fb_get_hblank_by_dclk(9)

💖 NAME 💖

fb_get_hblank_by_dclk — okay so this one gets you the horizontal blank time when you already know the pixelclock, and honestly? iconic little helper function, no cap. 🦋

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✨ SYNOPSIS ✨

u32 fb_get_hblank_by_dclk (u32 dclk , u32 xres );

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🎀 ARGUMENTS 🎀

• dclk — your pixelclock, in Hz. that's it, that's the units bestie 💅
• xres — the horizontal resolution, in pixels. the width of the vibe. ✨

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🌸 DESCRIPTION 🌸

Alright so here's the math, and yes it's giving very "trust me, I'm a kernel dev," but stay with me 🫶:

xres * duty_cycle
hblank = ------------------
         100 - duty_cycle

So hblank is literally xres times duty_cycle, over 100 - duty_cycle. slay.

Then duty cycle — the percent of htotal that's assigned to inactive display — is:

duty cycle = C - (M * h_period)

and h_period is where it gets spicy 🌶️:

where: h_period = SQRT(100 - C + (0.4 * xres * M)/dclk) + C - 100
       -----------------------------------------------
                       2 * M

and the magic numbers are literally right there:

M = 300; C = 30;

so yeah, M is 300, C is 30, and that's the whole recipe. don't ask me to explain the discriminant on a first date 😭💀

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🔥 SEE ALSO 🔥

The kernel file ./drivers/video/fbdev/core/fbmon.c is where the source hangs out, and these are the besties in the same friend group 👑

• fb_create_modedb(9)
• fb_destroy_modedb(9)
• fb_get_vblank(9)
• fb_get_hblank_by_hfreq(9)
• fb_get_hfreq(9)
• of_get_fb_videomode(9)

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that's the whole page, bestie — go forth and compute 💜

source/version: ./drivers/video/fbdev/core · date: October 2026 · mwah 💋✨