RE: [PATCH] can: flexcan: fix timeout when set small bitrate

[Joakim Zhang]

Hi Marc,

> -----Original Message-----
> From: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx>
> Sent: 2019å1æ31æ 17:12
> To: Joakim Zhang <qiangqing.zhang@xxxxxxx>; linux-can@xxxxxxxxxxxxxxx
> Cc: wg@xxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; dl-linux-imx <linux-imx@xxxxxxx>; Aisheng
> Dong <aisheng.dong@xxxxxxx>
> Subject: Re: [PATCH] can: flexcan: fix timeout when set small bitrate
> 
> On 1/31/19 9:48 AM, Joakim Zhang wrote:
> >> Which SoC are you using? Which clock rate has the flexcan IP core?
> >
> >   We tested on i.MX6 series boards and all met this issue. And ipg clock rate
> is 66MHZ, per clock rate is 30MHZ.
> 
> ok
> 
> >
> >>> It is caused by calling of flexcan_chip_unfreeze() timeout.
> >>>
> >>> Originally the code is using usleep_range(10, 20) for unfreeze
> >>> operation, but the patch (8badd65 can: flexcan: avoid calling
> >>> usleep_range from interrupt context) changed it into udelay(10)
> >>> which is only a half delay of before, there're also some other delay
> changes.
> >>>
> >>> After only changed unfreeze delay back to udelay(20), the issue is gone.
> >>> So other timeout values are kept the same as 8badd65 changed.
> >>
> >> Can you change FLEXCAN_TIMEOUT_US instead?
> >
> >   Of course, we can change FLEXCAN_TIMEOUT_US to 100, but this will
> extend the time of enable/disable/softreset.
> > Which method do you think is better?
> 
> If you double to FLEXCAN_TIMEOUT_US to 100, the loops in question will spin
> at maximum the double time. But the loops are left as soon as the condition is
> satisfied.
> 
> It will fix your problem with the 10 kbit/s bitrate. But if there is some kind of
> problem with the IP core it will still fail, it just takes double amount of time
> (100 Âs + overhead) until the function returns.
> 
> I don't see any harm in looping longer:
> - The previous good case is unchanged.
> - The error case takes double amount of time.
> - Your problem is hopefully fixed.

Thanks for your explanation, I will cook a patch then resend.

Best Regards,
Joakim Zhang
> Marc
> 
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