Re: [PATCH v1 0/4] can: ctucanfd: clenup acoording to the actual rules and documentation linking

[Marc Kleine-Budde]

On 03.05.2022 16:32:32, Andrew Dennison wrote:
> > > When value is configurable then for (uncommon) number
> > > of buffers which is not power of two, there will be likely
> > > a problem with way how buffers queue is implemented
> >
> 
> Only power of 2 makes sense to me: I didn't consider those corner
> cases but the driver could just round down to the next power of 2 and
> warn about a misconfiguration of the IP core.

+1

> I added the dynamic detection because the IP core default had changed
> to 2 TX buffers and this broke some hard coded assumptions in the
> driver in a rather obscure way that had me debugging for a bit...

The mainline driver uses a hard coded default of 4 still... Can you
provide that patch soonish?

> > You can make use of more TX buffers, if you implement (fully
> > hardware based) TX IRQ coalescing (== handle more than one TX
> > complete interrupt at a time) like in the mcp251xfd driver, or BQL
> > support (== send more than one TX CAN frame at a time). I've played
> > a bit with BQL support on the mcp251xfd driver (which is attached by
> > SPI), but with mixed results. Probably an issue with proper
> > configuration.
> 
> Reducing CAN IRQ load would be good.

IRQ coalescing comes at the price of increased latency, but if you have
a timeout in hardware you can configure the latencies precisely.

> > > We need 2 * priv->ntxbufs range to distinguish empty and full
> > > queue... But modulo is not nice either so I probably come with
> > > some other solution in a longer term. In the long term, I want to
> > > implement virtual queues to allow multiqueue to use dynamic Tx
> > > priority of up to 8 the buffers...
> >
> > ACK, multiqueue TX support would be nice for things like the
> > Earliest TX Time First scheduler (ETF). 1 TX queue for ETF, the
> > other for bulk messages.
> 
> Would be nice, I have multi-queue in the CAN layer I wrote for a
> little RTOS (predates socketcan) and have used for a while.

Out of interest:
What are the use cases? How did you decide which queue to use?

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
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