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Date:   Fri, 16 Mar 2018 11:17:41 -0400 (EDT)
From:   David Miller <davem@...hat.com>
To:     f.fainelli@...il.com
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net] net: systemport: Rewrite __bcm_sysport_tx_reclaim()

From: Florian Fainelli <f.fainelli@...il.com>
Date: Tue, 13 Mar 2018 14:45:07 -0700

> There is no need for complex checking between the last consumed index
> and current consumed index, a simple subtraction will do.
> 
> This also eliminates the possibility of a permanent transmit queue stall
> under the following conditions:
> 
> - one CPU bursts ring->size worth of traffic (up to 256 buffers), to the
>   point where we run out of free descriptors, so we stop the transmit
>   queue at the end of bcm_sysport_xmit()
> 
> - because of our locking, we have the transmit process disable
>   interrupts which means we can be blocking the TX reclamation process
> 
> - when TX reclamation finally runs, we will be computing the difference
>   between ring->c_index (last consumed index by SW) and what the HW
>   reports through its register
> 
> - this register is masked with (ring->size - 1) = 0xff, which will lead
>   to stripping the upper bits of the index (register is 16-bits wide)
> 
> - we will be computing last_tx_cn as 0, which means there is no work to
>   be done, and we never wake-up the transmit queue, leaving it
>   permanently disabled
> 
> A practical example is e.g: ring->c_index aka last_c_index = 12, we
> pushed 256 entries, HW consumer index = 268, we mask it with 0xff = 12,
> so last_tx_cn == 0, nothing happens.
> 
> Fixes: 80105befdb4b ("net: systemport: add Broadcom SYSTEMPORT Ethernet MAC driver")
> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>

Indeed if you mask the indexes too early this can happen.

Applied and queued up for -stable, thanks Florian.

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