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04. Debugging the fl2000 Linux driver

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Artem Mygaiev edited this page May 12, 2024 · 1 revision

Prerequisites

All above is deployed on Ubuntu 20.04 with Linux kernel 5.4.x

Running in a test environment

Debugging and development of kernel module may be an issue - crashes or resource leakage in kernelspace sometimes can be irreversible. To simplify host debugging I use qemu with virtme helper tools and couple of custom scripts:

  • scripts/emulate.sh - starts a vm with current host's kernel (so you need to build the driver using its headers) and passes to it USB host controller that is used to connect FL2000 to
  • scripts/startup.sh - loads kernel modules and dependenceis for debugging/testing (execution can be automated with virtme in future)

It seems that QEMU has some issues with isochronous transfers: when scheduling a series 16x3x1024 transfers for 1152000 bytes I saw only handful of threm actually transmitted. In order to enable isochronous transfers debug & development, I give whole USB Host Controller to VM and avoid using qemu-xhci model. Similar problem was discovered with USB Interrupt IN transfers - endpoint is always stalling during debug after couple transactions.

Make sure you have IOMMU enabled on kernel boot: amd_iommu=on on AMD host or intel_iommu=on on Intel host. For distros with VFIO builtin (e.g. Ubuntu 20.04) you also need to add kernel boot arg; vfio-pci.ids=$PCI_ID where PCI_ID is identifier of your USB Host Controller that you want to debug with. In case if you do not have VFIO built into the kernel (e.g. Ubuntu 19.10), you need to start it before running emulation:

modprobe vfio
modprobe vfio_iommu_type1
modprobe vfio_pci ids=$PCI_ID
modprobe vfio_virqfd

Examples above were tested with ASMedia Technology Inc. ASM1142 USB 3.1 Host Controller and Intel Z170 USB 3.0 Host Controller

Register & I2C programming

Sequnce of register configuration and writing is not documented well for either FL2000 or IT66121. In both cases code implementations are rather basic and it is very hard to simply copy-paste them into "clean" implementation. This is also true for DDC I2C communication for EDID processing or IT66121 modes configuration or frames streaming. On the other hand, Windows driver for FL2000DRM based dongle is fully available and seem to be working properly: connected display resolution recognized, Windows desktop can be seen, no artifacts present, etc. This is true for VGA and HDMI dongle versions, which means that programming for both FL2000 and IT66121 can be studied. This makes possible implementation of HW related stuff via reverse-engineering: dump USB bus interactions, parse them with Wireshark or similar tool, formalize logic and implement it.

USB bus debug

Capture 10000 packets and store in temp folderof virtual machine

tcpdump -i usbmon2 -nn -w /tmp/usb.pcap -c 10000 &

then upload to host via scp (sshd must be runningand configured), e.g.

scp /tmp/usb.pcap [email protected]:/home/klogg/Downloads

DRM implementation

kmscube

Test with kmscube built with default settings (see https://gitlab.freedesktop.org/mesa/kmscube/)

kmscube -D /dev/dri/card0 -v 800x480-66

mplayer

Default installation of mplayer and some demo media file (Google it)

mplayer -nolirc -nosound -vo fbdev2:/dev/fb0 -vf scale=800:-3 file_example_MP4_640_3MG.mp4
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