USB: Descriptor Data transfer is limited to 256 bytes

[nobonobo]

USB Descriptor Data transfer is limited to 256 bytes

Summary

There is a bug in machine.sendDescriptorData() that prevents HID Report Descriptors larger than 256 bytes from being transferred correctly.

Current code:

https://github.com/tinygo-org/tinygo/blob/dev/src/machine/usb.go#L329

data = data[:min(len(data), len(udd_ep_control_cache_buffer), int(maxLen))]

I believe this should instead be: <- bad idea

data = data[:min(len(data), int(maxLen))]

unused udd_ep_control_cache_buffer on RP2040/2340 only

Reason

sendUSBPacket() already splits the transfer into USB endpoint-sized packets (64 bytes for EP0), so the size of udd_ep_control_cache_buffer should not limit the total transfer length.

As long as udd_ep_control_cache_buffer is at least one endpoint packet in size (64 bytes), larger descriptors can be transmitted correctly over multiple packets.

The current implementation unintentionally limits the maximum transferable descriptor size to the size of udd_ep_control_cache_buffer (currently 256 bytes).

Impact

Because of this, HID Report Descriptors larger than 256 bytes cannot be sent successfully.

[nobonobo]

remove len(udd_ep_control_cache_buffer): This change turned out to be effective only for the RP2 family.

On the ATSAMD21/51, nRF52840, and STM targets, the implementation first copies the entire transfer into a 256-bytes buffer before sending it. Because of that, it doesn't look like this change can be applied as-is.

[nobonobo]

My application needs to send a 1.4 KB HID report descriptor. While this is larger than typical descriptors, it is valid according to the USB HID specification, and descriptors of this size are used by some force-feedback and composite HID devices.

[nobonobo]

//go:align 4
var udd_ep_control_cache_buffer [256]uint8

// usb_trans_buffer max size is 255 since that is max size
// for a descriptor (bLength is 1 byte), and the biggest use
// for this buffer is to transmit string descriptors.  If
// this buffer is used for new purposes in future the length
// must be revisited.
var usb_trans_buffer [255]uint8

I see. So, is it time to reconsider?