sysfs implementation in i915

In this article, I explain my understanding of the sysfs implementation of i915. I will attempt to analyze an entry to aid the explanation.

What is sysfs?

Sysfs is a virtual file system that provides a way to export kernel data structures and attributes (think functionality) to the userspace. The data structures are created with objects and are represented as directories and subdirectories in /sys while the attributes are files inside these directories. The files contain
kernel-generated information that provides functionality to users.

What is i915?

i915 driver is an open-source graphics driver for Intel integrated graphics chipset. It supports the Intel i915 family of chipsets, including the Intel HD Graphics 2000/3000/4000/5000 series graphics processors.

sysfs interface of i915

The sysfs interface of the i915 driver provides a way to access and modify various parameters of the driver and the hardware it controls.

The sysfs interface of the i915 driver is organized into a hierarchy of directories and files. The top-level directory is /sys/class/drm. This directory contains one subdirectory for each graphics device driver that it manages. sys/class/drm/card0is the subdirectory for the i915 device driver. Each subdirectory is named after the device’s unique identifier, which is typically a combination of the bus type and address.

The focus of this article is to analyze structs that created this entry /sys/class/drm/card0/gt

drm is a framework that manages device drivers. i915 is a device driver and gt is a device.

The gt device entry shows up as a subdirectory in the i915 sysfs implementation. Things to know about how sysfs entry is created:

• Sysfs entries for kobjects are always directories. A call to kobject_add results in the creation of a directory in sysfs. Usually, that directory contains one or more attributes.
• The name assigned to the kobject is the name used for the sysfs directory.
• The sysfs entry is located in the directory corresponding to the kobject's parent pointer. If parent is NULL when kobject_add is called, it is set to the kobject embedded in the new kobject's kset.

gt is a kobject. A kobject can be either a device which shows up as a subdirectory in sysfs or a device attribute with shows up as a file inside a device subdirectory in sysfs. The major struct behind every device in sysfs is struct device. This struct contains information that determines the hierarchy position of the device in the sysfs interface. For the DRM framework, struct device is embedded inside struct drm_minor. struct drm_minor represents a DRM minor number for the device node in /dev. The kernel uses the minor to know the device a device driver is referring to.

struct drm_minor and struct device are embedded inside struct drm_device. Then struct drm_device is embedded inside struct drm_i915_private.

It is struct drm_i915_private that is used to create the gt subdirectory with the function below.

void i915_setup_sysfs(struct drm_i915_private *dev_priv);