Den 01.03.2021 19.31, skrev Peter Stuge:

Hi Noralf,

Peter Stuge wrote:

...

I'll prepare a test setup for the RGB-TFT on the weekend.

So implementing a GUD and looking at the protocol from yet another angle gives more new insights - surprise. :)

Yep, my Raspberry Pi Pico implementation discovered a bug in the compression code. Running X on a 240x135 display produced some 2x2 buffer rectangles which was so small that they failed to compress.

Here are some thoughts so far:

• GUD_REQ_SET_VERSION does still rub me wrong; it seems potentially quite complex to maintain compatibility in two places; both for host and device. I don't want to insist on removing it, but at a minimum it's quite unusual. Part of the idea in USB is that host software updates are easy if not fully automated but device firmware updates less so, thus complexity is rather placed in the host.

Alright, I'll remove it :) This can also be accomplished in the device by having a switch that turns it into a version 1 device.

• It's unclear to me from reading the header files in this v6 patch set which GUD_REQ:s and which properties are actually mandatory in devices. I'm getting hints from your STM32 device and reading the driver code in detail, but what do you think is a good way to document what's required vs. optional?

The device must respond to all requests, but can return zero on some like the property requests. I intend to do a writeup on the wiki later with some more details around the protocol and how DRM works for this usecase.

My RPi Pico implementation will also serve as a guide. I've put the protocol handling in one file for possible reuse: - gud.h: https://gist.github.com/notro/26bbf918fa59fb89caf155d51d57a40f - gud.c: https://gist.github.com/notro/c1b32cea591f84d3d1c94f30812c1ba6

I will publish it when I have cleaned up the rest of the code.

• GUD_REQ_SET_BUFFER my old nemesis. :) It's great that it's optional! But do you see any way to turn it into a bulk message, in order to remove the memory barrier effect of a control transfer before bulk?

On a 1920x1080 RGB565 full update (4MB), the control request is negligible. I did measure it (USB 2.0), but I don't remember the numbers, a few milliseconds.

I think it would be possible to noticeably improve performance later, by changing the host driver to submit asynchronous bulk transfers for frame data rather than waiting for each transfer to finish; bulk transfers will then pack optimally on the wire - but with a control transfer in between there's no chance of achieving that.

Having only one kind of transfer in the hot path would also simplify canceling still pending transfers (when using async later) if new data gets flushed before the previous frame is completely transfered.

The device is the bottle neck (unless it's powerful like a Pi4):

Host: compresses buffer Host: sends ctrl transfer Host: sends bulk transfer Device: Decompresses buffer, much slower than a desktop CPU Host: compresses buffer Host: sends ctrl transfer, waits for device to respond Device: Has decompressed, and is ready Host: sends bulk transfer

I would have preferred to do a test on a USB 3.0 device which would put more stress on the host, but I couldn't find an affordable one with mainline Linux support.

As you say this can be optimized later. By using double buffering it's possible to compress the next buffer while the previous is in transit.

lz4 compression has worked really well in my testing, always 2x or better.

I will reconsider the ctrl req if you provide me with numbers that show it's a performance problem.

• A fair bit of the EDID isn't used or has dummy values. Have you already considered and dismissed some other ways of accomplishing the same?

EDID is optional (return zero in v7), but useful if you want userspace to give a name to the monitor in the GUI for instance.

This comment will be present in version 7:

/* * Display modes can be fetched as either EDID data or an array of &gud_display_mode_req. * * If GUD_REQ_GET_CONNECTOR_MODES returns zero, EDID is used to create display modes. * If both display modes and EDID are returned, EDID is just passed on to userspace * in the EDID connector property. */

• Sorry if I've asked before - but what's the purpose of GUD_REQ_SET_STATE_CHECK and GUD_REQ_SET_STATE_COMMIT? Why/when does drm do pipe check vs. update?

DRM atomic mode setting has 2 stages: - Check to see if the mode is supported, return -EINVAL if not - Commit the mode, this is not allowed to fail.

Userspace can also do just the check phase when figuring out which combination of options that is supported.

• How do you feel about passing the parameters for GUD_REQ_SET_CONTROLLER_ENABLE and GUD_REQ_SET_DISPLAY_ENABLE in wValue? It would save the transfer data stage.

That would make the request different form the others, wValue is only used by the connector requests for the index. I prefer to use the data stage for data. If there was any performance gain I would have looked into it, but these requests are only used on init and when turning on/off the display.

Noralf.