linux-kernel - Re: [PATCH v2 RFC net-next 09/18] net: mvpp2: add FCA RXQ non occupied descriptor threshold

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Date:   Sun, 24 Jan 2021 13:01:36 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     stefanc@...vell.com
Cc:     netdev@...r.kernel.org, thomas.petazzoni@...tlin.com,
        davem@...emloft.net, nadavh@...vell.com, ymarkman@...vell.com,
        linux-kernel@...r.kernel.org, kuba@...nel.org, mw@...ihalf.com,
        andrew@...n.ch, atenart@...nel.org
Subject: Re: [PATCH v2 RFC net-next 09/18] net: mvpp2: add FCA RXQ non
 occupied descriptor threshold

On Sun, Jan 24, 2021 at 01:43:58PM +0200, stefanc@...vell.com wrote:
> diff --git a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
> index 9d69752..0f5069f 100644
> --- a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
> +++ b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
> @@ -1154,6 +1154,9 @@ static void mvpp2_interrupts_mask(void *arg)
>  	mvpp2_thread_write(port->priv,
>  			   mvpp2_cpu_to_thread(port->priv, smp_processor_id()),
>  			   MVPP2_ISR_RX_TX_MASK_REG(port->id), 0);
> +	mvpp2_thread_write(port->priv,
> +			   mvpp2_cpu_to_thread(port->priv, smp_processor_id()),
> +			   MVPP2_ISR_RX_ERR_CAUSE_REG(port->id), 0);

I wonder if this should be refactored:

	u32 thread = mvpp2_cpu_to_thread(port->priv, smp_processor_id());

	mvpp2_thread_write(port->priv, thread,
			   MVPP2_ISR_RX_TX_MASK_REG(port->id), 0);
	mvpp2_thread_write(port->priv, thread,
			   MVPP2_ISR_RX_ERR_CAUSE_REG(port->id), 0);

to avoid having to recompute mvpp2_cpu_to_thread() for each write?

However, looking deeper...

static void mvpp2_interrupts_mask(void *arg)
{
	struct mvpp2_port *port = arg;
	u32 thread;
	int cpu;

	cpu = smp_processor_id();
	if (cpu > port->priv->nthreads)
		return

	thread = mvpp2_cpu_to_thread(port->priv, cpu);
	...

and I wonder about that condition - "cpu > port->priv->nthreads". If
cpu == port->priv->nthreads, then mvpp2_cpu_to_thread() will return
zero, just like the cpu=0 case. This leads me to suspect that this
comparison off by one.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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