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

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Message-Id: <20230329-iwlwifi-ptp-avoid-64-bit-div-v1-1-ad8db8d66bc2@kernel.org>
Date:   Wed, 29 Mar 2023 10:05:44 -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] wifi: iwlwifi: mvm: Avoid 64-bit division 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.

Using div_u64() would resolve this issue, but Arnd points out that this
can be quite expensive and the timestamp is being read at nanosecond
granularity. Nick pointed out that the result of this division is being
stored to a 32-bit type anyways, so truncate gp2_10ns first then do the
division, which elides the need for libcalls.

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/
Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Nathan Chancellor <nathan@...nel.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..cdd6d69c5b68 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 = (u32)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>