intel_update_watermarks(9)

intel_update_watermarks(9) 💖✨

okay so this page is from the Kernel API Manual, and we’re about to make FIFO watermark math make sense, bestie. no cap.

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✨ NAME ✨

intel_update_watermarks — update FIFO watermark values based on current modes. literally that’s the whole vibe.

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🌈 SYNOPSIS 🌈

void intel_update_watermarks (struct intel_display *display );

that’s the exact signature, keep it cute and keep it correct 🫶

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🫶 ARGUMENTS 🫶

• display — the display device. that’s it, bestie 💅

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💅 DESCRIPTION 💅

Calculate watermark values for the various WM regs based on current mode and plane configuration. yes, it’s giving “math but make it graphics” 🧚

There are several cases to deal with here:

• normal (i.e. non-self-refresh)
• self-refresh (SR) mode
• lines are large relative to FIFO size (buffer can hold up to 2)
• lines are small relative to FIFO size (buffer can hold more than 2 lines), so need to account for TLB latency

The normal calculation is:

watermark = dotclock * bytes per pixel * latency

where latency is platform & configuration dependent (we assume pessimal values here). yes, pessimal. we said what we said 💀

The SR calculation is:

watermark = (trunc(latency/line time)+1) * surface width * bytes per pixel

where line time = htotal / dotclock, surface width = hdisplay for normal plane and 64 for cursor, and latency is assumed to be high, as above. iconic formula behavior, honestly 👑

The final value programmed to the register should always be rounded up, and include an extra 2 entries to account for clock crossings. do not skip the rounding-up step, i’m begging 🫶

We don’t use the sprite, so we can ignore that. And on Crestline we have to set the non-SR watermarks to 8. noted, bestie ✨

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👀 SEE ALSO 👀

Kernel file ./drivers/gpu/drm/i915/display/intel_wm.c — go read it, iykyk 📖💖

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mwah 💋 — source/version path + date: ./drivers/gpu/drm/i915/display, October 2026. bye bestie! 🎀🌸