linux-kernel - Re: [PATCH 04/15] can: m_can: Use transmit event FIFO watermark level interrupt

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Date:   Tue, 13 Dec 2022 18:19:46 +0100
From:   Markus Schneider-Pargmann <msp@...libre.com>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
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

Hi Marc,

On Thu, Dec 01, 2022 at 05:59:53PM +0100, 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.

Just a quick comment here after trying this, I decided against it.
- I can't modify the watermark levels once the chip is active.
- Using interrupt (un)masking I can change the behavior for tx and rx
  with a single register write instead of two to the two fifo
  configuration registers.

You will see this in the second part of the series then.

Best,
Markus