[PATCH net-next v6 0/5] Coalesce mac ocp write/modify calls to reduce spinlock contention

[Mirsad Goran Todorovac]

The motivation for these helpers was the locking overhead of 130 consecutive
r8168_mac_ocp_write() calls in the RTL8411b reset after the NIC gets confused
if the PHY is powered-down.

To quote Heiner:

    On RTL8411b the RX unit gets confused if the PHY is powered-down.
    This was reported in [0] and confirmed by Realtek. Realtek provided
    a sequence to fix the RX unit after PHY wakeup.

A series of about 130 r8168_mac_ocp_write() calls is performed to program the
RTL registers for recovery, each doing an expensive spin_lock_irqsave() and
spin_unlock_irqrestore().

Each mac ocp write is made of:

    static void __r8168_mac_ocp_write(struct rtl8169_private *tp, u32 reg,
                      u32 data)
    {
        if (rtl_ocp_reg_failure(reg))
            return;

        RTL_W32(tp, OCPDR, OCPAR_FLAG | (reg << 15) | data);
    }

    static void r8168_mac_ocp_write(struct rtl8169_private *tp, u32 reg,
                    u32 data)
    {
        unsigned long flags;

        raw_spin_lock_irqsave(&tp->mac_ocp_lock, flags);
        __r8168_mac_ocp_write(tp, reg, data);
        raw_spin_unlock_irqrestore(&tp->mac_ocp_lock, flags);
    }

Register programming is done through RTL_W32() macro which expands into

    #define RTL_W32(tp, reg, val32) writel((val32), tp->mmio_addr + (reg))

which is further (on Alpha):

    extern inline void writel(u32 b, volatile void __iomem *addr)
    {
        mb();
        __raw_writel(b, addr);
    }

or on i386/x86_64:

    #define build_mmio_write(name, size, type, reg, barrier) \
    static inline void name(type val, volatile void __iomem *addr) \
    { asm volatile("mov" size " %0,%1": :reg (val), \
    "m" (*(volatile type __force *)addr) barrier); }

    build_mmio_write(writel, "l", unsigned int, "r", :"memory")

This obviously involves iat least a compiler barrier.

mb() expands into something like this i.e. on x86_64:

    #define mb()    asm volatile("lock; addl $0,0(%%esp)" ::: "memory")

This means a whole lot of memory bus stalls: for spin_lock_irqsave(),
memory barrier, writel(), and spin_unlock_irqrestore().

With about 130 of these sequential calls to r8168_mac_ocp_write() this looks like
a lock storm that will stall all of the cores and CPUs on the same memory controller
for certain time I/O takes to finish.

In a sequential case of RTL register programming, the writes to RTL registers
can be coalesced under a same raw spinlock. This can dramatically decrease the
number of bus stalls in a multicore or multi-CPU system.

Macro helpers r8168_mac_ocp_write_seq() and r8168_mac_ocp_modify_seq() are
provided to reduce lock contention:

    static void rtl_hw_start_8411_2(struct rtl8169_private *tp)
    {

        ...

        /* The following Realtek-provided magic fixes an issue with the RX unit
         * getting confused after the PHY having been powered-down.
         */

        static const struct recover_8411b_info init_zero_seq[] = {
            { 0xFC28, 0x0000 }, { 0xFC2A, 0x0000 }, { 0xFC2C, 0x0000 },
            ...
        };

        ...

        r8168_mac_ocp_write_seq(tp, init_zero_seq);

        ...

    }

The hex data is preserved intact through s/r8168_mac_ocp_write[(]tp,/{ / and s/[)];/ },/
functions that only changed the function names and the ending of the line, so the actual
hex data is unchanged.

To repeat, the reason for the introduction of the original commit
was to enable recovery of the RX unit on the RTL8411b which was confused by the
powered-down PHY. This sequence of r8168_mac_ocp_write() calls amplifies the problem
into a series of about 500+ memory bus locks, most waiting for the main memory read,
modify and write under a LOCK. The memory barrier in RTL_W32 should suffice for
the programming sequence to reach RTL NIC registers.

[0] https://bugzilla.redhat.com/show_bug.cgi?id=1692075

v6:
 proceeded according to Jacob Keller's suggestions by creating a cover page and reducing
 the text within the commits. Applying to the net-next tree as Heiner Kallweit requested.

v5:
 attempted some new optimisations, which were rejected, but not all and not completely.

v4:
 fixed complaints as advised by Heiner and checkpatch.pl.
 split the patch into five sections to be more easily manipulated and reviewed
 introduced r8168_mac_ocp_write_seq()
 applied coalescing of mac ocp writes/modifies for 8168H, 8125 and 8125B

v3:
 removed register/mask pair array sentinels, so using ARRAY_SIZE().
 avoided duplication of RTL_W32() call code as advised by Heiner.

Mirsad Goran Todorovac (5):
  r8169: Coalesce r8169_mac_ocp_write/modify calls to reduce spinlock
    stalls
  r8169: Coalesce RTL8411b PHY power-down recovery calls to reduce
    spinlock stalls
  r8169: Coalesce mac ocp write and modify for 8168H start to reduce
    spinlocks
  r8169: Coalesce mac ocp commands for 8125 and 8125B start to reduce
    spinlock contention
  r8169: Coalesce mac ocp commands for rtl_hw_init_8125 to reduce
    spinlocks

 drivers/net/ethernet/realtek/r8169_main.c | 304 +++++++++++-----------
 1 file changed, 150 insertions(+), 154 deletions(-)

-- 
2.34.1