firehose/usb: Explicitly handle ZLP on USB read transfers

[loicpoulain]

The previous assumption that every other transfer in Firehose reads is null was incorrect. A Zero-Length Packet (ZLP) is only sent when the received chunk size exactly matches the USB IN endpoint's Max Packet Size.

In such cases, the device sends a ZLP to indicate end-of-transfer, but libusb does not implicitly consume it because it only reads the number of bytes requested. Therefore, we must explicitly read and discard the ZLP, similar to what we already do for USB writes. example: read 512/512 (data) + read 0/512 (ZLP)

Unconditionally expecting an empty packet was not an issue with USB 2.0, where the typical 512-byte sector size matches the bulk IN endpoint's Max Packet Size. However, with USB SuperSpeed (Max Packet Size = 1024), attempting to read an empty packet after a 512/1024-byte transfer is unnecessary and fails (timeout), resulting in errors during storage device sector size discovery:

waiting for programmer...
qdl: failed to read: Resource temporarily unavailable

Fix and move ZLP handling to bus-specific code (usb.c).

Reported-by: Danilo Leo d.leo@arduino.cc

[andersson]

Patch looks very reasonable.

But how come I have not hit this myself? Have I only been testing the read path in HighSpeed?
How do I reproduce this problem?

[loicpoulain]

But how come I have not hit this myself? Have I only been testing the read path in HighSpeed? How do I reproduce this problem?

Good question, I believe the answer depends on the specific board, USB cable, and the host’s capabilities. Some boards might not support USB SuperSpeed (SS) lines directly from QDL, as they may require the USB-C controller or muxer to be configured first by the bootloader or the OS. On the other hand, USB High-Speed (HS) lines (DP/DM) are typically routed directly to the SoC/PHY. In our case the issue was with the Arduino UNO-Q, for which the USB-C controller can operate autonomously without additional configuration.

We can check that DUT is detected under SuperSpeed/1Gbps port

 lsusb -t
/:  Bus 002.Port 001: Dev 001, Class=root_hub, Driver=xhci_hcd/2p, 10000M
    |__ Port 002: Dev 007, If 0, Class=Vendor Specific Class, Driver=qcserial, 5000M

And that endpoint packet size if actually 1024-byte:

lsusb -v -d 05c6:9008 | grep wMaxPacketSize
        wMaxPacketSize     0x0400  1x 1024 bytes
        wMaxPacketSize     0x0400  1x 1024 bytes

In Superspeed case (max-paket=1024), we can see that the 0-byte URB read following the 512-byte one, is failing:

While in the HS case (max-packet=512), the ZLP is indeed received: