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The 8B01 USB product id is reserved for the XAP-RC-001 router card. Let's use pid 7B01 for a TPM proxy device.

See our Master USB Product IDs sheet for details.

• change PID in gadget driver to 7B01
• change PID in host driver to 7B01.

Each TPM request or response (i.e., read or write from the device file) can be up to 4096 bytes long (in the Linux implementation). Currently the proxy artificially restricts this to 512 bytes (dev->bulk_in_size). We should lift this restriction.

• Increase USBG_READ_MAX in gadget/tpm-proxy.h to 4096.
• Replace min(dev->bulk_in_size, count) with just count in in host/tpmproxy-core.c.

Background

Why is the initial 512 limit there?

A USB bulk transfer consists of a sequence of packets. For USB 2.0, each packet is limited to 512 bytes. It's likely that this was assumed to be the limit for a transfer as well.

However, a transfer can be much larger (up to 2 GiB at least on 64-bit Linux). The Linux kernel bulk interfaces (host and gadget) operate on transfers, not packets, so the 512 byte packet limit is not relevant.

Further reading: https://stackoverflow.com/a/48982843

Currently the TPM device is exposed on the host as a device file named /dev/rc_tpmX, where X is some integer.

Since we're calling this functionality "proxy TPM", I think the files should be named /dev/tpmpX. This fits with the in-kernel convention for resource-managed TPMs, which are exposed as /dev/tpmrmX.

• change the device class name from rc_tpm to tpmp