Date:   Fri, 2 Dec 2022 10:23:06 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Markus Schneider-Pargmann <msp@...libre.com>
Cc:     Chandrasekar Ramakrishnan <rcsekar@...sung.com>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        linux-can@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/15] can: m_can: Use transmit event FIFO watermark
 level interrupt

On 01.12.2022 17:59:51, Markus Schneider-Pargmann wrote:
> On Thu, Dec 01, 2022 at 12:00:33PM +0100, Marc Kleine-Budde wrote:
> > On 01.12.2022 11:12:20, Markus Schneider-Pargmann wrote:
> > > > > For the upcoming receive side patch I already added a hrtimer. I may try
> > > > > to use the same timer for both directions as it is going to do the exact
> > > > > same thing in both cases (call the interrupt routine). Of course that
> > > > > depends on the details of the coalescing support. Any objections on
> > > > > that?
> > > > 
> > > > For the mcp251xfd I implemented the RX and TX coalescing independent of
> > > > each other and made it configurable via ethtool's IRQ coalescing
> > > > options.
> > > > 
> > > > The hardware doesn't support any timeouts and only FIFO not empty, FIFO
> > > > half full and FIFO full IRQs and the on chip RAM for mailboxes is rather
> > > > limited. I think the mcan core has the same limitations.
> > > 
> > > Yes and no, the mcan core provides watermark levels so it has more
> > > options, but there is no hardware timer as well (at least I didn't see
> > > anything usable).
> > 
> > Are there any limitations to the water mark level?
> 
> Anything specific? I can't really see any limitation. You can set the
> watermark between 1 and 32. I guess we could also always use it instead
> of the new-element interrupt, but I haven't tried that yet. That may
> simplify the code.

Makes sense.

> > > > The configuration for the mcp251xfd looks like this:
> > > > 
> > > > - First decide for classical CAN or CAN-FD mode
> > > > - configure RX and TX ring size
> > > >   9263c2e92be9 ("can: mcp251xfd: ring: add support for runtime configurable RX/TX ring parameters")
> > > >   For TX only a single FIFO is used.
> > > >   For RX up to 3 FIFOs (up to a depth of 32 each).
> > > >   FIFO depth is limited to power of 2.
> > > >   On the mcan cores this is currently done with a DT property.
> > > >   Runtime configurable ring size is optional but gives more flexibility
> > > >   for our use-cases due to limited RAM size.
> > > > - configure RX and TX coalescing via ethtools
> > > >   Set a timeout and the max CAN frames to coalesce.
> > > >   The max frames are limited to half or full FIFO.
> > > 
> > > mcan can offer more options for the max frames limit fortunately.
> > > 
> > > > 
> > > > How does coalescing work?
> > > > 
> > > > If coalescing is activated during reading of the RX'ed frames the FIFO
> > > > not empty IRQ is disabled (the half or full IRQ stays enabled). After
> > > > handling the RX'ed frames a hrtimer is started. In the hrtimer's
> > > > functions the FIFO not empty IRQ is enabled again.
> > > 
> > > My rx path patches are working similarly though not 100% the same. I
> > > will adopt everything and add it to the next version of this series.
> > > 
> > > > 
> > > > I decided not to call the IRQ handler from the hrtimer to avoid
> > > > concurrency, but enable the FIFO not empty IRQ.
> > > 
> > > mcan uses a threaded irq and I found this nice helper function I am
> > > currently using for the receive path.
> > > 	irq_wake_thread()
> > > 
> > > It is not widely used so I hope this is fine. But this hopefully avoids
> > > the concurrency issue. Also I don't need to artificially create an IRQ
> > > as you do.
> > 
> > I think it's Ok to use the function. Which IRQs are enabled after you
> > leave the RX handler? The mcp251xfd driver enables only a high watermark
> > IRQ and sets up the hrtimer. Then we have 3 scenarios:
> > - high watermark IRQ triggers -> IRQ is handled,
> > - FIFO level between 0 and high water mark -> no IRQ triggered, but
> >   hrtimer will run, irq_wake_thread() is called, IRQ is handled
> > - FIFO level 0 -> no IRQ triggered, hrtimer will run. What do you do in
> >   the IRQ handler? Check if FIFO is empty and enable the FIFO not empty
> >   IRQ?
> 
> I am currently doing the normal IRQ handler run. It checks the
> "Interrupt Register" at the beginning. This register does not show the
> interrupts that fired, it shows the status. So even though the watermark
> interrupt didn't trigger when called by a timer, RF0N 'new message'
> status bit is still set if there is something new in the FIFO.

That covers scenario 2 from above.

> Of course it is the same for the transmit status bits.

ACK - The TX complete event handling is a 95% copy/paste of the RX
handling.

> So there is no need to read the FIFO fill levels directly, just the
> general status register.

What do you do if the hrtimer fires and there's no CAN frame waiting in
the FIFO?

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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