netdev - Re: [PATCH v3] Renesas Ethernet AVB driver

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Date:	Wed, 22 Apr 2015 05:04:13 +0000
From:	MITSUHIRO KIMURA <mitsuhiro.kimura.kc@...esas.com>
To:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	"pawel.moll@....com" <pawel.moll@....com>,
	"mark.rutland@....com" <mark.rutland@....com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"galak@...eaurora.org" <galak@...eaurora.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"richardcochran@...il.com" <richardcochran@...il.com>
CC:	"linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
	MASARU NAGAI <masaru.nagai.vx@...esas.com>
Subject: Re: [PATCH v3] Renesas Ethernet AVB driver

Hello Sergei.

 (2015/04/15 6:37:28), Sergei Shtylyov wrote:
> >> +		if (!ravb_tx_free(ndev, q)) {
> >> +			netif_warn(priv, tx_queued, ndev, "TX FD exhausted.\n");
> >> +			netif_stop_queue(ndev);
> >> +			spin_unlock_irqrestore(&priv->lock, flags);
> >> +			return NETDEV_TX_BUSY;
> >> +		}
> >> +	}
> >> +	entry = priv->cur_tx[q] % priv->num_tx_ring[q];
> >> +	priv->cur_tx[q]++;
> >> +	spin_unlock_irqrestore(&priv->lock, flags);
> >> +
> >> +	if (skb_put_padto(skb, ETH_ZLEN))
> >> +		return NETDEV_TX_OK;
> >> +
> >> +	priv->tx_skb[q][entry] = skb;
> >> +	buffer = PTR_ALIGN(priv->tx_buffers[q][entry], RAVB_ALIGN);
> >> +	memcpy(buffer, skb->data, skb->len);
> 
> > ~1500 bytes memcpy(), not good...
> 
>     I'm looking in the manual and not finding the hard requirement to have the
> buffer address aligned to 128 bytes (RAVB_ALIGN), sigh... Kimura-san?

There are the hardware requirement that the frame data must be aligned with
a 32-bit boundary in the URAM, see section 45A.3.3.1 Data Representation
in the manual.
I think that the original skb->data is almost aligned with 2 bytes boundary
by NET_IP_ALING, so we copied original skb->data to the local aligned buffer.

In addition, see section 45A.3.3.12 Tips for Optimizing Performance in Handling
Descriptors, it mentioned that frame data is accessed in blocks up to 128 bytes
and the number of 128 byte borders (addresses H'xxx00 and H'xxx80) and frame data
inside should be minimized.
So we set RAVB_ALIGN to 128 bytes.

Best Regards,
Mitsuhiro Kimura