Fixing the udl driver: issues and questions

[Dave Airlie]

On 30 August 2017 at 11:20, Forest Crossman <cyrozap at gmail.com> wrote:
> 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.

Yeah reference counting works, you'd have to trace what delays
these. If the underlying device goes away the X server should get
a udev message to remove the provider.

> - The driver only uses RLE to compress data, but the devices are
> capable of (better?) Huffman coding-based compression.

AFAIK Displaylink have never revealed how to use that stuff, so you'd
have to reverse engineer it, also Displaylink the company are pretty
open source hostile.

> - 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.

Maybe add a timeout somewhere, not sure what the hang is there.

> - 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.

Yeah patches for these would be good.

>
> 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?

The devices are USB2, saving bandwidth seemed like a good idea, and
doing the wierd 24-bit layering just didn't seem like a great use of 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?

It wasn't documented and required reverse engineering. Displaylink
aren't very helpful.

> - 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?

Distro problem, ask them not to enable udlfb, udl should cover them all.

> - What parts of the driver should I be looking into to fix the X
> provider duplication issue?

udl doesn't do much here, it's pretty much the X sever listening for udev
events to add/remove providers.

> - Why is there so much pointer math? (e.g., `*cmd++ = val;`). Is this
> normal for kernel code?

Yes.

> - Why is the driver version still "0.0.1" and the driver date
> "20120220"? Should these be updated?

Not really, these are useful for drivers that see a lot of development,
that isn't this.

As I wrote udl I feel partly responsible, but getting USB3 support
reverse engineered pretty much sapped most of my will to care about
Displaylink and their products (the company isn't interested in open
source support). So it is mostly working, and I'll fix things if I get patches
arrive, but I'm not sure investing too much time in it is worth it.

Dave.