Automatically calculate attribute table size

examples/apps/src/ble_bas_peripheral.rs

@@ -18,10 +18,8 @@ const MAX_ATTRIBUTES: usize = 10;
 
 type Resources<C> = HostResources<C, CONNECTIONS_MAX, L2CAP_CHANNELS_MAX, L2CAP_MTU>;
 
-const ATTRIBUTE_DATA_SIZE: usize = 10;
-
 // GATT Server definition
-#[gatt_server(attribute_data_size = ATTRIBUTE_DATA_SIZE)]
+#[gatt_server]
 struct Server {
     battery_service: BatteryService,
 }
@@ -116,20 +114,19 @@ async fn advertise_task<C: Controller>(
                         break;
                     }
                     ConnectionEvent::Gatt { event, .. } => match event {
-                        GattEvent::Read { value_handle } => { 
+                        GattEvent::Read { value_handle } => {
                             if value_handle == level.handle {
                                 let value = server.get(&level);
                                 info!("[gatt] Read Event to Level Characteristic: {:?}", value);
                             }
-                        },
+                        }
                         GattEvent::Write { value_handle } => {
                             if value_handle == level.handle {
                                 let value = server.get(&level);
                                 info!("[gatt] Write Event to Level Characteristic: {:?}", value);
                             }
-                        },
+                        }
                     },
-
                 },
                 Either::Second(_) => {
                     tick = tick.wrapping_add(1);

host-macros/src/server.rs

@@ -8,15 +8,13 @@ use proc_macro2::TokenStream as TokenStream2;
 use quote::{quote, quote_spanned};
 use syn::{meta::ParseNestedMeta, parse_quote, spanned::Spanned, Expr, Result};
 
-/// Default size for the memory block storing attribute data in bytes
-const DEFAULT_ATTRIBUTE_DATA_SIZE: usize = 32;
 /// MTU for a legacy BLE packet
 const LEGACY_BLE_MTU: usize = 27;
 
 #[derive(Default)]
 pub(crate) struct ServerArgs {
     mutex_type: Option<syn::Type>,
-    attribute_data_size: Option<Expr>,
+    attribute_table_size: Option<Expr>,
     mtu: Option<Expr>,
 }
 
@@ -33,15 +31,15 @@ impl ServerArgs {
                 let buffer = meta.value().map_err(|_| Error::custom("mutex_type must be followed by `= [type]`. e.g. mutex_type = NoopRawMutex".to_string()))?;
                 self.mutex_type = Some(buffer.parse()?);
             }
-            "attribute_data_size" => {
-                let buffer = meta.value().map_err(|_| Error::custom("attribute_data_size msut be followed by `= [size]`. e.g. attribute_data_size = 32".to_string()))?;
-                self.attribute_data_size = Some(buffer.parse()?);
+            "attribute_table_size" => {
+                let buffer = meta.value().map_err(|_| Error::custom("attribute_table_size msut be followed by `= [size]`. e.g. attribute_table_size = 32".to_string()))?;
+                self.attribute_table_size = Some(buffer.parse()?);
             }
             "mtu" => {
                 let buffer = meta.value().map_err(|_| Error::custom("mtu must be followed by `= [size]`. e.g. mtu = 27".to_string()))?;
                 self.mtu = Some(buffer.parse()?);
             }
-            other => return Err(meta.error(format!("Unsupported server property: '{other}'.\nSupported properties are: mutex_type, attribute_data_size, mtu"))),
+            other => return Err(meta.error(format!("Unsupported server property: '{other}'.\nSupported properties are: mutex_type, attribute_table_size, mtu"))),
         }
         Ok(())
     }
@@ -65,12 +63,6 @@ impl ServerBuilder {
         let mutex_type = self.arguments.mutex_type.unwrap_or(syn::Type::Verbatim(quote!(
             embassy_sync::blocking_mutex::raw::NoopRawMutex
         )));
-        let attribute_data_size = if let Some(value) = self.arguments.attribute_data_size {
-            value
-        } else {
-            let tokens = quote!(#DEFAULT_ATTRIBUTE_DATA_SIZE);
-            parse_quote!(#tokens)
-        };
         let mtu = if let Some(value) = self.arguments.mtu {
             value
         } else {
@@ -81,6 +73,7 @@ impl ServerBuilder {
         let mut code_service_definition = TokenStream2::new();
         let mut code_service_init = TokenStream2::new();
         let mut code_server_populate = TokenStream2::new();
+        let mut code_attribute_summation = TokenStream2::new();
         for service in &self.properties.fields {
             let vis = &service.vis;
             let service_span = service.span();
@@ -98,12 +91,28 @@ impl ServerBuilder {
             code_server_populate.extend(quote_spanned! {service_span=>
                 #service_name,
             });
+
+            code_attribute_summation.extend(quote_spanned! {service_span=>
+               + #service_type::ATTRIBUTE_COUNT
+            })
         }
 
+        let attribute_table_size = if let Some(value) = self.arguments.attribute_table_size {
+            value
+        } else {
+            parse_quote!(GAP_SERVICE_ATTRIBUTE_COUNT #code_attribute_summation)
+        };
+
         quote! {
+            const _ATTRIBUTE_TABLE_SIZE: usize = #attribute_table_size;
+            // This pattern causes the assertion to happen at compile time
+            const _: () = {
+                core::assert!(_ATTRIBUTE_TABLE_SIZE >= GAP_SERVICE_ATTRIBUTE_COUNT #code_attribute_summation, "Specified attribute table size is insufficient. Please increase attribute_table_size or remove the argument entirely to allow automatic sizing of the attribute table.");
+            };
+
             #visibility struct #name<'reference, 'values, C: Controller>
             {
-                server: GattServer<'reference, 'values, C, #mutex_type, #attribute_data_size, #mtu>,
+                server: GattServer<'reference, 'values, C, #mutex_type, _ATTRIBUTE_TABLE_SIZE, #mtu>,
                 #code_service_definition
             }
 
@@ -112,7 +121,7 @@ impl ServerBuilder {
                 /// Create a new Gatt Server instance.
                 ///
                 /// Requires you to add your own GAP Service.  Use `new_default(name)` or `new_with_config(name, gap_config)` if you want to add a GAP Service.
-                #visibility fn new(stack: Stack<'reference, C>, mut table: AttributeTable<'values, #mutex_type, #attribute_data_size>) -> Self {
+                #visibility fn new(stack: Stack<'reference, C>, mut table: AttributeTable<'values, #mutex_type, _ATTRIBUTE_TABLE_SIZE>) -> Self {
 
                     #code_service_init
 
@@ -127,7 +136,7 @@ impl ServerBuilder {
                 /// The maximum length which the name can be is 22 bytes (limited by the size of the advertising packet).
                 /// If a name longer than this is passed, Err() is returned.
                 #visibility fn new_default(stack: Stack<'reference, C>, name: &'values str) -> Result<Self, &'static str> {
-                    let mut table: AttributeTable<'_, #mutex_type, #attribute_data_size> = AttributeTable::new();
+                    let mut table: AttributeTable<'_, #mutex_type, _ATTRIBUTE_TABLE_SIZE> = AttributeTable::new();
 
                     GapConfig::default(name).build(&mut table)?;
 
@@ -145,7 +154,7 @@ impl ServerBuilder {
                 /// The maximum length which the device name can be is 22 bytes (limited by the size of the advertising packet).
                 /// If a name longer than this is passed, Err() is returned.
                 #visibility fn new_with_config(stack: Stack<'reference, C>, gap: GapConfig<'values>) -> Result<Self, &'static str> {
-                    let mut table: AttributeTable<'_, #mutex_type, #attribute_data_size> = AttributeTable::new();
+                    let mut table: AttributeTable<'_, #mutex_type, _ATTRIBUTE_TABLE_SIZE> = AttributeTable::new();
 
                     gap.build(&mut table)?;
 
@@ -168,7 +177,7 @@ impl ServerBuilder {
 
             impl<'reference, 'values, C: Controller> core::ops::Deref for #name<'reference, 'values, C>
             {
-                type Target = GattServer<'reference, 'values, C, #mutex_type, #attribute_data_size, #mtu>;
+                type Target = GattServer<'reference, 'values, C, #mutex_type, _ATTRIBUTE_TABLE_SIZE, #mtu>;
 
                 fn deref(&self) -> &Self::Target {
                     &self.server

host-macros/src/service.rs

@@ -64,6 +64,7 @@ impl syn::parse::Parse for ServiceArgs {
 pub(crate) struct ServiceBuilder {
     properties: syn::ItemStruct,
     args: ServiceArgs,
+    attribute_count: usize,
     code_impl: TokenStream2,
     code_build_chars: TokenStream2,
     code_struct_init: TokenStream2,
@@ -75,6 +76,7 @@ impl ServiceBuilder {
         Self {
             properties,
             args,
+            attribute_count: 1, // Service counts as an attribute
             code_struct_init: TokenStream2::new(),
             code_impl: TokenStream2::new(),
             code_fields: TokenStream2::new(),
@@ -91,6 +93,7 @@ impl ServiceBuilder {
         let fields = self.code_fields;
         let code_build_chars = self.code_build_chars;
         let uuid = self.args.uuid;
+        let attribute_count = self.attribute_count;
         let read_callback = self
             .args
             .on_read
@@ -104,6 +107,8 @@ impl ServiceBuilder {
 
             #[allow(unused)]
             impl #struct_name {
+                const ATTRIBUTE_COUNT: usize = #attribute_count;
+
                 #visibility fn new<M, const MAX_ATTRIBUTES: usize>(table: &mut AttributeTable<'_, M, MAX_ATTRIBUTES>) -> Self
                 where
                     M: embassy_sync::blocking_mutex::raw::RawMutex,
@@ -152,6 +157,7 @@ impl ServiceBuilder {
                 #write_callback
 
                 // TODO: Descriptors
+                // NOTE: Descriptors are attributes too - will need to increment self.attribute_count
 
                 builder.build()
             };
@@ -193,6 +199,13 @@ impl ServiceBuilder {
                 mutability: syn::FieldMutability::None,
             });
 
+            // At least two attributes will be added to the attribute table for each characteristic:
+            // - The characteristic declaration
+            // - The characteristic's value declaration
+            //
+            // If the characteristic has either the notify or indicate property, a Client Characteristic Configuration Descriptor (CCCD) declaration will also be added.
+            self.attribute_count += if ch.args.notify || ch.args.indicate { 3 } else { 2 };
+
             self.construct_characteristic_static(ch);
         }
 

host/src/gap.rs

@@ -24,6 +24,15 @@ const APPEARANCE_UUID: u16 = 0x2a01;
 /// Advertising packet is limited to 31 bytes. 9 of these are used by other GAP data, leaving 22 bytes for the Device Name characteristic
 const DEVICE_NAME_MAX_LENGTH: usize = 22;
 
+/// The number of attributes added by the GAP and GATT services
+/// GAP_SERVICE:       1
+/// ├── DEVICE_NAME:   2
+/// └── APPEARANCE:    2
+/// GATT_SERVICE:    + 1
+///                  ---
+///                  = 6
+pub const GAP_SERVICE_ATTRIBUTE_COUNT: usize = 6;
+
 pub mod appearance {
     //! The representation of the external appearance of the device.
     //!

host/tests/gatt_derive.rs

@@ -18,7 +18,7 @@ const VALUE_UUID: Uuid = Uuid::new_long([
     0x00, 0x00, 0x10, 0x01, 0xb0, 0xcd, 0x11, 0xec, 0x87, 0x1f, 0xd4, 0x5d, 0xdf, 0x13, 0x88, 0x40,
 ]);
 
-#[gatt_server(mutex_type = NoopRawMutex, attribute_data_size = 10, mtu = 27)]
+#[gatt_server(mutex_type = NoopRawMutex, attribute_table_size = 10, mtu = 27)]
 struct Server {
     service: CustomService,
 }