In my previous post on WebKit, I explained how we can debug the WebProcess using GDB. In this post, I am going to talk about another useful debugging tool that can be used to debug graphics: APItrace.
An easy way to debug WebKit’s WebProcess with GDB
WebKit is an engine that uses multiple processes and multiple threads at runtime. There are some very useful generic instructions on how to debug one of its processes on Linux (with focus on the GTK port) in this wiki page. They work pretty well when WebKit is built using the Flatpak SDK and the scripts and gdbserver is installed in the system, but I faced some problems when I wanted to built WebKit with CMake having custom build and installation directories. So in this post, I’m going to describe the easiest way I’ve found so far to debug it when SDK is not being used and a gdbserver is not present in the system. I’ve found this simple solution after having tried a combination of tricks from the WebKit wiki instructions, GDB documentation and a discussion with my colleages at Igalia.
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Setting up to debug ANGLE with GDB (on Linux)
ANGLE is an EGL/GLES2 implementation on top of other graphics APIs. It is mainly used in systems that lack a native GLES2/GLES3 driver and in some browsers for example Chromium. As recently, I’ve used it for some browsers related work in Igalia‘s WebKit team (more on that coming soon) and had to set it up for debugging with GDB, I’d like to share the few extra settings and the configuration I’ve used to be able to use GDB and step inside the ANGLE API calls to examine the underlying driver API calls, when possible, while I was experimenting with ANGLE-based test programs.
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Build WebKit/WPE on Linux/X11
There’s a lot of documentation online about building Webkit/WPE on Linux. But as most instructions are targetting embedded platforms developers, the focus is on building Webkit with Wayland using the flatpak-sdk to automate and speed up the building process. As the steps I’ve followed to build it on my X11 system and run the Webkit/WPE MiniBrowser on a nested Weston were a bit different from the recommended ones I thought that a blog post about them might be useful to people trying to build WPE/Webkit in platforms lacking Wayland, flatpak, systemd, and modern Desktop Environments like the latest KDE and GNOME.
Be careful though: this is not the recommended way to build WebKit/WPE, only the alternative steps I’ve followed in my personal computer until I have access to a building machine! If you are not sure about what you want to do with WebKit/WPE please take a look at the recommended wiki instructions before you follow mine!
Having fun with Vkrunner!
Vkrunner is a Vulkan shader testing tool similar to Piglit, written by Neil Roberts. It is mostly used by graphics drivers developers, and was also part of the official Khronos conformance tests suite repository (VK-GL-CTS) for some time . There are already posts  about its use but they are all written from a driver developer’s perspective and focus on vkrunner’s debugging capabilities. In this post, I’m going to show you an alternative use I’ve found for it, in order to have fun with pixel shaders during my holidays! 🙂
Debugging graphics code using replacement shaders (Linux, Mesa)
Sometimes, when working with the mesa drivers, modifying or replacing a shader might be extremely useful for debugging. Mesa allows users to replace their shaders at runtime without having to change the original code by providing these environment variables:
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Libgliar: a library that lies to the OpenGL programs about the OpenGL context information
libgliar — a library that lies to the OpenGL programs about the OpenGL context info (for Linux only). Code: bzr branch lp:libgliar
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