bool
+
 config ASHMEM
 	bool "Enable the Anonymous Shared Memory Subsystem"
 	depends on SHMEM
+	select ANDROID_STAGING
+	help
+	  The ashmem subsystem is a new shared memory allocator, similar to
+	  POSIX SHM but with different behavior and sporting a simpler
+	  file-based API.
+
+	  It is, in theory, a good memory allocator for low-memory devices,
+	  because it can discard shared memory units when under memory pressure.
+
+config ASHMEM_RUST
+	bool "Enable the Anonymous Shared Memory Subsystem in Rust"
+	depends on SHMEM && RUST && !ASHMEM
+	select ANDROID_STAGING
 	help
 	  The ashmem subsystem is a new shared memory allocator, similar to
 	  POSIX SHM but with different behavior and sporting a simpler

drivers/staging/android/Makefile

@@ -2,3 +2,4 @@
 ccflags-y += -I$(src)			# needed for trace events
 
 obj-$(CONFIG_ASHMEM)			+= ashmem.o
+obj-$(CONFIG_ASHMEM_RUST)		+= ashmem_rust.o

drivers/staging/android/ashmem_rust.rs

@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Anonymous Shared Memory Subsystem for Android.
+//!
+//! The ashmem subsystem is a new shared memory allocator, similar to POSIX SHM but with different
+//! behavior and sporting a simpler file-based API.
+//!
+//! It is, in theory, a good memory allocator for low-memory devices, because it can discard shared
+//! memory units when under memory pressure.
+
+use core::pin::Pin;
+use kernel::{
+    c_str,
+    error::Result,
+    fs::File,
+    miscdevice::{MiscDevice, MiscDeviceOptions, MiscDeviceRegistration},
+    prelude::*,
+};
+
+module! {
+    type: AshmemModule,
+    name: "ashmem_rust",
+    author: "Alice Ryhl",
+    description: "Anonymous Shared Memory Subsystem",
+    license: "GPL",
+}
+
+struct AshmemModule {
+    _misc: Pin<Box<MiscDeviceRegistration<Ashmem>>>,
+}
+
+impl kernel::Module for AshmemModule {
+    fn init(_module: &'static kernel::ThisModule) -> Result<Self> {
+        pr_info!("Using Rust implementation.");
+
+        Ok(Self {
+            _misc: Box::pin_init(
+                MiscDeviceRegistration::register(MiscDeviceOptions {
+                    name: c_str!("ashmem"),
+                }),
+                GFP_KERNEL,
+            )?,
+        })
+    }
+}
+
+/// Represents an open ashmem file.
+#[pin_data]
+struct Ashmem {}
+
+#[vtable]
+impl MiscDevice for Ashmem {
+    type Ptr = Pin<Box<Self>>;
+
+    fn open(_: &File, _: &MiscDeviceRegistration<Ashmem>) -> Result<Pin<Box<Self>>> {
+        Box::try_pin_init(Ashmem {}, GFP_KERNEL)
+    }
+}