Re: API to support hardware priorities

[Marc Kleine-Budde <mkl@pengutronix.de>]

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On 11/09/2015 02:32 PM, Marc Kleine-Budde wrote:
> Let's move this discussion to the mailing list. I'll translate your
> questions.
> 
> On 11/09/2015 12:36 PM, Benedikt Spranger wrote:
>> C-CAN und D-CAN basierte Hardware (wie auch andere) unterstützt ja das
>> priorisierende Versenden von CAN-Paketen. Gibt es Bestrebungen, dieses
>> Feature in der API abzubilden?
> 
> Several CAN IP cores support prioritisation of CAN frames. Is it planned
> to support this?

There are two buffers before a CAN frame gets send to the wire:
First there is the queueing in the network layer. You can replace the
FIFO of the interface by a multi prio queue and filter your CAN frames
by CAN-ID.

The second buffer might be the in the hardware. For now each driver
ensures that the buffer in configured as a FIFO. Some CAN IP cores
support sending CAN frames based on the CAN-ID (lowest ID first), some
have a priority value, some cores even combine these two.

Several thoughts on this:

IIRC the kernel networking driver API supports several TX queues per
networking device. We might be exploited this to implement a bulk large
FIFO queue and a high prio no just one buffer TX queue per driver.

We can introduce a per device knob to switch a CAN driver from true TX
FIFO mode to CAN-ID based priority TX mode. The driver's TX and
TX-complete routines have to be adopted as they now assume that the CAN
frames are send in true FIFO order. The feature can be toggled via
netlink/canconfig.

>> Im konkreten Fall werden neben Bulk-Daten azyklisch auch zeitkritische
>> CAN-Pakete versendet. Hierbei ist eine FIFO-Abarbeitung störend. Das
>> Mailbox-basierte Design von C-CAN/D-CAN unterstützt ja neben der
>> Verwendung der Mailboxen als FIFO auch die dedizierte Konfiguration auf
>> eine CAN-Id/Maske.
> 
> Bene want to send bulk data and time critical cyclic data. Using a
> hardware FIFO introduces unwanted latencies. He further mentions that
> it's possible to configure the mailboxes to serve a CAN-ID/mask.

Are we talking about RX or TX here?

Do you mean the mailbox feature, where you queue a frame into hardware,
but sending is delayed until a certain CAN-ID is RX'ed?

Marc

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