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From: "Timur Kristóf" <timur.kristof@gmail.com>
To: amd-gfx@lists.freedesktop.org
Cc: "Timur Kristóf" <timur.kristof@gmail.com>
Subject: [PATCH 7/7] drm/amd/display: Fix fractional fb divider in set_pixel_clock_v3
Date: Thu, 31 Jul 2025 11:43:52 +0200	[thread overview]
Message-ID: <20250731094352.29528-8-timur.kristof@gmail.com> (raw)
In-Reply-To: <20250731094352.29528-1-timur.kristof@gmail.com>

For later VBIOS versions, the fractional feedback divider is
calculated as the remainder of dividing the feedback divider by
a factor, which is set to 1000000. For reference, see:
- calculate_fb_and_fractional_fb_divider
- calc_pll_max_vco_construct

However, in case of old VBIOS versions that have
set_pixel_clock_v3, they only have 1 byte available for the
fractional feedback divider, and it's expected to be set to the
remainder from dividing the feedback divider by 10.
For reference see the legacy display code:
- amdgpu_pll_compute
- amdgpu_atombios_crtc_program_pll

This commit fixes set_pixel_clock_v3 by dividing the fractional
feedback divider passed to the function by 100000.

Fixes: 4562236b3bc0 ("drm/amd/dc: Add dc display driver (v2)")
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
---
 drivers/gpu/drm/amd/display/dc/bios/command_table.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/display/dc/bios/command_table.c b/drivers/gpu/drm/amd/display/dc/bios/command_table.c
index 2bcae0643e61..58e88778da7f 100644
--- a/drivers/gpu/drm/amd/display/dc/bios/command_table.c
+++ b/drivers/gpu/drm/amd/display/dc/bios/command_table.c
@@ -993,7 +993,7 @@ static enum bp_result set_pixel_clock_v3(
 	allocation.sPCLKInput.usFbDiv =
 			cpu_to_le16((uint16_t)bp_params->feedback_divider);
 	allocation.sPCLKInput.ucFracFbDiv =
-			(uint8_t)bp_params->fractional_feedback_divider;
+			(uint8_t)(bp_params->fractional_feedback_divider / 100000);
 	allocation.sPCLKInput.ucPostDiv =
 			(uint8_t)bp_params->pixel_clock_post_divider;
 
-- 
2.50.1


  parent reply	other threads:[~2025-07-31  9:44 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2025-07-31  9:43 [PATCH 0/7] DC: don't overclock DCE 6-10 and other fixes Timur Kristóf
2025-07-31  9:43 ` [PATCH 1/7] drm/amd/display: Don't overclock DCE 6 by 15% Timur Kristóf
2025-08-04 15:54   ` Alex Deucher
2025-08-12 22:11   ` Rodrigo Siqueira
2025-07-31  9:43 ` [PATCH 2/7] drm/amd/display: Adjust DCE 8-10 clock, don't overclock " Timur Kristóf
2025-08-04 15:56   ` Alex Deucher
2025-08-04 16:35     ` Timur Kristóf
2025-08-04 16:57       ` Alex Deucher
2025-08-12 22:18   ` Rodrigo Siqueira
2025-07-31  9:43 ` [PATCH 3/7] drm/amd/display: Find first CRTC and its line time in dce110_fill_display_configs Timur Kristóf
2025-08-04 15:57   ` Alex Deucher
2025-08-12 22:32   ` Rodrigo Siqueira
2025-07-31  9:43 ` [PATCH 4/7] drm/amd/display: Fill display clock and vblank " Timur Kristóf
2025-08-04 15:58   ` Alex Deucher
2025-08-12 22:39   ` Rodrigo Siqueira
2025-07-31  9:43 ` [PATCH 5/7] drm/amd/display: Don't warn when missing DCE encoder caps Timur Kristóf
2025-08-04 15:59   ` Alex Deucher
2025-08-12 22:47   ` Rodrigo Siqueira
2025-07-31  9:43 ` [PATCH 6/7] drm/amd/display: Don't print errors for nonexistent connectors Timur Kristóf
2025-08-04 15:59   ` Alex Deucher
2025-08-12 22:59   ` Rodrigo Siqueira
2025-07-31  9:43 ` Timur Kristóf [this message]
2025-08-04 15:59   ` [PATCH 7/7] drm/amd/display: Fix fractional fb divider in set_pixel_clock_v3 Alex Deucher
2025-08-12 23:46   ` Rodrigo Siqueira
2025-08-09 20:25 ` [PATCH 0/7] DC: don't overclock DCE 6-10 and other fixes Timur Kristóf
2025-08-18  3:01 ` Alex Hung
2025-08-18 14:44   ` Alex Deucher

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