From: Ben Hutchings <bhutchings@solarflare.com>
To: David Miller <davem@davemloft.net>,
Geert Uytterhoeven <geert@linux-m68k.org>,
Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: <linux-net-drivers@solarflare.com>,
Linux Kernel Development <linux-kernel@vger.kernel.org>,
<linux-s390@vger.kernel.org>, netdev <netdev@vger.kernel.org>
Subject: [PATCHv2 net 2/2] sfc: Reduce RX scatter buffer size, and reduce alignment if appropriate
Date: Mon, 13 May 2013 23:01:22 +0100 [thread overview]
Message-ID: <1368482482.3305.50.camel@bwh-desktop.uk.solarflarecom.com> (raw)
In-Reply-To: <1368482311.3305.48.camel@bwh-desktop.uk.solarflarecom.com>
efx_start_datapath() asserts that we can fit 2 RX scatter buffers plus
a software structure, each appropriately aligned, into a single page.
Where L1_CACHE_BYTES == 256 and PAGE_SIZE == 4096, which is the case
on s390, this assertion fails.
The current scatter buffer size is also not a multiple of 64 or 128,
which are more common cache line sizes. If we can make both the start
and end of a scatter buffer cache-aligned, this will reduce the need
for read-modify-write operations on inter- processor links.
Fix the alignment by reducing EFX_RX_USR_BUF_SIZE to 2048 - 256 ==
1792. (We could use 2048 - L1_CACHE_BYTES, but EFX_RX_USR_BUF_SIZE
also affects user-level networking where a larger amount of
housekeeping data may be needed. Although this version of the driver
does not support user-level networking, I prefer to keep scattering
behaviour consistent with the out-of-tree version.)
This still doesn't fix the s390 build because like most architectures
it has NET_IP_ALIGN == 2. When NET_IP_ALIGN != 0 we cannot achieve
cache line alignment at either the start or end of a scatter buffer,
so there is actually no point in padding the buffers to a multiple of
the cache line size. All we need is 4-byte alignment of the network
header, so do that.
Adjust the assertions accordingly.
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
---
drivers/net/ethernet/sfc/efx.c | 6 ++++--
drivers/net/ethernet/sfc/net_driver.h | 16 ++++++++++++++--
drivers/net/ethernet/sfc/rx.c | 2 +-
3 files changed, 19 insertions(+), 5 deletions(-)
diff --git a/drivers/net/ethernet/sfc/efx.c b/drivers/net/ethernet/sfc/efx.c
index 999289b..39e4cb3 100644
--- a/drivers/net/ethernet/sfc/efx.c
+++ b/drivers/net/ethernet/sfc/efx.c
@@ -643,9 +643,11 @@ static void efx_start_datapath(struct efx_nic *efx)
efx->rx_scatter = false;
efx->rx_buffer_order = 0;
} else if (efx->type->can_rx_scatter) {
+ BUILD_BUG_ON(EFX_RX_USR_BUF_SIZE % L1_CACHE_BYTES);
BUILD_BUG_ON(sizeof(struct efx_rx_page_state) +
- NET_IP_ALIGN + EFX_RX_USR_BUF_SIZE >
- PAGE_SIZE / 2);
+ 2 * ALIGN(NET_IP_ALIGN + EFX_RX_USR_BUF_SIZE,
+ EFX_RX_BUF_ALIGNMENT) >
+ PAGE_SIZE);
efx->rx_scatter = true;
efx->rx_dma_len = EFX_RX_USR_BUF_SIZE;
efx->rx_buffer_order = 0;
diff --git a/drivers/net/ethernet/sfc/net_driver.h b/drivers/net/ethernet/sfc/net_driver.h
index 5efddf3..39d6bd77 100644
--- a/drivers/net/ethernet/sfc/net_driver.h
+++ b/drivers/net/ethernet/sfc/net_driver.h
@@ -72,8 +72,20 @@
/* Maximum possible MTU the driver supports */
#define EFX_MAX_MTU (9 * 1024)
-/* Size of an RX scatter buffer. Small enough to pack 2 into a 4K page. */
-#define EFX_RX_USR_BUF_SIZE 1824
+/* Size of an RX scatter buffer. Small enough to pack 2 into a 4K page,
+ * and should be a multiple of the cache line size.
+ */
+#define EFX_RX_USR_BUF_SIZE (2048 - 256)
+
+/* If possible, we should ensure cache line alignment at start and end
+ * of every buffer. Otherwise, we just need to ensure 4-byte
+ * alignment of the network header.
+ */
+#if NET_IP_ALIGN == 0
+#define EFX_RX_BUF_ALIGNMENT L1_CACHE_BYTES
+#else
+#define EFX_RX_BUF_ALIGNMENT 4
+#endif
/* Forward declare Precision Time Protocol (PTP) support structure. */
struct efx_ptp_data;
diff --git a/drivers/net/ethernet/sfc/rx.c b/drivers/net/ethernet/sfc/rx.c
index 99f70dd..a7dfe36 100644
--- a/drivers/net/ethernet/sfc/rx.c
+++ b/drivers/net/ethernet/sfc/rx.c
@@ -94,7 +94,7 @@ static inline void efx_sync_rx_buffer(struct efx_nic *efx,
void efx_rx_config_page_split(struct efx_nic *efx)
{
efx->rx_page_buf_step = ALIGN(efx->rx_dma_len + NET_IP_ALIGN,
- L1_CACHE_BYTES);
+ EFX_RX_BUF_ALIGNMENT);
efx->rx_bufs_per_page = efx->rx_buffer_order ? 1 :
((PAGE_SIZE - sizeof(struct efx_rx_page_state)) /
efx->rx_page_buf_step);
--
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.
next prev parent reply other threads:[~2013-05-13 22:01 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-05-13 21:58 [PATCHv2 net 1/2] sfc: Delete EFX_PAGE_IP_ALIGN, equivalent to NET_IP_ALIGN Ben Hutchings
2013-05-13 22:01 ` Ben Hutchings [this message]
2013-05-14 7:09 ` [PATCHv2 net 2/2] sfc: Reduce RX scatter buffer size, and reduce alignment if appropriate Geert Uytterhoeven
2013-05-14 18:32 ` David Miller
2013-07-23 20:19 ` Ben Hutchings
2013-05-14 8:32 ` [PATCHv2 net 1/2] sfc: Delete EFX_PAGE_IP_ALIGN, equivalent to NET_IP_ALIGN David Laight
2013-05-14 13:48 ` Ben Hutchings
2013-05-14 18:32 ` David Miller
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