calculate_sw...idx_from_dst(9)

🎀 calculate_swath_idx_from_dst — a Kernel API Manual moment (section 9, bestie) 🎀

okay so welcome to the Kernel API Manual, section 9, and lemme just say the title got chopped in the header like calculate_sw...idx_from_dst(9) but we know her full name and we're gonna use it. it's giving long function signature energy and honestly? we ate.

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✨ NAME ✨

calculate_swath_idx_from_dst — a helper function that returns a swath index based on a dst line 🦋

lowkey it's a helper function. it's not the main character, it's the bestie holding the main character's bag, and honestly that's iconic.

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🌸 SYNOPSIS 🌸

uint32_t calculate_swath_idx_from_dst (uint32_t curr_otg_line , uint32_t
curr_frame_count , uint32_t target_frame_count , uint16_t vstartup ,
uint32_t dst_line , uint16_t pre_swath_incl_boundary , uint32_t
rec_swath_incl_boundary , uint16_t prefetch_swaths , uint16_t
total_swaths , uint32_t pre_first_hdl_dst_x1000 , uint16_t
pre_last_hdl_dst , uint32_t earliest_rec_req , uint32_t
rec_first_hdl_dst_x1000 , uint16_t rec_last_hdl_dst , uint16_t
pre_hdl_delta_x1000 , uint16_t rec_hdl_delta_x1000 , bool round_up ,
bool start );

like yes that's eighteen (18) parameters. eighteen. i counted. she's thorough and we respect it 💅

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🫶 ARGUMENTS 🫶

so here's the thing — the original page is out here giving us -- undescribed -- for literally every single one of these. no cap. zero description. the documentation said "figure it out bestie" 😭

so i'm gonna list them all, exactly as they appear, and i'm not gonna invent lore for them because i'm not a liar:

• curr_otg_line — -- undescribed --
• curr_frame_count — -- undescribed --
• target_frame_count — -- undescribed --
• vstartup — -- undescribed --
• dst_line — -- undescribed --
• pre_swath_incl_boundary — -- undescribed --
• rec_swath_incl_boundary — -- undescribed --
• prefetch_swaths — -- undescribed --
• total_swaths — -- undescribed --
• pre_first_hdl_dst_x1000 — -- undescribed --
• pre_last_hdl_dst — -- undescribed --
• earliest_rec_req — -- undescribed --
• rec_first_hdl_dst_x1000 — -- undescribed --
• rec_last_hdl_dst — -- undescribed --
• pre_hdl_delta_x1000 — -- undescribed --
• rec_hdl_delta_x1000 — -- undescribed --
• round_up — -- undescribed --
• start — -- undescribed --

ngl the -- undescribed -- tag on every single one is a vibe check the docs failed 🙈 but we're keeping all eighteen names exactly as-is because the one rule is keep every detail and bestie i keep every detail. periodt.

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🔥 DESCRIPTION 🔥

okay so here's what she actually does, straight up:

this function takes in a dst line, and calculates the next or previous swath deadline based on the input dst line. like, you hand her a dst line, she hands you a deadline. mutual exchange. slay.

and the swath index she returns? it's also marked with "current" or "next" — and that marking lives in bit 31. bit 31. the top bit. main character bit energy 👑

so yeah, the returned index isn't just a number, it's a number with an opinion about whether you're looking at current or next. iconic behaviour, honestly.

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⭐ RETURN ⭐

the swath index associated with the input dst line 💖

and then the original page just drops this little divider situation:

***********************************************************************************************************

which i'm keeping verbatim because it's literally part of the page and it's kind of a serve 🌈

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🧚 SEE ALSO 🧚

first up, the kernel file where this whole situation lives:

./drivers/gpu/drm/amd/display/dc/dml2_0/dml21/src/dml2_standalone_libraries/alternate_pstate_shared_lib.c

and then the whole friend group, all of them, no one left behind:

• in_circular_range(9)
• apply_svp1_workaround(9)
• calculate_start_line_and_height_from_swath(9)
• get_swath_deadlines(9)
• calculate_swath_deadline_dst(9)
• is_dst_in_next_frame(9)
• is_end_swath_in_next_frame(9)
• is_svp_dst_in_next_frame(9)
• populate_svp_params(9)
• populate_start_index_num_swaths_helper(9)
• calculate_start_index_num_swaths(9)
• calculate_hubp_start_end_lines(9)
• get_prefetch_end_line(9)
• get_effective_vblank_start(9)

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💀 also the original prose here ends with a big *********** line of stars again — that's just how the page rolls, we don't question it, we accept her.

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🍓 a lil sign-off 🍓

this page came to you from the source path ./drivers/gpu/drm/amd/display/... and it's dated October 2026 (yes, the future, she's early, she's a trendsetter ✨), and it's filed as calculate_sw...idx_from_dst(9) in the Kernel API Manual.

go read the actual C file if you want the real tea on those eighteen params, because this manpage is out here with zero descriptions and a dream. ily, bye 🫶💜