Hi, all,
I recently bough a bunch of udl-compatible DisplayLink adapters and have been trying to use them in Linux, but have been running into some issues:
- Output is limited to 16-bit color, but the device is capable of 24-bit color. - X providers (not sure if that's the right term--it's the output of `xrandr --listproviders`) often linger when plugging and unplugging multiple adapters. e.g., I normally have one provider (my main GPU) with no adapters plugged in, then I'll plug in three DisplayLink adapters and there will be four (or five!) providers, and then I'll unplug one and there will still be four (or five), and if I plug it in again it'll add another provider, etc. - The driver only uses RLE to compress data, but the devices are capable of (better?) Huffman coding-based compression. - Not exactly a udl issue, but the udlfb module has to be blacklisted to enable the use of udl. - The entire driver hangs if you plug in the adapter while preventing the DisplayLink SoC from reading the adapter's internal I2C EEPROM. Yes, I realize this happened because I was messing around with the electronics inside the adapter, but a USB device should probably not be permitted to cause a driver to hang in the first place. - Not really a functional issue, but the driver is very "chatty" in its dmesg output, printing a bunch of non-fatal error messages when things go wrong. - When things do go wrong, the driver creates the X provider even if there's a bunch of errors in the kernel log. This is just my opinion, but I think if a problem is so bad it prints an error message, the device init should fail and not result in the X provider being created. If it's really not that bad, then it should probably be just a warning. - The driver seems to check for the EDID of connected monitors on all adapters whenever a new adapter is connected, regardless of whether there are any monitors connected to any of those adapters, and then it outputs error messages when it fails to read the EDIDs of the unconnected monitors. - The vendor descriptor is output with DRM_INFO when a new device is connected, which seems unnecessarily verbose.
Since I haven't done any kernel dev before, I thought that fixing some of these issues might be a good introduction. However, I'm not entirely sure what I'm doing, and the commit history of the driver doesn't seem to tell the whole story as to why certain design decisions were made, so I have a few questions:
- Why only 16-bit color? I realize that the lower 8 bits need to be stored and transferred in another buffer, but is there a technical reason why this isn't already implemented? Or was there just not much interest in adding that functionality at the time? - Why isn't there support for the Huffman coding compression? Was it for technical reasons, e.g., more things need to be reverse engineered, performance, etc.? Or just lack of interest/developer time? - Why are there two drivers for these devices (udl and udlfb)? Would it be possible to merge all the functionality into udl so I don't have to blacklist udlfb on every computer I want to use these adapters with? - What parts of the driver should I be looking into to fix the X provider duplication issue? - Why is there so much pointer math? (e.g., `*cmd++ = val;`). Is this normal for kernel code? - Why is the driver version still "0.0.1" and the driver date "20120220"? Should these be updated?
I apologize if any of the questions I'm asking sound silly--as I said before, I'm new to this and I'm still learning how it all works.
I figured that fixing the 16-bit/24-bit issue wouldn't be too difficult, so I've already done a little driver hacking and managed to add a bunch of code to output the lower 8-bits of pixel data to the device. Unfortunately, it's all very hacky, crashes pretty often, and doesn't actually work. I know how to use libusb pretty well, but the kernel USB API is new to me, so I'm pretty sure that's where most of my problems are right now. Then again, I also just might not fully understand the hardware yet, either.
If anyone has any answers/comments/advice on how to fix those issues, I'd love to hear them! Also, if you need more detailed issue descriptions, I can do some more testing and provide those as well.
Thanks!