Fixing the udl driver: issues and questions

[Forest Crossman <cyrozap@gmail.com>]

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!
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