Re: [PATCH 1/1] usb: cdns3: fix NCM gadget RX speed 20x slow than expection at iMX8QM

[Peter Chen <peter.chen@kernel.org>]

On 23-05-18 11:49:45, Frank Li wrote:
> At iMX8QM platform, enable NCM gadget and run 'iperf3 -s'.
> At host, run 'iperf3 -V -c fe80::6863:98ff:feef:3e0%enxc6e147509498'
> 
> [  5]   0.00-1.00   sec  1.55 MBytes  13.0 Mbits/sec   90   4.18 KBytes
> [  5]   1.00-2.00   sec  1.44 MBytes  12.0 Mbits/sec   75   4.18 KBytes
> [  5]   2.00-3.00   sec  1.48 MBytes  12.4 Mbits/sec   75   4.18 KBytes
> 
> Expected speed should be bigger than 300Mbits/sec.
> 
> The root cause of this performance drop was found to be data corruption
> happening at 4K borders in some Ethernet packets, leading to TCP
> checksum errors. This corruption occurs from the position
> (4K - (address & 0x7F)) to 4K. The u_ether function's allocation of
> skb_buff reserves 64B, meaning all RX addresses resemble 0xXXXX0040.
> 
> Force trb_burst_size to 16 can fix this problem.
> 
> Cc: stable@vger.kernel.org
> Fixes: 7733f6c32e36 ("usb: cdns3: Add Cadence USB3 DRD Driver")
> Signed-off-by: Frank Li <Frank.Li@nxp.com>
> ---
>  drivers/usb/cdns3/cdns3-gadget.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/usb/cdns3/cdns3-gadget.c b/drivers/usb/cdns3/cdns3-gadget.c
> index ccfaebca6faa..1dcadef933e3 100644
> --- a/drivers/usb/cdns3/cdns3-gadget.c
> +++ b/drivers/usb/cdns3/cdns3-gadget.c
> @@ -2097,6 +2097,19 @@ int cdns3_ep_config(struct cdns3_endpoint *priv_ep, bool enable)
>  	else
>  		priv_ep->trb_burst_size = 16;
>  
> +	/*
> +	 * In versions preceding DEV_VER_V2, for example, iMX8QM, there exit the bugs
> +	 * in the DMA. These bugs occur when the trb_burst_size exceeds 16 and the
> +	 * address is not aligned to 128 Bytes (which is a product of the 64-bit AXI
> +	 * and AXI maximum burst length of 16 or 0xF+1, dma_axi_ctrl0[3:0]). This
> +	 * results in data corruption when it crosses the 4K border. The corruption
> +	 * specifically occurs from the position (4K - (address & 0x7F)) to 4K.
> +	 *
> +	 * So force trb_burst_size to 16 at such platform.
> +	 */
> +	if (priv_dev->dev_ver < DEV_VER_V2)
> +		priv_ep->trb_burst_size = 16;
> +
>  	mult = min_t(u8, mult, EP_CFG_MULT_MAX);
>  	buffering = min_t(u8, buffering, EP_CFG_BUFFERING_MAX);
>  	maxburst = min_t(u8, maxburst, EP_CFG_MAXBURST_MAX);
> -- 
> 2.34.1
> 

Acked-by: Peter Chen <peter.chen@kernel.org>

-- 

Thanks,
Peter Chen