linux-kernel - Re: [Intel-wired-lan] [PATCH 2/2] igb: Make DMA faster when CPU is active on the PCIe link

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Date:   Wed, 11 May 2022 14:49:12 +0200
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Kai-Heng Feng <kai.heng.feng@...onical.com>
Cc:     jesse.brandeburg@...el.com, anthony.l.nguyen@...el.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>,
        intel-wired-lan@...ts.osuosl.org, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [Intel-wired-lan] [PATCH 2/2] igb: Make DMA faster when CPU is
 active on the PCIe link

Dear Kai-Hang,


Thank you for the patch.


Am 11.05.22 um 14:28 schrieb Kai-Heng Feng:
> We found Intel I210 can only achieve ~750Mbps Tx speed on some
> platforms. The RR2DCDELAY shows around 0x2xxx DMA delay, which will be

Please give an example platform, where it works and where it does not.

How did you test transfer speed?

> significantly lower when 1) ASPM is disabled or 2) SoC package c-state
> stays above PC3. When the RR2DCDELAY is around 0x1xxx the Tx speed can
> reach to ~950Mbps.
> 
> According to the I210 datasheet "8.26.1 PCIe Misc. Register - PCIEMISC",
> "DMA Idle Indication" doesn't seem to tie to DMA coalesce anymore, so
> set it to 1b for "DMA is considered idle when there is no Rx or Tx AND
> when there are no TLPs indicating that CPU is active detected on the
> PCIe link (such as the host executes CSR or Configuration register read
> or write operation)" and performing Tx should also fall under "active
> CPU on PCIe link" case.
> 
> In addition to that, commit b6e0c419f040 ("igb: Move DMA Coalescing init
> code to separate function.") seems to wrongly changed from enabling
> E1000_PCIEMISC_LX_DECISION to disabling it, also fix that.

Please split this into a separate commit with Fixes tag, and maybe the 
commit author in Cc.


Kind regards,

Paul


> Signed-off-by: Kai-Heng Feng <kai.heng.feng@...onical.com>
> ---
>   drivers/net/ethernet/intel/igb/igb_main.c | 12 +++++-------
>   1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
> index 34b33b21e0dcd..eca797dded429 100644
> --- a/drivers/net/ethernet/intel/igb/igb_main.c
> +++ b/drivers/net/ethernet/intel/igb/igb_main.c
> @@ -9897,11 +9897,10 @@ static void igb_init_dmac(struct igb_adapter *adapter, u32 pba)
>   	struct e1000_hw *hw = &adapter->hw;
>   	u32 dmac_thr;
>   	u16 hwm;
> +	u32 reg;
>   
>   	if (hw->mac.type > e1000_82580) {
>   		if (adapter->flags & IGB_FLAG_DMAC) {
> -			u32 reg;
> -
>   			/* force threshold to 0. */
>   			wr32(E1000_DMCTXTH, 0);
>   
> @@ -9934,7 +9933,6 @@ static void igb_init_dmac(struct igb_adapter *adapter, u32 pba)
>   			/* Disable BMC-to-OS Watchdog Enable */
>   			if (hw->mac.type != e1000_i354)
>   				reg &= ~E1000_DMACR_DC_BMC2OSW_EN;
> -
>   			wr32(E1000_DMACR, reg);
>   
>   			/* no lower threshold to disable
> @@ -9951,12 +9949,12 @@ static void igb_init_dmac(struct igb_adapter *adapter, u32 pba)
>   			 */
>   			wr32(E1000_DMCTXTH, (IGB_MIN_TXPBSIZE -
>   			     (IGB_TX_BUF_4096 + adapter->max_frame_size)) >> 6);
> +		}
>   
> -			/* make low power state decision controlled
> -			 * by DMA coal
> -			 */
> +		if (hw->mac.type >= e1000_i210 ||
> +		    (adapter->flags & IGB_FLAG_DMAC)) {
>   			reg = rd32(E1000_PCIEMISC);
> -			reg &= ~E1000_PCIEMISC_LX_DECISION;
> +			reg |= E1000_PCIEMISC_LX_DECISION;
>   			wr32(E1000_PCIEMISC, reg);
>   		} /* endif adapter->dmac is not disabled */
>   	} else if (hw->mac.type == e1000_82580) {