sfc: Use write-combining to reduce TX latency

Based on work by Neil Turton <nturton@solarflare.com> and
Kieran Mansley <kmansley@solarflare.com>.

The BIU has now been verified to handle 3- and 4-dword writes within a
single 128-bit register correctly.  This means we can enable write-
combining and only insert write barriers between writes to distinct
registers.

This has been observed to save about 0.5 us when pushing a TX
descriptor to an empty TX queue.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

diff --git a/drivers/net/sfc/efx.c b/drivers/net/sfc/efx.c
index d563049859a815fac3e640d556649a4f01161bdd..b8bd936374f29e5142deb4d75f4ec700af42a5bf 100644 (file)
--- a/drivers/net/sfc/efx.c
+++ b/drivers/net/sfc/efx.c
@@ -1104,8 +1104,8 @@ static int efx_init_io(struct efx_nic *efx)
                rc = -EIO;
                goto fail3;
        }
-       efx->membase = ioremap_nocache(efx->membase_phys,
-                                      efx->type->mem_map_size);
+       efx->membase = ioremap_wc(efx->membase_phys,
+                                 efx->type->mem_map_size);
        if (!efx->membase) {
                netif_err(efx, probe, efx->net_dev,
                          "could not map memory BAR at %llx+%x\n",

diff --git a/drivers/net/sfc/io.h b/drivers/net/sfc/io.h
index dc45110b2456492a0292754270f94915905e6c74..d9d8c2ef1074a3de6278eff70d7894cc8cc65991 100644 (file)
--- a/drivers/net/sfc/io.h
+++ b/drivers/net/sfc/io.h
@@ -48,9 +48,9 @@
  *   replacing the low 96 bits with zero does not affect functionality.
  * - If the host writes to the last dword address of such a register
  *   (i.e. the high 32 bits) the underlying register will always be
- *   written.  If the collector does not hold values for the low 96
- *   bits of the register, they will be written as zero.  Writing to
- *   the last qword does not have this effect and must not be done.
+ *   written.  If the collector and the current write together do not
+ *   provide values for all 128 bits of the register, the low 96 bits
+ *   will be written as zero.
  * - If the host writes to the address of any other part of such a
  *   register while the collector already holds values for some other
  *   register, the write is discarded and the collector maintains its
@@ -103,6 +103,7 @@ static inline void efx_writeo(struct efx_nic *efx, efx_oword_t *value,
        _efx_writed(efx, value->u32[2], reg + 8);
        _efx_writed(efx, value->u32[3], reg + 12);
 #endif
+       wmb();
        mmiowb();
        spin_unlock_irqrestore(&efx->biu_lock, flags);
 }
@@ -125,6 +126,7 @@ static inline void efx_sram_writeq(struct efx_nic *efx, void __iomem *membase,
        __raw_writel((__force u32)value->u32[0], membase + addr);
        __raw_writel((__force u32)value->u32[1], membase + addr + 4);
 #endif
+       wmb();
        mmiowb();
        spin_unlock_irqrestore(&efx->biu_lock, flags);
 }
@@ -139,6 +141,7 @@ static inline void efx_writed(struct efx_nic *efx, efx_dword_t *value,
 
        /* No lock required */
        _efx_writed(efx, value->u32[0], reg);
+       wmb();
 }
 
 /* Read a 128-bit CSR, locking as appropriate. */
@@ -237,12 +240,14 @@ static inline void _efx_writeo_page(struct efx_nic *efx, efx_oword_t *value,
 
 #ifdef EFX_USE_QWORD_IO
        _efx_writeq(efx, value->u64[0], reg + 0);
+       _efx_writeq(efx, value->u64[1], reg + 8);
 #else
        _efx_writed(efx, value->u32[0], reg + 0);
        _efx_writed(efx, value->u32[1], reg + 4);
-#endif
        _efx_writed(efx, value->u32[2], reg + 8);
        _efx_writed(efx, value->u32[3], reg + 12);
+#endif
+       wmb();
 }
 #define efx_writeo_page(efx, value, reg, page)                         \
        _efx_writeo_page(efx, value,                                    \

diff --git a/drivers/net/sfc/mcdi.c b/drivers/net/sfc/mcdi.c
index 8bba8955f310ce351e605967a458c5d767df08aa..5e118f0d2479231e1f0069062c6cd9d5582d423a 100644 (file)
--- a/drivers/net/sfc/mcdi.c
+++ b/drivers/net/sfc/mcdi.c
@@ -94,14 +94,15 @@ static void efx_mcdi_copyin(struct efx_nic *efx, unsigned cmd,
 
        efx_writed(efx, &hdr, pdu);
 
-       for (i = 0; i < inlen; i += 4)
+       for (i = 0; i < inlen; i += 4) {
                _efx_writed(efx, *((__le32 *)(inbuf + i)), pdu + 4 + i);
-
-       /* Ensure the payload is written out before the header */
-       wmb();
+               /* use wmb() within loop to inhibit write combining */
+               wmb();
+       }
 
        /* ring the doorbell with a distinctive value */
        _efx_writed(efx, (__force __le32) 0x45789abc, doorbell);
+       wmb();
 }
 
 static void efx_mcdi_copyout(struct efx_nic *efx, u8 *outbuf, size_t outlen)