netdev - Re: [PATCH net-next v9 2/2] net: ethernet: Add driver for Sunplus SP7021

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Date:   Wed, 27 Apr 2022 00:52:34 +0200
From:   Francois Romieu <romieu@...zoreil.com>
To:     Wells Lu <wellslutw@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, robh+dt@...nel.org,
        netdev@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, p.zabel@...gutronix.de,
        pabeni@...hat.com, krzk+dt@...nel.org, roopa@...dia.com,
        andrew@...n.ch, edumazet@...gle.com, wells.lu@...plus.com
Subject: Re: [PATCH net-next v9 2/2] net: ethernet: Add driver for Sunplus
 SP7021

Wells Lu <wellslutw@...il.com> :
[...]
> +int spl2sw_rx_poll(struct napi_struct *napi, int budget)
> +{
[...]
> +	wmb();	/* make sure settings are effective. */
> +	mask = readl(comm->l2sw_reg_base + L2SW_SW_INT_MASK_0);
> +	mask &= ~MAC_INT_RX;
> +	writel(mask, comm->l2sw_reg_base + L2SW_SW_INT_MASK_0);
> +
> +	napi_complete(napi);
> +	return 0;
> +}
> +
> +int spl2sw_tx_poll(struct napi_struct *napi, int budget)
> +{
[...]
> +	wmb();			/* make sure settings are effective. */
> +	mask = readl(comm->l2sw_reg_base + L2SW_SW_INT_MASK_0);
> +	mask &= ~MAC_INT_TX;
> +	writel(mask, comm->l2sw_reg_base + L2SW_SW_INT_MASK_0);
> +
> +	napi_complete(napi);
> +	return 0;
> +}
> +
> +irqreturn_t spl2sw_ethernet_interrupt(int irq, void *dev_id)
> +{
[...]
> +	if (status & MAC_INT_RX) {
> +		/* Disable RX interrupts. */
> +		mask = readl(comm->l2sw_reg_base + L2SW_SW_INT_MASK_0);
> +		mask |= MAC_INT_RX;
> +		writel(mask, comm->l2sw_reg_base + L2SW_SW_INT_MASK_0);
[...]
> +		napi_schedule(&comm->rx_napi);
> +	}
> +
> +	if (status & MAC_INT_TX) {
> +		/* Disable TX interrupts. */
> +		mask = readl(comm->l2sw_reg_base + L2SW_SW_INT_MASK_0);
> +		mask |= MAC_INT_TX;
> +		writel(mask, comm->l2sw_reg_base + L2SW_SW_INT_MASK_0);
> +
> +		if (unlikely(status & MAC_INT_TX_DES_ERR)) {
[...]
> +		} else {
> +			napi_schedule(&comm->tx_napi);
> +		}
> +	}

The readl/writel sequence in rx_poll (or tx_poll) races with the irq
handler performing MAC_INT_TX (or MAC_INT_RX) work. If the readl
returns the same value to both callers, one of the writel will be
overwritten.

-- 
Ueimor