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From:   Tim van der Staaij | Zign <Tim.vanderstaaij@zigngroup.com>
To:     Sascha Hauer <s.hauer@pengutronix.de>
CC:     Shawn Guo <shawnguo@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Vinod Koul <vkoul@kernel.org>,
        Pengutronix Kernel Team <kernel@pengutronix.de>,
        "dmaengine@vger.kernel.org" <dmaengine@vger.kernel.org>,
        Fabio Estevam <festevam@gmail.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] dmaengine: imx-sdma: fix deadlock in interrupt handler
Thread-Topic: [PATCH] dmaengine: imx-sdma: fix deadlock in interrupt handler
Thread-Index: AQHZ7HFXCtmZOIdMP0ek3TzK7ZAfK7Amm5EAgASqfACAAXywOw==
Date:   Tue, 26 Sep 2023 07:54:18 +0000
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Hi Sascha,=0A=
=0A=
> I think this is a false positive. The i.MX UART driver makes sure that=0A=
the console UART never uses DMA, so it shouldn't happen that the DMA=0A=
driver issuing console messages calls back into the DMA driver.=0A=
>=0A=
> Could you give the following patch a test?=0A=
=0A=
Thank you for looking into this. I thought I had an idea of what was going =
on=0A=
but it seems that was based on some wrong assumptions (I'm mostly a Linux u=
ser=0A=
and not familiar with kernel code yet).=0A=
=0A=
I tested with your patch and it does indeed fix the issue on my machine. No=
te=0A=
that I have been testing this in a similar way as you did. The log message=
=0A=
which triggers this issue in practice is a rare occurrence on my system=0A=
because of its condition, so I added a dev_warn_once() to=0A=
sdma_update_channel_loop() just outside of the conditional, which fires as=
=0A=
soon as some data is received through DMA. This consistently reproduces the=
=0A=
lockdep warning without your patch, so I'm confident that the patch works.=
=0A=
=0A=
> I inserted a dev_info() into the imx-sdma driver and got said circular=0A=
locking warning. With my patch applied it's gone. Nevertheless I would=0A=
wait for Tims feedback and resend it with some more people on Cc. I=0A=
never used lockdep_set_subclass() and I am not sure if it's legal to=0A=
put it into the UART startup function like I did.=0A=
=0A=
Sounds good, could you submit the patch and keep me in cc?=0A=
=0A=
Thanks,=0A=
Tim=0A=