BUILD bug hidden in SFC driver.

[Stephen Hemminger <stephen@networkplumber.org>]

While examining the use of VLA in DPDK, ran into a bug in sfc driver.

If DPDK is built with -Wvla, then the RTE_BUILD_BUG_ON() macro won't work
as written. Experimenting with a better more portable version of that macro
as:
	#define RTE_BUILD_BUG_ON(e) _Static_assert(!(e),  #e)

revealed that the SFC driver was calling RTE_BUILD_BUG_ON with non constant
expression.

../drivers/net/sfc/sfc_ef100_tx.c: In function ‘sfc_ef100_tx_pkt_descs_max’:
../lib/eal/include/rte_common.h:585:20: warning: comparison of integer expressions of different signedness: ‘unsigned int’ and ‘int’ [-Wsign-compare]
  585 |                 _a < _b ? _a : _b; \
      |                    ^
../lib/eal/include/rte_common.h:498:46: note: in definition of macro ‘RTE_BUILD_BUG_ON’
  498 | #define RTE_BUILD_BUG_ON(e) _Static_assert(!(e),  #e)
      |                                              ^
../drivers/net/sfc/sfc_ef100_tx.c:566:34: note: in expansion of macro ‘RTE_MIN’
  566 |                                  RTE_MIN((unsigned int)EFX_MAC_PDU_MAX,
      |                                  ^~~~~~~
../lib/eal/include/rte_common.h:585:32: warning: operand of ‘?:’ changes signedness from ‘int’ to ‘unsigned int’ due to unsignedness of other operand [-Wsign-compare]
  585 |                 _a < _b ? _a : _b; \
      |                                ^~
../lib/eal/include/rte_common.h:498:46: note: in definition of macro ‘RTE_BUILD_BUG_ON’
  498 | #define RTE_BUILD_BUG_ON(e) _Static_assert(!(e),  #e)
      |                                              ^
../drivers/net/sfc/sfc_ef100_tx.c:566:34: note: in expansion of macro ‘RTE_MIN’
  566 |                                  RTE_MIN((unsigned int)EFX_MAC_PDU_MAX,
      |                                  ^~~~~~~
../lib/eal/include/rte_common.h:498:44: error: expression in static assertion is not constant
  498 | #define RTE_BUILD_BUG_ON(e) _Static_assert(!(e),  #e)
      |                                            ^~~~
../drivers/net/sfc/sfc_ef100_tx.c:565:17: note: in expansion of macro ‘RTE_BUILD_BUG_ON’


The problem is that Gcc does not evaluate a ternary operator expression
with all constants at compile time to produce a constant value! Apparently,
the language standards leave this as ambiguous.

If the code is expanded into two assertions as:

diff --git a/drivers/net/sfc/sfc_ef100_tx.c b/drivers/net/sfc/sfc_ef100_tx.c
index 1b6374775f07..25e6633d6679 100644
--- a/drivers/net/sfc/sfc_ef100_tx.c
+++ b/drivers/net/sfc/sfc_ef100_tx.c
@@ -562,9 +562,8 @@ sfc_ef100_tx_pkt_descs_max(const struct rte_mbuf *m)
                 * Make sure that the first segment does not need fragmentation
                 * (split into many Tx descriptors).
                 */
-               RTE_BUILD_BUG_ON(SFC_EF100_TX_SEND_DESC_LEN_MAX <
-                                RTE_MIN((unsigned int)EFX_MAC_PDU_MAX,
-                                SFC_MBUF_SEG_LEN_MAX));
+               RTE_BUILD_BUG_ON(SFC_EF100_TX_SEND_DESC_LEN_MAX < EFX_MAC_PDU_MAX);
+               RTE_BUILD_BUG_ON(SFC_EF100_TX_SEND_DESC_LEN_MAX < SFC_MBUF_SEG_LEN_MAX);
        }
 
        if (m->ol_flags & sfc_dp_mport_override) {

Then a new problem arises:
In file included from ../lib/mbuf/rte_mbuf.h:36,
                 from ../drivers/net/sfc/sfc_ef100_tx.c:12:
../drivers/net/sfc/sfc_ef100_tx.c: In function ‘sfc_ef100_tx_pkt_descs_max’:
../lib/eal/include/rte_common.h:498:29: error: static assertion failed: "SFC_EF100_TX_SEND_DESC_LEN_MAX < SFC_MBUF_SEG_LEN_MAX"
  498 | #define RTE_BUILD_BUG_ON(e) _Static_assert(!(e),  #e)
      |                             ^~~~~~~~~~~~~~
../drivers/net/sfc/sfc_ef100_tx.c:566:17: note: in expansion of macro ‘RTE_BUILD_BUG_ON’
  566 |                 RTE_BUILD_BUG_ON(SFC_EF100_TX_SEND_DESC_LEN_MAX < SFC_MBUF_SEG_LEN_MAX);
      |                 ^~~~~~~~~~~~~~~~

Building a little program to unwind the #defines reveals:

SFC_EF100_TX_SEND_DESC_LEN_MAX = 16383
EFX_MAC_PDU_MAX = 9240
SFC_MBUF_SEG_LEN_MAX = 65535

I.e:
	RTE_BUILD_BUG_ON(16383 < RTE_MIN(9240, 65535));
	

Therefore the current driver should be getting build bug, but the existing macro
hides it.