Re: AW: AW: flexcan missing error state transitions - Marc Kleine-Budde

From: Marc Kleine-Budde <mkl@pengutronix.de>
To: "Jonas Mark (ST-FIR/ENG1)" <Mark.Jonas@de.bosch.com>,
	Andri Yngvason <andri.yngvason@marel.com>,
	"linux-can@vger.kernel.org" <linux-can@vger.kernel.org>,
	"wg@grandegger.com" <wg@grandegger.com>
Cc: "hs@denx.de" <hs@denx.de>,
	"RUAN Tingquan (ST-FIR/ENG1-Zhu)" <Tingquan.Ruan@cn.bosch.com>,
	"ZHU Yi (ST-FIR/ENG1-Zhu)" <Yi.Zhu5@cn.bosch.com>
Subject: Re: AW: AW: flexcan missing error state transitions
Date: Tue, 22 Aug 2017 16:06:36 +0200	[thread overview]
Message-ID: <71679863-5b47-faf2-ca44-27eb1abf16c6@pengutronix.de> (raw)
In-Reply-To: <e54b0db7ec524bcca65e1975aaf13b2b@SI-MBX1030.de.bosch.com>

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On 08/22/2017 03:50 PM, Jonas Mark (ST-FIR/ENG1) wrote:
> Hello Andri,
> 
>>> Which polling interval do you think would be appropriate? It
>>> somehow has to be
>> a reasonable trade-off between latency and CPU usage. That's a good
>> question. I don't think that there is a really good answer to it. 
>> It would be nice to see intermittent state-transitions happening,
>> so I would say as fast as possible without affecting CPU usage to a
>> significant degree. The polling timer should not be a high
>> resolution timer or anything that would preempt an rt task.
>> 
>> This value could also depend on the bitrate as that will affect the
>> rate at which the error-counter can change. E.g. on 125 k rate, you
>> will receive at most one frame per 1 ms. This means that the error
>> state will change at at least 64 ms intervals, so we would have to
>> sample at 32 ms intervals to catch all the transitions. For 1 M
>> this would be around 2 ms.
>> 
>> Note that this is all somewhat speculative as I do not know exactly
>> how fast the error counters are incremented/decremented.
> 
> We agree that there is a sensible lower boundary for the polling
> interval which depends on the bitrate. And there is a upper boundary
> which depends on the application.
> 
> We are currently thinking about whether it would be possible to do
> the polling only in case the controller is actually experiencing
> trouble.

Why not enable bus errors on broken controllers. Disable once a bus
error hits and re-enable them after a configurable timeout with a proper
default. I have a patch somewhere, although it applies only against the
pre rx-offload version of the driver.

Marc

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

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