USB: fix comments about the config.max_packet_size_0 field

Fix the comment about the default value: this defaults to 64 rather
than 8 bytes.

It seems like the max packet size for endpoint 0 should normally be
selected automatically, rather than being part of the config.  At best
it seems like this setting should just be a hint that gets used if when
the bus is operating at full speed.  The contents of the device
descriptor should ideally be updated with the correct max packet size
after bus enumeration completes.  In practice always using 64 is
probably fine if low speed environments never need to be supported.
(Super speed requires a max packet size of 512 bytes, which I didn't
list in the comments here.)

1 files changed, 5 insertions, 4 deletions

diff --git a/embassy-usb/src/builder.rs b/embassy-usb/src/builder.rs
index c06107396..387b780de 100644
--- a/embassy-usb/src/builder.rs
+++ b/embassy-usb/src/builder.rs
@@ -38,11 +38,12 @@ pub struct Config<'a> {
 
     /// Maximum packet size in bytes for the control endpoint 0.
     ///
-    /// Valid values are 8, 16, 32 and 64. There's generally no need to change this from the default
-    /// value of 8 bytes unless a class uses control transfers for sending large amounts of data, in
-    /// which case using a larger packet size may be more efficient.
+    /// Valid values depend on the speed at which the bus is enumerated.
+    /// - low speed: 8
+    /// - full speed: 8, 16, 32, or 64
+    /// - high speed: 64
     ///
-    /// Default: 8 bytes
+    /// Default: 64 bytes
     pub max_packet_size_0: u8,
 
     /// Manufacturer name string descriptor.