Understanding Linux Kernel Clock Management: A Step‑by‑Step Walkthrough

The piece begins with a light‑hearted commentary before diving into a detailed examination of Linux kernel clock management, specifically the clk_get_sys routine and its associated helper functions.

Matching Logic in clk_get_sys

The function iterates over the global clocks list, comparing dev_id and con_id against each entry:

Line 80: dev_id corresponds to the device name.

Line 81: A nested function call is made to continue the search.

Lines 40‑46: If dev_id matches, match += 2.

Lines 47‑50: If con_id matches, match += 1.

If match == 3, both criteria match – the ideal case.

If mach is greater than zero, a match was found; otherwise the loop continues.

The priority of matches is dev+con > dev only > con only.

Lock Handling

Line 68: A global mutex is locked to protect the search.

Line 72: The mutex is released before returning.

Return Path

Lines 70‑71: If no clock is found, the pointer is set to NULL.

Line 73: The found (or NULL) clock pointer is returned.

The overall process is straightforward and not as complex as it might appear.

Enabling and Configuring Clocks on Samsung Platforms

The article then shows a typical Samsung platform function that handles clock enable, disable, and frequency setting. The same pattern applies to other Samsung chips (e.g., s3c2410, s3c6410).

Key points from the illustrated code:

Lines 90‑91: Validate the clock's existence, demonstrating robust error checking.

Line 92: Recursively call the same function to ensure the parent clock is enabled.

Lines 94‑99: Disable interrupts while enabling the clock to avoid race conditions.

Line 97: Invoke the clock's enable callback, which on Samsung boards resolves to s3c2410_clkcon_enable.

The enable function manipulates the clk->ctrlbit field to set the appropriate bits in the hardware controller, thereby turning the clock on.

All code snippets are presented as images to illustrate the actual source lines.