netdev - Re: [PATCH net-next v1] net: stmmac: xgmac: increase length limit of descriptor ring

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Date: Thu, 20 Jun 2024 15:06:57 +0300
From: Serge Semin <fancer.lancer@...il.com>
To: Furong Xu <0x1207@...il.com>
Cc: "David S. Miller" <davem@...emloft.net>, 
	Alexandre Torgue <alexandre.torgue@...s.st.com>, Jose Abreu <joabreu@...opsys.com>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Maxime Coquelin <mcoquelin.stm32@...il.com>, 
	Joao Pinto <jpinto@...opsys.com>, Simon Horman <horms@...nel.org>, netdev@...r.kernel.org, 
	linux-stm32@...md-mailman.stormreply.com, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, xfr@...look.com, rock.xu@....com
Subject: Re: [PATCH net-next v1] net: stmmac: xgmac: increase length limit of
 descriptor ring

Hi Furong

On Thu, Jun 20, 2024 at 04:52:00PM +0800, Furong Xu wrote:
> DWXGMAC CORE supports a ring length of 65536 descriptors, bump max length
> from 1024 to 65536

What XGMAC IP-core version are you talking about? The DW XGMAC
IP-core databooks I have define the upper limit much lesser than that.

Do you understand that specifying 65K descriptors will cause a huge
amount of memory consumed, right? Each descriptor is equipped with at
least 1-page buffer. If QoS/XGMAC SPH is enabled then each descriptor
is equipped with a second buffer. So 65K-descriptor will cause
allocation of at least 65536 * (4 * 4) bytes + 65536 * PAGE_SIZE
bytes. So it's ~256MB for the smallest possible 4K-pages. Not to
mention that there can be more than one queue, two buffers assigned to
each descriptor and more than a single page allocated for each buffer
in case of jumbos. All of that will multiply the basic ~256MB memory
consumption.

Taking all of the above into account, what is the practical reason of
having so many descriptors allocated? Are you afraid your CPU won't
keep up with some heavy incoming traffic?

Just a note about GMACs. The only GMAC having the ring-length limited
is DW QoS Eth (v4.x/v5.x). It may have up to 1K descriptors in the
ring. DW GMAC v3.73a doesn't have the descriptors array length constraint.
The last descriptor is marked by a special flag TDESC0.21 and
RDESC1.15, after meeting which the DMA-engine gets back to the first
descriptor in the ring.

-Serge(y)

> 
> Signed-off-by: Furong Xu <0x1207@...il.com>
> ---
>  .../net/ethernet/stmicro/stmmac/dwxgmac2.h    |  2 ++
>  .../ethernet/stmicro/stmmac/stmmac_ethtool.c  | 24 +++++++++++++++----
>  2 files changed, 22 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h b/drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h
> index 6a2c7d22df1e..264f4f876c74 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h
> +++ b/drivers/net/ethernet/stmicro/stmmac/dwxgmac2.h
> @@ -11,6 +11,8 @@
>  
>  /* Misc */
>  #define XGMAC_JUMBO_LEN			16368
> +#define XGMAC_DMA_MAX_TX_SIZE		65536
> +#define XGMAC_DMA_MAX_RX_SIZE		65536
>  
>  /* MAC Registers */
>  #define XGMAC_TX_CONFIG			0x00000000
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c
> index 18468c0228f0..3ae465c5a712 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c
> @@ -491,9 +491,16 @@ static void stmmac_get_ringparam(struct net_device *netdev,
>  				 struct netlink_ext_ack *extack)
>  {
>  	struct stmmac_priv *priv = netdev_priv(netdev);

> +	u32 dma_max_rx_size = DMA_MAX_RX_SIZE;
> +	u32 dma_max_tx_size = DMA_MAX_TX_SIZE;
>  
> -	ring->rx_max_pending = DMA_MAX_RX_SIZE;
> -	ring->tx_max_pending = DMA_MAX_TX_SIZE;
> +	if (priv->plat->has_xgmac) {
> +		dma_max_rx_size = XGMAC_DMA_MAX_RX_SIZE;
> +		dma_max_tx_size = XGMAC_DMA_MAX_TX_SIZE;
> +	}
> +
> +	ring->rx_max_pending = dma_max_rx_size;
> +	ring->tx_max_pending = dma_max_tx_size;

Do you understand the consequence of this change, right?
De

>  	ring->rx_pending = priv->dma_conf.dma_rx_size;
>  	ring->tx_pending = priv->dma_conf.dma_tx_size;
>  }
> @@ -503,12 +510,21 @@ static int stmmac_set_ringparam(struct net_device *netdev,
>  				struct kernel_ethtool_ringparam *kernel_ring,
>  				struct netlink_ext_ack *extack)
>  {
> +	struct stmmac_priv *priv = netdev_priv(netdev);
> +	u32 dma_max_rx_size = DMA_MAX_RX_SIZE;
> +	u32 dma_max_tx_size = DMA_MAX_TX_SIZE;
> +
> +	if (priv->plat->has_xgmac) {
> +		dma_max_rx_size = XGMAC_DMA_MAX_RX_SIZE;
> +		dma_max_tx_size = XGMAC_DMA_MAX_TX_SIZE;
> +	}
> +
>  	if (ring->rx_mini_pending || ring->rx_jumbo_pending ||
>  	    ring->rx_pending < DMA_MIN_RX_SIZE ||
> -	    ring->rx_pending > DMA_MAX_RX_SIZE ||
> +	    ring->rx_pending > dma_max_rx_size ||
>  	    !is_power_of_2(ring->rx_pending) ||
>  	    ring->tx_pending < DMA_MIN_TX_SIZE ||
> -	    ring->tx_pending > DMA_MAX_TX_SIZE ||
> +	    ring->tx_pending > dma_max_tx_size ||
>  	    !is_power_of_2(ring->tx_pending))
>  		return -EINVAL;
>  
> -- 
> 2.34.1
> 
>