Re: [EXT] Re: [PATCH v4 net-next 10/19] net: mvpp2: add FCA RXQ non occupied descriptor threshold

[Russell King - ARM Linux admin]

On Wed, Jan 27, 2021 at 06:41:32PM +0000, Stefan Chulski wrote:
> 
>  >
> > > From: Stefan Chulski <stefanc@xxxxxxxxxxx>
> > >
> > > RXQ non occupied descriptor threshold would be used by Flow Control
> > > Firmware feature to move to the XOFF mode.
> > > RXQ non occupied threshold would change interrupt cause that polled by
> > > CM3 Firmware.
> > > Actual non occupied interrupt masked and won't trigger interrupt.
> > 
> > Does this mean that this change enables a feature, but it is unused due to a
> > masked interrupt?
> 
> Firmware poll RXQ non occupied cause register to indicate if number of registers bellow threshold.
> We do not trigger any interrupt, just poll this bit in CM3. So this cause always masked.

The functional spec for A8040 says that the register at 0xF2005520
is "RX Exceptions Interrupt Mask" and the bit description talks about
it controlling interrupt signal generation. However, the bit that
allows RX Exceptions to be raised in MVPP2_ISR_RX_TX_MASK_REG is clear,
so it won't proceed beyond the next level up.

So, I think the commit description needs to say something like:

"The firmware needs to monitor the RX Non-occupied descriptor bits for
 flow control to move to XOFF mode. These bits need to be unmasked to
 be functional, but they will not raise interrupts as we leave the
 RX exception summary bit in MVPP2_ISR_RX_TX_MASK_REG clear."

I think that's essentially what you're trying to describe - please
change if not.

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