linux-kernel - Re: Question regarding RT patches for ARM

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Date:   Wed, 17 May 2017 18:02:13 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Eric Nelson <eric@...int.com>
Cc:     Fawad Lateef <fawadlateef@...il.com>,
        GuJiangfei <qiledexin@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kernelnewbies <kernelnewbies@...nelnewbies.org>,
        linux-rt-users@...r.kernel.org
Subject: Re: Question regarding RT patches for ARM

On 2017-05-17 07:45:39 [-0700], Eric Nelson wrote:
> Hi Fawad,
Hi Fawad,

> On 05/17/2017 06:40 AM, Fawad Lateef wrote:
> > > > I'm not sure about the unhandled page fault, but the stalls may be
> > > > because of the SDMA driver.
> > > > 
> > > > See this patch in the Freescale/NXP community for kernel 4.1:
> > > > https://github.com/Freescale/meta-freescale/blob/master/recipes-kernel/linux/linux-fslc-imx-rt/0003-Work-around-CPU-stalls-in-the-imx-sdma-driver.patch

That is something I see for the first time. The description says that it
leads to this stall but it fails to explain _how_ it got there.

> > > Thanks for the patch link. I will give it a try.
> > 
> > Talked to hardware manufacturer and they applied the patch.
> > 
> > Still that SDMA is only used for NOR flash and only u-boot and its
> > environment are stored in nor flash. So not sure if that is going to
> > help in any case.
> > 
> 
> SDMA is used for much more than NOR flash (SSI, USB, and sometimes
> UARTs).
> 
> > So I would really like few more suggestions OR confirmations about ARM
> > (cortex-A9 based imx6) (preemept)-RT support/patches are stable and
> > can be used for final products.
> > 
> 
> I don't have any experience with the RT patches on 4.9 or later
> kernels, but do know that they're being used on i.MX6 with the
> vendor kernels (3.10.x, 4.1.x).

I just looked into a few boot logs and it seems the Phyflex board (imx6
quad) bootet v4.9.27-rt18, run cyclictest including some tests and
nothing produced this "unhandled page fault" you mention here. This
includes the previous RT version in the v4.9 line.

Of this "page fault" happens on boot, it has to do something without
your .config + hw combination. If it happens at rune-time it is probably
triggered by a driver or an application is triggering this.

> Regards,
> 
> 
> Eric

Sebastian