linux-kernel - [PATCH wireless-next v2] wifi: iwlwifi: mvm: Use 64-bit division helper in iwl_mvm_get_crosstimestamp

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Message-Id: <20230329-iwlwifi-ptp-avoid-64-bit-div-v2-1-22b988eb009b@kernel.org>
Date:   Wed, 29 Mar 2023 10:32:46 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     gregory.greenman@...el.com, kvalo@...nel.org
Cc:     nathan@...nel.org, ndesaulniers@...gle.com, trix@...hat.com,
        johannes.berg@...el.com, avraham.stern@...el.com,
        krishnanand.prabhu@...el.com, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, patches@...ts.linux.dev,
        Arnd Bergmann <arnd@...db.de>,
        "kernelci.org bot" <bot@...nelci.org>
Subject: [PATCH wireless-next v2] wifi: iwlwifi: mvm: Use 64-bit division
 helper in iwl_mvm_get_crosstimestamp_fw()

There is a 64-bit division in iwl_mvm_get_crosstimestamp_fw(), which
results in a link failure when building 32-bit architectures with clang:

  ld.lld: error: undefined symbol: __udivdi3
  >>> referenced by ptp.c
  >>>               drivers/net/wireless/intel/iwlwifi/mvm/ptp.o:(iwl_mvm_phc_get_crosstimestamp) in archive vmlinux.a

GCC has optimizations for division by a constant that clang does not
implement, so this issue is not visible when building with GCC.

Use the 64-bit division helper div_u64(), which takes a u64 dividend and
u32 divisor, which matches this situation and prevents the emission of a
libcall for the division.

Fixes: 21fb8da6ebe4 ("wifi: iwlwifi: mvm: read synced time from firmware if supported")
Reported-by: Arnd Bergmann <arnd@...db.de>
Link: https://github.com/ClangBuiltLinux/linux/issues/1826
Reported-by: "kernelci.org bot" <bot@...nelci.org>
Link: https://lore.kernel.org/6423173a.620a0220.3d5cc.6358@mx.google.com/
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
Changes in v2:
- Use division helper div_u64() instead of truncating dividend before
  division (Johannes).
- Link to v1: https://lore.kernel.org/r/20230329-iwlwifi-ptp-avoid-64-bit-div-v1-1-ad8db8d66bc2@kernel.org
---
 drivers/net/wireless/intel/iwlwifi/mvm/ptp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/wireless/intel/iwlwifi/mvm/ptp.c b/drivers/net/wireless/intel/iwlwifi/mvm/ptp.c
index 5c2bfc8ed88d..e89259de6f4c 100644
--- a/drivers/net/wireless/intel/iwlwifi/mvm/ptp.c
+++ b/drivers/net/wireless/intel/iwlwifi/mvm/ptp.c
@@ -116,7 +116,7 @@ iwl_mvm_get_crosstimestamp_fw(struct iwl_mvm *mvm, u32 *gp2, u64 *sys_time)
 
 	gp2_10ns = (u64)le32_to_cpu(resp->gp2_timestamp_hi) << 32 |
 		le32_to_cpu(resp->gp2_timestamp_lo);
-	*gp2 = gp2_10ns / 100;
+	*gp2 = div_u64(gp2_10ns, 100);
 
 	*sys_time = (u64)le32_to_cpu(resp->platform_timestamp_hi) << 32 |
 		le32_to_cpu(resp->platform_timestamp_lo);

---
base-commit: 2af3b2a631b194a43551ce119cb71559d8f6b54b
change-id: 20230329-iwlwifi-ptp-avoid-64-bit-div-1c4717f73f8a

Best regards,
-- 
Nathan Chancellor <nathan@...nel.org>