netdev - Re: [PATCH 0/1] Alternative, restart tx after tx used bit read

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Date:   Tue, 11 Apr 2023 19:07:15 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Ingo Rohloff <ingo.rohloff@...terbach.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Lars-Peter Clausen <lars@...afoo.de>
Cc:     robert.hancock@...ian.com, Nicolas.Ferre@...rochip.com,
        claudiu.beznea@...rochip.com, davem@...emloft.net,
        netdev@...r.kernel.org, tomas.melin@...sala.com
Subject: Re: [PATCH 0/1] Alternative, restart tx after tx used bit read

On Fri,  7 Apr 2023 23:33:48 +0200 Ingo Rohloff wrote:
> Analysis:
> Commit 404cd086f29e867f ("net: macb: Allocate valid memory for TX and RX BD
> prefetch") mentions:
> 
>     GEM version in ZynqMP and most versions greater than r1p07 supports
>     TX and RX BD prefetch. The number of BDs that can be prefetched is a
>     HW configurable parameter. For ZynqMP, this parameter is 4.
> 
> I think what happens is this:
> Example Scenario (SW == linux kernel, HW == cadence ethernet IP).
> 1) SW has written TX descriptors 0..7
> 2) HW is currently transmitting TX descriptor 6.
>    HW has already prefetched TX descriptors 6,7,8,9.
> 3) SW writes TX descriptor 8 (clearing TX_USED)
> 4) SW writes the TSTART bit.
>    HW ignores this, because it is still transmitting.
> 5) HW transmits TX descriptor 7.
> 6) HW reaches descriptor 8; because this descriptor
>    has already been prefetched, HW sees a non-active
>    descriptor (TX_USED set) and stops transmitting.

This sounds broken, any idea if this is how the IP is supposed to work
or it may be an integration issue in Zynq?  The other side of this
question is how expensive the workaround is - a spin lock and two extra
register reads on completion seems like a lot.

Roman, Lars, have you seen Tx stalls on your macb setups?