Commit 65f0b417 authored by Ben Hutchings's avatar Ben Hutchings
Browse files

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: default avatarBen Hutchings <bhutchings@solarflare.com>
parent 6d84b986

drivers/net/sfc/efx.c

+2 −2
Original line number Diff line number Diff line
@@ -1104,7 +1104,7 @@ static int efx_init_io(struct efx_nic *efx) 
		rc = -EIO;

		goto fail3;

	}

	efx->membase = ioremap_nocache(efx->membase_phys,

	efx->membase = ioremap_wc(efx->membase_phys,

				  efx->type->mem_map_size);

	if (!efx->membase) {

		netif_err(efx, probe, efx->net_dev,

drivers/net/sfc/io.h

+9 −4
Original line number Diff line number Diff line
@@ -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,					\

drivers/net/sfc/mcdi.c

+5 −4
Original line number Diff line number Diff line
@@ -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 */

		/* 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)

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