Re: [PATCH net-next v1 1/6] lan743x: boost performance on cpu archs w/o dma cache snooping

On Fri, 29 Jan 2021 14:52:35 -0500 Sven Van Asbroeck wrote:

From: Sven Van Asbroeck <redacted>

The buffers in the lan743x driver's receive ring are always 9K,
even when the largest packet that can be received (the mtu) is
much smaller. This performs particularly badly on cpu archs
without dma cache snooping (such as ARM): each received packet
results in a 9K dma_{map|unmap} operation, which is very expensive
because cpu caches need to be invalidated.

Careful measurement of the driver rx path on armv7 reveals that
the cpu spends the majority of its time waiting for cache
invalidation.

Optimize as follows:

1. set rx ring buffer size equal to the mtu. this limits the
   amount of cache that needs to be invalidated per dma_map().

2. when dma_unmap()ping, skip cpu sync. Sync only the packet data
   actually received, the size of which the chip will indicate in
   its rx ring descriptors. this limits the amount of cache that
   needs to be invalidated per dma_unmap().

These optimizations double the rx performance on armv7.
Third parties report 3x rx speedup on armv8.

Performance on dma cache snooping architectures (such as x86)
is expected to stay the same.

Tested with iperf3 on a freescale imx6qp + lan7430, both sides
set to mtu 1500 bytes, measure rx performance:

Before:
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-20.00  sec   550 MBytes   231 Mbits/sec    0
After:
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-20.00  sec  1.33 GBytes   570 Mbits/sec    0

Test by Anders Roenningen (anders@ronningen.priv.no) on armv8,
    rx iperf3:
Before 102 Mbits/sec
After  279 Mbits/sec

Signed-off-by: Sven Van Asbroeck <redacted>

You may need to rebase to see this:

drivers/net/ethernet/microchip/lan743x_main.c:2123:41: warning: restricted __le32 degrades to integer