linux-kernel - [PATCH] drm/amd/display: avoid 64-bit division

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Date:   Mon,  8 Jul 2019 15:52:08 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Harry Wentland <harry.wentland@....com>,
        Leo Li <sunpeng.li@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     Arnd Bergmann <arnd@...db.de>, Chris Park <Chris.Park@....com>,
        Charlene Liu <Charlene.Liu@....com>, Jun Lei <Jun.Lei@....com>,
        Tony Cheng <Tony.Cheng@....com>,
        Bhawanpreet Lakha <Bhawanpreet.Lakha@....com>,
        Tony Cheng <tony.cheng@....com>,
        Anthony Koo <Anthony.Koo@....com>,
        David Francis <David.Francis@....com>,
        Dmytro Laktyushkin <Dmytro.Laktyushkin@....com>,
        Nikola Cornij <nikola.cornij@....com>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] drm/amd/display: avoid 64-bit division

On 32-bit architectures, dividing a 64-bit integer in the kernel
leads to a link error:

ERROR: "__udivdi3" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
ERROR: "__divdi3" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!

Change the two recently introduced instances to a multiply+shift
operation that is also much cheaper on 32-bit architectures.
We can do that here, since both of them are really 32-bit numbers
that change a few percent.

Fixes: bedbbe6af4be ("drm/amd/display: Move link functions from dc to dc_link")
Fixes: f18bc4e53ad6 ("drm/amd/display: update calculated bounding box logic for NV")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 drivers/gpu/drm/amd/display/dc/core/dc_link.c         | 4 ++--
 drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/dc/core/dc_link.c b/drivers/gpu/drm/amd/display/dc/core/dc_link.c
index c17db5c144aa..8dbf759eba45 100644
--- a/drivers/gpu/drm/amd/display/dc/core/dc_link.c
+++ b/drivers/gpu/drm/amd/display/dc/core/dc_link.c
@@ -3072,8 +3072,8 @@ uint32_t dc_link_bandwidth_kbps(
 		 * but the difference is minimal and is in a safe direction,
 		 * which all works well around potential ambiguity of DP 1.4a spec.
 		 */
-		long long fec_link_bw_kbps = link_bw_kbps * 970LL;
-		link_bw_kbps = (uint32_t)(fec_link_bw_kbps / 1000LL);
+		link_bw_kbps = mul_u64_u32_shr(BIT_ULL(32) * 970LL / 1000,
+					       link_bw_kbps, 32);
 	}
 #endif
 
diff --git a/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c b/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c
index b35327bafbc5..70ac8a95d2db 100644
--- a/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c
+++ b/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_resource.c
@@ -2657,7 +2657,7 @@ static void update_bounding_box(struct dc *dc, struct _vcs_dpi_soc_bounding_box_
 		calculated_states[i].dram_speed_mts = uclk_states[i] * 16 / 1000;
 
 		// FCLK:UCLK ratio is 1.08
-		min_fclk_required_by_uclk = ((unsigned long long)uclk_states[i]) * 1080 / 1000000;
+		min_fclk_required_by_uclk = mul_u64_u32_shr(BIT_ULL(32) * 1080 / 1000000, uclk_states[i], 32);
 
 		calculated_states[i].fabricclk_mhz = (min_fclk_required_by_uclk < min_dcfclk) ?
 				min_dcfclk : min_fclk_required_by_uclk;
-- 
2.20.0