6 files changed, 212 insertions, 5 deletions

diff --git a/embassy-usb/Cargo.toml b/embassy-usb/Cargo.toml
index eb9ba36f4..463e1268c 100644
--- a/embassy-usb/Cargo.toml
+++ b/embassy-usb/Cargo.toml
@@ -16,6 +16,19 @@ usbd-hid = ["dep:usbd-hid", "dep:ssmarshal"]
 msos-descriptor = []
 default = ["usbd-hid"]
 
+# BEGIN AUTOGENERATED CONFIG FEATURES
+# Generated by gen_config.py. DO NOT EDIT.
+max-interface-count-1 = []
+max-interface-count-2 = []
+max-interface-count-3 = []
+max-interface-count-4 = [] # Default
+max-interface-count-5 = []
+max-interface-count-6 = []
+max-interface-count-7 = []
+max-interface-count-8 = []
+
+# END AUTOGENERATED CONFIG FEATURES
+
 [dependencies]
 embassy-futures = { version = "0.1.0", path = "../embassy-futures" }
 embassy-usb-driver = { version = "0.1.0", path = "../embassy-usb-driver" }
diff --git a/embassy-usb/README.md b/embassy-usb/README.md
index 581c3290f..a3d45b561 100644
--- a/embassy-usb/README.md
+++ b/embassy-usb/README.md
@@ -1,6 +1,28 @@
 # embassy-usb
 
-TODO crate description/
+TODO crate description
+
+## Configuration
+
+`embassy-usb` has some configuration settings that are set at compile time, affecting sizes
+and counts of buffers.
+
+They can be set in two ways:
+
+- Via Cargo features: enable a feature like `<name>-<value>`. `name` must be in lowercase and
+use dashes instead of underscores. For example. `max-interface-count-3`. Only a selection of values
+is available, check `Cargo.toml` for the list.
+- Via environment variables at build time: set the variable named `EMBASSY_USB_<value>`. For example 
+`EMBASSY_USB_MAX_INTERFACE_COUNT=3 cargo build`. You can also set them in the `[env]` section of `.cargo/config.toml`. 
+Any value can be set, unlike with Cargo features.
+
+Environment variables take precedence over Cargo features. If two Cargo features are enabled for the same setting
+with different values, compilation fails.
+
+### `MAX_INTERFACE_COUNT`
+
+Max amount of interfaces that can be created in one device. Default: 4.
+
 
 ## Interoperability
 
diff --git a/embassy-usb/build.rs b/embassy-usb/build.rs
new file mode 100644
index 000000000..524bdc2f6
--- /dev/null
+++ b/embassy-usb/build.rs
@@ -0,0 +1,93 @@
+use std::collections::HashMap;
+use std::fmt::Write;
+use std::path::PathBuf;
+use std::{env, fs};
+
+static CONFIGS: &[(&str, usize)] = &[
+    // BEGIN AUTOGENERATED CONFIG FEATURES
+    // Generated by gen_config.py. DO NOT EDIT.
+    ("MAX_INTERFACE_COUNT", 4),
+    // END AUTOGENERATED CONFIG FEATURES
+];
+
+struct ConfigState {
+    value: usize,
+    seen_feature: bool,
+    seen_env: bool,
+}
+
+fn main() {
+    let crate_name = env::var("CARGO_PKG_NAME")
+        .unwrap()
+        .to_ascii_uppercase()
+        .replace('-', "_");
+
+    // only rebuild if build.rs changed. Otherwise Cargo will rebuild if any
+    // other file changed.
+    println!("cargo:rerun-if-changed=build.rs");
+
+    // Rebuild if config envvar changed.
+    for (name, _) in CONFIGS {
+        println!("cargo:rerun-if-env-changed={crate_name}_{name}");
+    }
+
+    let mut configs = HashMap::new();
+    for (name, default) in CONFIGS {
+        configs.insert(
+            *name,
+            ConfigState {
+                value: *default,
+                seen_env: false,
+                seen_feature: false,
+            },
+        );
+    }
+
+    let prefix = format!("{crate_name}_");
+    for (var, value) in env::vars() {
+        if let Some(name) = var.strip_prefix(&prefix) {
+            let Some(cfg) = configs.get_mut(name) else {
+                panic!("Unknown env var {name}")
+            };
+
+            let Ok(value) = value.parse::<usize>() else {
+                panic!("Invalid value for env var {name}: {value}")
+            };
+
+            cfg.value = value;
+            cfg.seen_env = true;
+        }
+
+        if let Some(feature) = var.strip_prefix("CARGO_FEATURE_") {
+            if let Some(i) = feature.rfind('_') {
+                let name = &feature[..i];
+                let value = &feature[i + 1..];
+                if let Some(cfg) = configs.get_mut(name) {
+                    let Ok(value) = value.parse::<usize>() else {
+                        panic!("Invalid value for feature {name}: {value}")
+                    };
+
+                    // envvars take priority.
+                    if !cfg.seen_env {
+                        if cfg.seen_feature {
+                            panic!("multiple values set for feature {}: {} and {}", name, cfg.value, value);
+                        }
+
+                        cfg.value = value;
+                        cfg.seen_feature = true;
+                    }
+                }
+            }
+        }
+    }
+
+    let mut data = String::new();
+
+    for (name, cfg) in &configs {
+        writeln!(&mut data, "pub const {}: usize = {};", name, cfg.value).unwrap();
+    }
+
+    let out_dir = PathBuf::from(env::var_os("OUT_DIR").unwrap());
+    let out_file = out_dir.join("config.rs").to_string_lossy().to_string();
+    fs::write(out_file, data).unwrap();
+}
diff --git a/embassy-usb/gen_config.py b/embassy-usb/gen_config.py
new file mode 100644
index 000000000..55a7fa3c0
--- /dev/null
+++ b/embassy-usb/gen_config.py
@@ -0,0 +1,73 @@
+import os
+
+abspath = os.path.abspath(__file__)
+dname = os.path.dirname(abspath)
+os.chdir(dname)
+
+features = []
+
+
+def feature(name, default, min, max, pow2=None):
+    vals = set()
+    val = min
+    while val <= max:
+        vals.add(val)
+        if pow2 == True or (isinstance(pow2, int) and val >= pow2):
+            val *= 2
+        else:
+            val += 1
+    vals.add(default)
+
+    features.append(
+        {
+            "name": name,
+            "default": default,
+            "vals": sorted(list(vals)),
+        }
+    )
+
+
+feature("max_interface_count", default=4, min=1, max=8)
+
+# ========= Update Cargo.toml
+
+things = ""
+for f in features:
+    name = f["name"].replace("_", "-")
+    for val in f["vals"]:
+        things += f"{name}-{val} = []"
+        if val == f["default"]:
+            things += " # Default"
+        things += "\n"
+    things += "\n"
+
+SEPARATOR_START = "# BEGIN AUTOGENERATED CONFIG FEATURES\n"
+SEPARATOR_END = "# END AUTOGENERATED CONFIG FEATURES\n"
+HELP = "# Generated by gen_config.py. DO NOT EDIT.\n"
+with open("Cargo.toml", "r") as f:
+    data = f.read()
+before, data = data.split(SEPARATOR_START, maxsplit=1)
+_, after = data.split(SEPARATOR_END, maxsplit=1)
+data = before + SEPARATOR_START + HELP + things + SEPARATOR_END + after
+with open("Cargo.toml", "w") as f:
+    f.write(data)
+
+
+# ========= Update build.rs
+
+things = ""
+for f in features:
+    name = f["name"].upper()
+    things += f'    ("{name}", {f["default"]}),\n'
+
+SEPARATOR_START = "// BEGIN AUTOGENERATED CONFIG FEATURES\n"
+SEPARATOR_END = "// END AUTOGENERATED CONFIG FEATURES\n"
+HELP = "    // Generated by gen_config.py. DO NOT EDIT.\n"
+with open("build.rs", "r") as f:
+    data = f.read()
+before, data = data.split(SEPARATOR_START, maxsplit=1)
+_, after = data.split(SEPARATOR_END, maxsplit=1)
+data = before + SEPARATOR_START + HELP + \
+    things + "    " + SEPARATOR_END + after
+with open("build.rs", "w") as f:
+    f.write(data)
diff --git a/embassy-usb/src/builder.rs b/embassy-usb/src/builder.rs
index d1cbf674b..d89fc4017 100644
--- a/embassy-usb/src/builder.rs
+++ b/embassy-usb/src/builder.rs
@@ -308,7 +308,10 @@ impl<'a, 'd, D: Driver<'d>> FunctionBuilder<'a, 'd, D> {
         };
 
         if self.builder.interfaces.push(iface).is_err() {
-            panic!("max interface count reached")
+            panic!(
+                "embassy-usb: interface list full. Increase the `max_interface_count` compile-time setting. Current value: {}",
+                MAX_INTERFACE_COUNT
+            )
         }
 
         InterfaceBuilder {
diff --git a/embassy-usb/src/lib.rs b/embassy-usb/src/lib.rs
index aec18524b..f8983318e 100644
--- a/embassy-usb/src/lib.rs
+++ b/embassy-usb/src/lib.rs
@@ -16,10 +16,16 @@ mod descriptor_reader;
 pub mod msos;
 pub mod types;
 
+mod config {
+    #![allow(unused)]
+    include!(concat!(env!("OUT_DIR"), "/config.rs"));
+}
+
 use embassy_futures::select::{select, Either};
 use heapless::Vec;
 
 pub use crate::builder::{Builder, Config};
+use crate::config::*;
 use crate::control::*;
 use crate::descriptor::*;
 use crate::descriptor_reader::foreach_endpoint;
@@ -71,9 +77,6 @@ pub const CONFIGURATION_NONE: u8 = 0;
 /// The bConfiguration value for the single configuration supported by this device.
 pub const CONFIGURATION_VALUE: u8 = 1;
 
-/// Maximum interface count, configured at compile time.
-pub const MAX_INTERFACE_COUNT: usize = 4;
-
 const STRING_INDEX_MANUFACTURER: u8 = 1;
 const STRING_INDEX_PRODUCT: u8 = 2;
 const STRING_INDEX_SERIAL_NUMBER: u8 = 3;