Re: [PATCH net-next v3 4/4] net: ocelot: add FDMA support

[Vladimir Oltean]

On Wed, Dec 01, 2021 at 10:29:26AM +0100, Clément Léger wrote:
> Le Mon, 29 Nov 2021 17:40:39 +0000,
> Vladimir Oltean <vladimir.oltean@xxxxxxx> a écrit :
>
> > On Mon, Nov 29, 2021 at 09:19:02AM +0100, Clément Léger wrote:
> > > > I'm not sure why you're letting the hardware grind to a halt first,
> > > > before refilling? I think since the CPU is the bottleneck anyway, you
> > > > can stop the extraction channel at any time you want to refill.
> > > > A constant stream of less data might be better than a bursty one.
> > > > Or maybe I'm misunderstanding some of the details of the hardware.
> > >
> > > Indeed, I can stop the extraction channel but that does not seems a
> > > good idea to stop the channel in a steady state. At least that's what I
> > > thought since it will make the receive "window" non predictable. Not
> > > sure how well it will play with various protocol but I will try
> > > implementing the refill we talked previously (ie when there an
> > > available threshold is reached).
> > (...)
> > > > I don't understand why you restart the injection channel from the TX
> > > > confirmation interrupt. It raised the interrupt to tell you that it hit
> > > > a NULL LLP because there's nothing left to send. If you restart it now and
> > > > no other transmission has happened in the meantime, won't it stop again?
> > >
> > > Actually, it is only restarted if there is some pending packets to
> > > send. With this hardware, packets can't be added while the FDMA is
> > > running and it must be stopped everytime we want to add a packet to the
> > > list. To avoid that, in the TX path, if the FDMA is stopped, we set the
> > > llp of the packet to NULL and start the chan. However, if the FDMA TX
> > > channel is running, we don't stop it, we simply add the next packets to
> > > the ring. However, the FDMA will stop on the previous NULL LLP. So when
> > > we hit a LLP, we might not be at the end of the list. This is why the
> > > next check verifies if we hit a NULL LLP and if there is still some
> > > packet to send.
> >
> > Oh, is that so? That would be pretty odd if the hardware is so dumb that
> > it doesn't detect changes made to an LLP on the go.
> >
> > The manual has this to say, and I'm not sure how to interpret it:
> >
> > | It is possible to update an active channels LLP pointer and pointers in
> > | the DCB chains. Before changing pointers software must schedule the
> > | channel for disabling (by writing FDMA_CH_DISABLE.CH_DISABLE[ch]) and
> > | then wait for the channel to set FDMA_CH_SAFE.CH_SAFE[ch]. When the
> > | pointer update is complete, soft must re-activate the channel by setting
> > | FDMA_CH_ACTIVATE.CH_ACTIVATE[ch]. Setting activate will cancel the
> > | deactivate-request, or if the channel has disabled itself in the
> > | meantime, it will re activate the channel.
> >
> > So it is possible to update an active channel's LLP pointer, but not
> > while it's active? Thank you very much!
>
> In the manual, this is also stated that:
>
> | The FDMA does not reload the current DCB when re- activated,
> | so if the LLP-field of the current DCB is modified, then software must
> | also modify FDMA_DCB_LLP[ch].
>
> The FDMA present on the next generation (sparx5) is *almost* the same
> but a new RELOAD register has been added and allows adding a DCB at the
> end of the linked list without stopping the FDMA, and then simply hit
> the RELOAD register to restart it if needed. Unfortunately, this is not
> the case for the ocelot one.
>
> >
> > If true, this will severely limit the termination performance one is
> > able to obtain with this switch, even with a faster CPU and PCIe.

Sadly I don't have the time or hardware to dig deeper into this, so I'll
have to trust you, even if it sounds like a severe limitation.

> > > > > +void ocelot_fdma_netdev_init(struct ocelot_fdma *fdma, struct net_device *dev)
> > > > > +{
> > > > > +	dev->needed_headroom = OCELOT_TAG_LEN;
> > > > > +	dev->needed_tailroom = ETH_FCS_LEN;
> > > >
> > > > The needed_headroom is in no way specific to FDMA, right? Why aren't you
> > > > doing it for manual register-based injection too? (in a separate patch ofc)
> > >
> > > Actually, If I switch to page based ring, This won't be useful anymore
> > > because the header will be written directly in the page and not anymore
> > > directly in the skb header.
> >
> > I don't understand this comment. You set up the needed headroom and
> > tailroom netdev variables to avoid reallocation on TX, not for RX.
> > And you use half page buffers for RX, not for TX.
>
> Ok, so indeed, I don't think it is needed for the register-based
> injection since the IFH is computed on the stack and pushed word by
> word into the fifo separately from the skb data. In the case of the
> FDMA, it is read from the start of the DCB DATAL adress so this is why
> this is needed. I could also put the IFH in a separate DCB and then
> split the data in a next DCB using SOF/EOF flags but I'm not sure it
> will be beneficial from a performance point of view. I could try that
> since the CPU is slow, it might be better in some case to let the FDMA
> handle this instead of usign the CPU to increase the SKB size and
> linearize it.
>
> >
> > > > I can't help but think how painful it is that with a CPU as slow as
> > > > yours, insult over injury, you also need to check for each packet
> > > > whether the device tree had defined the "fdma" region or not, because
> > > > you practically keep two traffic I/O implementations due to that sole
> > > > reason. I think for the ocelot switchdev driver, which is strictly for
> > > > MIPS CPUs embedded within the device, it should be fine to introduce a
> > > > static key here (search for static_branch_likely in the kernel).
> > >
> > > I thinked about it *but* did not wanted to add a key since it would be
> > > global. However, we could consider that there is always only one
> > > instance of the driver and indeed a static key is an option.
> > > Unfortunately, I'm not sure this will yield any noticeable performance
> > > improvement.
> >
> > What is the concern with a static key in this driver, exactly?
>
> Only that the static key will be global but this driver does not have
> anything global. If you have no concern about that, I'm ok to add one.

I don't see a downside to this, do you? Even if we get support for a
PCIe ocelot driver later on, we don't know how that is going to look, if
it's going to reuse the exact same xmit function, etc.