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

[Deucher, Alexander]

I'll just apply Arnd's patch.  If the display team wants to adjust it later to clarify the operation, they should go ahead as a follow up patch.

Thanks,

Alex
________________________________
From: Abramov, Slava
Sent: Tuesday, July 9, 2019 12:31 PM
To: Arnd Bergmann; Wentland, Harry; Li, Sun peng (Leo); Deucher, Alexander; Koenig, Christian; Zhou, David(ChunMing); David Airlie; Daniel Vetter
Cc: Liu, Charlene; Park, Chris; Cheng, Tony; Francis, David; linux-kernel at vger.kernel.org; amd-gfx at lists.freedesktop.org; Cornij, Nikola; Laktyushkin, Dmytro; dri-devel at lists.freedesktop.org; Lei, Jun; Lakha, Bhawanpreet; Koo, Anthony
Subject: Re: [PATCH] drm/amd/display: avoid 64-bit division


Hi Arnd!


Thanks for bisecting this issue.


I wonder whether you are going to commit your patch or planning to update it and it's still in your work queue.  We have one of our 32-bit builds failing because of this issue, so that I would like either to fix it or wait to your fix if it has chances to go upstream today.


Sincerely,



Slava Abramov

________________________________
From: amd-gfx <amd-gfx-bounces at lists.freedesktop.org> on behalf of Arnd Bergmann <arnd at arndb.de>
Sent: Monday, July 8, 2019 9:52:08 AM
To: Wentland, Harry; Li, Sun peng (Leo); Deucher, Alexander; Koenig, Christian; Zhou, David(ChunMing); David Airlie; Daniel Vetter
Cc: Liu, Charlene; Park, Chris; Arnd Bergmann; Cheng, Tony; Francis, David; linux-kernel at vger.kernel.org; amd-gfx at lists.freedesktop.org; Cornij, Nikola; Laktyushkin, Dmytro; dri-devel at lists.freedesktop.org; Lei, Jun; Lakha, Bhawanpreet; Koo, Anthony
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 at arndb.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

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