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sfc: Use write-combining to reduce TX latency

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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>
This commit is contained in:
Ben Hutchings
2011-02-22 17:26:10 +00:00
parent 6d84b986b2
commit 65f0b417de
3 changed files with 16 additions and 10 deletions
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13
drivers/net/sfc/io.h
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@@ -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, \