Re: [PATCH net v2] net: stmmac: set PP_FLAG_DMA_SYNC_DEV only if XDP is enabled

[Jon Hunter]

Hi Furong,

On 19/09/2024 13:10, Furong Xu wrote:
> Commit 5fabb01207a2 ("net: stmmac: Add initial XDP support") sets
> PP_FLAG_DMA_SYNC_DEV flag for page_pool unconditionally,
> page_pool_recycle_direct() will call page_pool_dma_sync_for_device()
> on every page even the page is not going to be reused by XDP program.
>
> When XDP is not enabled, the page which holds the received buffer
> will be recycled once the buffer is copied into new SKB by
> skb_copy_to_linear_data(), then the MAC core will never reuse this
> page any longer. Always setting PP_FLAG_DMA_SYNC_DEV wastes CPU cycles
> on unnecessary calling of page_pool_dma_sync_for_device().
>
> After this patch, up to 9% noticeable performance improvement was observed
> on certain platforms.
>
> Fixes: 5fabb01207a2 ("net: stmmac: Add initial XDP support")
> Signed-off-by: Furong Xu <0x1207@xxxxxxxxx>
> ---
>   drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index f3a1b179aaea..95d3d1081727 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -2022,7 +2022,7 @@ static int __alloc_dma_rx_desc_resources(struct stmmac_priv *priv,
>   	rx_q->queue_index = queue;
>   	rx_q->priv_data = priv;
>   
> -	pp_params.flags = PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC_DEV;
> +	pp_params.flags = PP_FLAG_DMA_MAP | (xdp_prog ? PP_FLAG_DMA_SYNC_DEV : 0);
>   	pp_params.pool_size = dma_conf->dma_rx_size;
>   	num_pages = DIV_ROUND_UP(dma_conf->dma_buf_sz, PAGE_SIZE);
>   	pp_params.order = ilog2(num_pages);


We have noticed a boot regression in both -next and mainline v6.12-rc1. 
Bisect is pointing to this commit. Reverting this commit fixes the problem.

This boot regression is seen on our Tegra234 Jetson AGX Orin platform 
that uses the drivers/net/ethernet/stmicro/stmmac/dwmac-tegra.c driver. 
We are booting with NFS and although the network interface does come up, 
we fail to mount the rootfs via NFS.

So it would appear that we need to set this flag for this device. Any 
thoughts?

Thanks
Jon

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