Re: [PATCH net] net: microchip: lan743x : bidirectional throughuput improvement

[Jacob Keller]

On 9/27/2023 4:16 AM, Vishvambar Panth S wrote:
> The LAN743x/PCI11xxx DMA descriptors are always 4 dwords long, but the
> device supports placing the descriptors in memory back to back or
> reserving space in between them using its DMA_DESCRIPTOR_SPACE (DSPACE)
> configurable hardware setting. Currently DSPACE is unnecessarily set to
> match the host's L1 cache line size, resulting in space reserved in
> between descriptors in most platforms and causing a suboptimal behavior
> (single PCIe Mem transaction per descriptor). By changing the setting
> to DSPACE=16 many descriptors can be packed in a single PCIe Mem
> transaction resulting in a massive performance improvement in
> bidirectional tests without any negative effects.
> Tested and verified improvements on x64 PC and several ARM platforms
> (typical data below)
> 

I assume here the choice of 16 is to get the 16 bytes from 4 dwords?

> Test setup 1: x64 PC with LAN7430 ---> x64 PC
> 
> iperf3 UDP bidirectional with DSPACE set to L1 CACHE Size:
> - - - - - - - - - - - - - - - - - - - - - - - - -
> [ ID][Role] Interval           Transfer     Bitrate
> [  5][TX-C]   0.00-10.00  sec   170 MBytes   143 Mbits/sec  sender
> [  5][TX-C]   0.00-10.04  sec   169 MBytes   141 Mbits/sec  receiver
> [  7][RX-C]   0.00-10.00  sec  1.02 GBytes   876 Mbits/sec  sender
> [  7][RX-C]   0.00-10.04  sec  1.02 GBytes   870 Mbits/sec  receiver
> 
> iperf3 UDP bidirectional with DSPACE set to 16 Bytes
> - - - - - - - - - - - - - - - - - - - - - - - - -
> [ ID][Role] Interval           Transfer     Bitrate
> [  5][TX-C]   0.00-10.00  sec  1.11 GBytes   956 Mbits/sec  sender
> [  5][TX-C]   0.00-10.04  sec  1.11 GBytes   951 Mbits/sec  receiver
> [  7][RX-C]   0.00-10.00  sec  1.10 GBytes   948 Mbits/sec  sender
> [  7][RX-C]   0.00-10.04  sec  1.10 GBytes   942 Mbits/sec  receiver
> 

Going from barely transmitting to hitting the line rate *and* improving
both Tx and Rx slightly is fantastic. Huge win just avoiding all that
unnecessary wasted PCIe bandwidth.

> Test setup 2 : RK3399 with LAN7430 ---> x64 PC
> 
> RK3399 Spec:
> The SOM-RK3399 is ARM module designed and developed by FriendlyElec.
> Cores: 64-bit Dual Core Cortex-A72 + Quad Core Cortex-A53
> Frequency: Cortex-A72(up to 2.0GHz), Cortex-A53(up to 1.5GHz)
> PCIe: PCIe x4, compatible with PCIe 2.1, Dual operation mode
> 
> iperf3 UDP bidirectional with DSPACE set to L1 CACHE Size:
> - - - - - - - - - - - - - - - - - - - - - - - - -
> [ ID][Role] Interval           Transfer     Bitrate
> [  5][TX-C]   0.00-10.00  sec   534 MBytes   448 Mbits/sec  sender
> [  5][TX-C]   0.00-10.05  sec   534 MBytes   446 Mbits/sec  receiver
> [  7][RX-C]   0.00-10.00  sec  1.12 GBytes   961 Mbits/sec  sender
> [  7][RX-C]   0.00-10.05  sec  1.11 GBytes   946 Mbits/sec  receiver
> 
> iperf3 UDP bidirectional with DSPACE set to 16 Bytes
> - - - - - - - - - - - - - - - - - - - - - - - - -
> [ ID][Role] Interval           Transfer     Bitrate
> [  5][TX-C]   0.00-10.00  sec   966 MBytes   810 Mbits/sec   sender
> [  5][TX-C]   0.00-10.04  sec   965 MBytes   806 Mbits/sec   receiver
> [  7][RX-C]   0.00-10.00  sec  1.11 GBytes   956 Mbits/sec   sender
> [  7][RX-C]   0.00-10.04  sec  1.07 GBytes   919 Mbits/sec   receiver
> 
> Fixes: 23f0703c125b ("lan743x: Add main source files for new lan743x driver")
> Signed-off-by: Vishvambar Panth S <vishvambarpanth.s@xxxxxxxxxxxxx>
> ---

Always fun to see a massive win like this! Even just the 2x improvement
on the RK3399 is huge, let alone 6x improvement on the x86 platform.

Reviewed-by: Jacob Keller <jacob.e.keller@xxxxxxxxx>

>  drivers/net/ethernet/microchip/lan743x_main.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/microchip/lan743x_main.h b/drivers/net/ethernet/microchip/lan743x_main.h
> index 52609fc13ad9..6dac6fef7d24 100644
> --- a/drivers/net/ethernet/microchip/lan743x_main.h
> +++ b/drivers/net/ethernet/microchip/lan743x_main.h
> @@ -1067,7 +1067,7 @@ struct lan743x_adapter {
>  #define DMA_DESCRIPTOR_SPACING_32       (32)
>  #define DMA_DESCRIPTOR_SPACING_64       (64)
>  #define DMA_DESCRIPTOR_SPACING_128      (128)
> -#define DEFAULT_DMA_DESCRIPTOR_SPACING  (L1_CACHE_BYTES)
> +#define DEFAULT_DMA_DESCRIPTOR_SPACING  (DMA_DESCRIPTOR_SPACING_16)
>  
>  #define DMAC_CHANNEL_STATE_SET(start_bit, stop_bit) \
>  	(((start_bit) ? 2 : 0) | ((stop_bit) ? 1 : 0))