netdev - ethtool build failure

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Date:   Sat, 6 Jun 2020 15:24:22 +0200
From:   Heiko Thiery <heiko.thiery@...il.com>
To:     Michal Kubecek <mkubecek@...e.cz>, netdev@...r.kernel.org
Subject: ethtool build failure

Hi Michael et all,

I'm digging in the reason for a failure when building ethtool with
buildroot [1].

I see the following error:
---
data/buildroot/buildroot-test/instance-0/output/host/bin/i686-linux-gcc
-DHAVE_CONFIG_H -I.  -I./uapi  -D_LARGEFILE_SOURCE
-D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Wall -D_LARGEFILE_SOURCE
-D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os   -static -c -o
netlink/desc-rtnl.o netlink/desc-rtnl.c
In file included from ./uapi/linux/ethtool_netlink.h:12,
                 from netlink/desc-ethtool.c:7:
./uapi/linux/ethtool.h:1294:19: warning: implicit declaration of
function '__KERNEL_DIV_ROUND_UP' [-Wimplicit-function-declaration]
  __u32 queue_mask[__KERNEL_DIV_ROUND_UP(MAX_NUM_QUEUE, 32)];
                   ^~~~~~~~~~~~~~~~~~~~~
./uapi/linux/ethtool.h:1294:8: error: variably modified 'queue_mask'
at file scope
  __u32 queue_mask[__KERNEL_DIV_ROUND_UP(MAX_NUM_QUEUE, 32)];
        ^~~~~~~~~~
---

The problems seems to be injected by the "warning: implicit
declaration of function".

When I move the __KERNEL_DIV_ROUND_UP macro right beside usage in
"uapi/linux/ethtool.h" the failure is gone.

---
diff --git a/uapi/linux/ethtool.h b/uapi/linux/ethtool.h
index d3dcb45..6710fa0 100644
--- a/uapi/linux/ethtool.h
+++ b/uapi/linux/ethtool.h
@@ -1288,6 +1288,11 @@ enum ethtool_sfeatures_retval_bits {
  * @queue_mask: Bitmap of the queues which sub command apply to
  * @data: A complete command structure following for each of the
queues addressed
  */
+/* ethtool.h epxects __KERNEL_DIV_ROUND_UP to be defined by <linux/kernel.h> */
+#include <linux/kernel.h>
+#ifndef __KERNEL_DIV_ROUND_UP
+#define __KERNEL_DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))
+#endif
 struct ethtool_per_queue_op {
        __u32   cmd;
        __u32   sub_command;
---

Has anyone an idea what goes wrong?

[1] http://autobuild.buildroot.net/results/23dffe2f9766c12badb6e42bf3f0356c6df0cbfb/build-end.log

-- 
Thanks,
Heiko