Date:   Tue, 19 May 2020 12:05:20 +0200
From:   Matteo Croce <mcroce@...hat.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Antoine Tenart <antoine.tenart@...tlin.com>,
        netdev <netdev@...r.kernel.org>,
        "gregory.clement@...tlin.com" <gregory.clement@...tlin.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Maxime Chevallier <maxime.chevallier@...tlin.com>,
        Nadav Haklai <nadavh@...vell.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        "miquel.raynal@...tlin.com" <miquel.raynal@...tlin.com>,
        Stefan Chulski <stefanc@...vell.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        "David S . Miller" <davem@...emloft.net>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [EXT] Re: [PATCH net-next 3/5] net: mvpp2: cls: Use RSS contexts
 to handle RSS tables

On Tue, May 19, 2020 at 11:54 AM Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> On Sat, May 09, 2020 at 09:20:50PM +0100, Russell King - ARM Linux admin wrote:
> > On Sat, May 09, 2020 at 08:52:46PM +0100, Russell King - ARM Linux admin wrote:
> > > It is highly likely that 895586d5dc32 is responsible for this breakage.
> > > I've been investigating this afternoon, and what I've found, comparing
> > > a kernel without 895586d5dc32 and with 895586d5dc32 applied is:
> > >
> > > - The table programmed into the hardware via mvpp22_rss_fill_table()
> > >   appears to be identical with or without the commit.
> > >
> > > - When rxhash is enabled on eth2, mvpp2_rss_port_c2_enable() reports
> > >   that c2.attr[0] and c2.attr[2] are written back containing:
> > >
> > >    - with 895586d5dc32, failing:    00200000 40000000
> > >    - without 895586d5dc32, working: 04000000 40000000
> > >
> > > - When disabling rxhash, c2.attr[0] and c2.attr[2] are written back as:
> > >
> > >    04000000 00000000
> > >
> > > The second value represents the MVPP22_CLS_C2_ATTR2_RSS_EN bit, the
> > > first value is the queue number, which comprises two fields.  The high
> > > 5 bits are 24:29 and the low three are 21:23 inclusive.  This comes
> > > from:
> > >
> > >        c2.attr[0] = MVPP22_CLS_C2_ATTR0_QHIGH(qh) |
> > >                      MVPP22_CLS_C2_ATTR0_QLOW(ql);
> > > #define     MVPP22_CLS_C2_ATTR0_QHIGH(qh)       (((qh) & 0x1f) << 24)
> > > #define     MVPP22_CLS_C2_ATTR0_QLOW(ql)        (((ql) & 0x7) << 21)
> > >
> > > So, the working case gives eth2 a queue id of 4.0, or 32 as per
> > > port->first_rxq, and the non-working case a queue id of 0.1, or 1.
> > >
> > > The allocation of queue IDs seems to be in mvpp2_port_probe():
> > >
> > >         if (priv->hw_version == MVPP21)
> > >                 port->first_rxq = port->id * port->nrxqs;
> > >         else
> > >                 port->first_rxq = port->id * priv->max_port_rxqs;
> > >
> > > Where:
> > >
> > >         if (priv->hw_version == MVPP21)
> > >                 priv->max_port_rxqs = 8;
> > >         else
> > >                 priv->max_port_rxqs = 32;
> > >
> > > Making the port 0 (eth0 / eth1) have port->first_rxq = 0, and port 1
> > > (eth2) be 32.  It seems the idea is that the first 32 queues belong to
> > > port 0, the second 32 queues belong to port 1, etc.
> > >
> > > mvpp2_rss_port_c2_enable() gets the queue number from it's parameter,
> > > 'ctx', which comes from mvpp22_rss_ctx(port, 0).  This returns
> > > port->rss_ctx[0].
> > >
> > > mvpp22_rss_context_create() is responsible for allocating that, which
> > > it does by looking for an unallocated priv->rss_tables[] pointer.  This
> > > table is shared amongst all ports on the CP silicon.
> > >
> > > When we write the tables in mvpp22_rss_fill_table(), the RSS table
> > > entry is defined by:
> > >
> > >             u32 sel = MVPP22_RSS_INDEX_TABLE(rss_ctx) |
> > >                           MVPP22_RSS_INDEX_TABLE_ENTRY(i);
> > >
> > > where rss_ctx is the context ID (queue number) and i is the index in
> > > the table.
> > >
> > > #define     MVPP22_RSS_INDEX_TABLE_ENTRY(idx)   (idx)
> > > #define     MVPP22_RSS_INDEX_TABLE(idx)         ((idx) << 8)
> > > #define     MVPP22_RSS_INDEX_QUEUE(idx)         ((idx) << 16)
> > >
> > > If we look at what is written:
> > >
> > > - The first table to be written has "sel" values of 00000000..0000001f,
> > >   containing values 0..3. This appears to be for eth1.  This is table 0,
> > >   RX queue number 0.
> > > - The second table has "sel" values of 00000100..0000011f, and appears
> > >   to be for eth2.  These contain values 0x20..0x23.  This is table 1,
> > >   RX queue number 0.
> > > - The third table has "sel" values of 00000200..0000021f, and appears
> > >   to be for eth3.  These contain values 0x40..0x43.  This is table 2,
> > >   RX queue number 0.
> > >
> > > Okay, so how do queue numbers translate to the RSS table?  There is
> > > another table - the RXQ2RSS table, indexed by the MVPP22_RSS_INDEX_QUEUE
> > > field of MVPP22_RSS_INDEX and accessed through the MVPP22_RXQ2RSS_TABLE
> > > register.  Before 895586d5dc32, it was:
> > >
> > >        mvpp2_write(priv, MVPP22_RSS_INDEX,
> > >                    MVPP22_RSS_INDEX_QUEUE(port->first_rxq));
> > >        mvpp2_write(priv, MVPP22_RXQ2RSS_TABLE,
> > >                    MVPP22_RSS_TABLE_POINTER(port->id));
> > >
> > > and after:
> > >
> > >        mvpp2_write(priv, MVPP22_RSS_INDEX, MVPP22_RSS_INDEX_QUEUE(ctx));
> > >        mvpp2_write(priv, MVPP22_RXQ2RSS_TABLE, MVPP22_RSS_TABLE_POINTER(ctx));
> > >
> > > So, before the commit, for eth2, that would've contained '32' for the
> > > index and '1' for the table pointer - mapping queue 32 to table 1.
> > > Remember that this is queue-high.queue-low of 4.0.
> > >
> > > After the commit, we appear to map queue 1 to table 1.  That again
> > > looks fine on the face of it.
> > >
> > > Section 9.3.1 of the A8040 manual seems indicate the reason that the
> > > queue number is separated.  queue-low seems to always come from the
> > > classifier, whereas queue-high can be from the ingress physical port
> > > number or the classifier depending on the MVPP2_CLS_SWFWD_PCTRL_REG.
> > >
> > > We set the port bit in MVPP2_CLS_SWFWD_PCTRL_REG, meaning that queue-high
> > > comes from the MVPP2_CLS_SWFWD_P2HQ_REG() register... and this seems to
> > > be where our bug comes from.
> > >
> > > mvpp2_cls_oversize_rxq_set() sets this up as:
> > >
> > >         mvpp2_write(port->priv, MVPP2_CLS_SWFWD_P2HQ_REG(port->id),
> > >                     (port->first_rxq >> MVPP2_CLS_OVERSIZE_RXQ_LOW_BITS));
> > >
> > >         val = mvpp2_read(port->priv, MVPP2_CLS_SWFWD_PCTRL_REG);
> > >         val |= MVPP2_CLS_SWFWD_PCTRL_MASK(port->id);
> > >         mvpp2_write(port->priv, MVPP2_CLS_SWFWD_PCTRL_REG, val);
> > >
> > > so, the queue-high for eth2 is _always_ 4, meaning that only queues
> > > 32 through 39 inclusive are available to eth2.  Yet, we're trying to
> > > tell the classifier to set queue-high, which will be ignored, to zero.
> > >
> > > So we end up directing traffic from eth2 not to queue 1, but to queue
> > > 33, and then we tell it to look up queue 33 in the RSS table.  However,
> > > RSS table has not been programmed for queue 33, and so it ends up
> > > (presumably) dropping the packets.
> > >
> > > It seems that mvpp22_rss_context_create() doesn't take account of the
> > > fact that the upper 5 bits of the queue ID can't actually be changed
> > > due to the settings in mvpp2_cls_oversize_rxq_set(), _or_ it seems
> > > that mvpp2_cls_oversize_rxq_set() has been missed in this commit.
> > > Either way, these two functions mutually disagree with what queue
> > > number should be used.
> > >
> > > So, 895586d5dc32 is indeed the cause of this problem.
> >
> > Looking deeper into what mvpp2_cls_oversize_rxq_set() and the MTU
> > validation is doing, it seems that MVPP2_CLS_SWFWD_P2HQ_REG() is
> > used for at least a couple of things.
> >
> > So, with the classifier having had RSS enabled and directing eth2
> > traffic to queue 1, we can not ignore the fact that we may have
> > packets appearing on queue 32 for this port.
> >
> > One of the things that queue 32 will be used for is if an over-sized
> > packet attempts to egress through eth2 - it seems that the A8040 has
> > the ability to forward frames between its ports.  However, afaik we
> > don't support that feature, and the kernel restricts the packet size,
> > so we should never violate the MTU validator and end up with such a
> > packet.  In any case, _if_ we were to attempt to transmit an oversized
> > packet, we have no support in the kernel to deal with that appearing
> > in the port's receive queue.
> >
> > Maybe it would be safe to clear the MVPP2_CLS_SWFWD_PCTRL_MASK() bit?
> >
> > My testing seems to confirm my findings above - clearing this bit
> > means that if I enable rxhash on eth2, the interface can then pass
> > traffic, as we are now directing traffic to RX queue 1 rather than
> > queue 33.  Traffic still seems to work with rxhash off as well.
> >
> > So, I think it's clear where the problem lies, but not what the correct
> > solution is; someone with more experience of packet classifiers (this
> > one?) needs to look at this - this is my first venture into these
> > things, and certainly the first time I've traced through how this is
> > trying to work (or not)...
>
> This is what I was using here to work around the problem, and what I
> mentioned above.
>
> diff --git a/drivers/net/ethernet/marvell/mvpp2/mvpp2_cls.c b/drivers/net/ethernet/marvell/mvpp2/mvpp2_cls.c
> index fd221d88811e..0dd3b65822dd 100644
> --- a/drivers/net/ethernet/marvell/mvpp2/mvpp2_cls.c
> +++ b/drivers/net/ethernet/marvell/mvpp2/mvpp2_cls.c
> @@ -1058,7 +1058,7 @@ void mvpp2_cls_oversize_rxq_set(struct mvpp2_port *port)
>                     (port->first_rxq >> MVPP2_CLS_OVERSIZE_RXQ_LOW_BITS));
>
>         val = mvpp2_read(port->priv, MVPP2_CLS_SWFWD_PCTRL_REG);
> -       val |= MVPP2_CLS_SWFWD_PCTRL_MASK(port->id);
> +       val &= ~MVPP2_CLS_SWFWD_PCTRL_MASK(port->id);
>         mvpp2_write(port->priv, MVPP2_CLS_SWFWD_PCTRL_REG, val);
>  }
>
>

Hi,

I will try this change and let you know if it works.

Thanks

-- 
Matteo Croce
per aspera ad upstream

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