netdev - Re: [PATCH v5 1/2] net: moxa: clear TX descriptor length bits

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Date:	Wed, 20 Aug 2014 10:10:10 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jonas Jensen <jonas.jensen@...il.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	f.fainelli@...il.com
Subject: Re: [PATCH v5 1/2] net: moxa: clear TX descriptor length bits

On Wed, 2014-08-20 at 16:18 +0200, Jonas Jensen wrote:
> TX buffer length is not cleared on ndo_start_xmit().
> Failing to do so can bug/hang the controller and
> cause TX interrupts to stop altogether.

> 
> diff --git a/drivers/net/ethernet/moxa/moxart_ether.c b/drivers/net/ethernet/moxa/moxart_ether.c
> index 5020fd4..aa45607 100644
> --- a/drivers/net/ethernet/moxa/moxart_ether.c
> +++ b/drivers/net/ethernet/moxa/moxart_ether.c
> @@ -348,7 +348,8 @@ static int moxart_mac_start_xmit(struct sk_buff *skb, struct net_device *ndev)
>  
>  	txdes1 = readl(desc + TX_REG_OFFSET_DESC1);
>  	txdes1 |= TX_DESC1_LTS | TX_DESC1_FTS;
> -	txdes1 &= ~(TX_DESC1_FIFO_COMPLETE | TX_DESC1_INTR_COMPLETE);
> +	txdes1 &= ~(TX_DESC1_FIFO_COMPLETE | TX_DESC1_INTR_COMPLETE |
> +		    TX_DESC1_BUF_SIZE_MASK);
>  	txdes1 |= (len & TX_DESC1_BUF_SIZE_MASK);
>  	writel(txdes1, desc + TX_REG_OFFSET_DESC1);
>  	writel(TX_DESC0_DMA_OWN, desc + TX_REG_OFFSET_DESC0);

Wouldnt it be faster to not use readl() at all here ?

readl() is only used to get TX_DESC1_END bit.

It seems you could replace the thing by :

txdes1 = TX_DESC1_LTS | TX_DESC1_FTS | (len & TX_DESC1_BUF_SIZE_MASK);
if (tx_head == TX_DESC_NUM_MASK)
	txdes1 |= TX_DESC1_END;



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