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Date:   Mon, 21 Nov 2022 09:48:05 +0100
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Subject: Re: [Linux-stm32] [PATCH 00/16] STM32 configure UART nodes for DMA
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To:     Amelie Delaunay <amelie.delaunay@foss.st.com>,
        Marek Vasut <marex@denx.de>,
        =?UTF-8?Q?Uwe_Kleine-K=c3=b6nig?= <u.kleine-koenig@pengutronix.de>,
        Erwan LE RAY <erwan.leray@foss.st.com>
CC:     <devicetree@vger.kernel.org>,
        Amelie DELAUNAY <amelie.delaunay@st.com>,
        Maxime Coquelin <mcoquelin.stm32@gmail.com>,
        <linux-kernel@vger.kernel.org>,
        Marcin Sloniewski <marcin.sloniewski@gmail.com>,
        Rob Herring <robh+dt@kernel.org>,
        Jagan Teki <jagan@amarulasolutions.com>,
        "Pengutronix Kernel Team" <kernel@pengutronix.de>,
        Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>,
        <linux-stm32@st-md-mailman.stormreply.com>,
        <linux-arm-kernel@lists.infradead.org>
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 <28c5e508-6893-8ca3-88cc-487a98871722@foss.st.com>
From:   Valentin CARON <valentin.caron@foss.st.com>
In-Reply-To: <28c5e508-6893-8ca3-88cc-487a98871722@foss.st.com>
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Hi Uwe,

We found the issue, thank you to have reported it.

stm32-usart driver was not tolerant to a probe defer from DMA when the 
earlycon is active.

You can find the patch here:
https://lore.kernel.org/lkml/20221118170602.1057863-1-valentin.caron@foss.st.com/

Valentin

On 11/9/22 14:48, Amelie Delaunay wrote:
> On 11/8/22 16:28, Marek Vasut wrote:
>> On 11/8/22 12:59, Uwe Kleine-König wrote:
>>> On Fri, Feb 04, 2022 at 04:41:55PM +0100, Erwan LE RAY wrote:
>>>> On 2/4/22 2:22 PM, Alexandre TORGUE wrote:
>>>>> Hi Ahmad
>>>>>
>>>>> On 2/3/22 18:25, Ahmad Fatoum wrote:
>>>>>> Hello Erwan,
>>>>>>
>>>>>> On 03.02.22 18:10, Erwan Le Ray wrote:
>>>>>>> Add DMA configuration to UART nodes in stm32mp15x (SOC level) and
>>>>>>> remove it at board level to keep current PIO behavior when needed.
>>>>>>> For stm32-ed1 and stm32-dkx boards, UART4 (console) and UART7
>>>>>>> (no HW flow control pin available) are kept in PIO mode, while 
>>>>>>> USART3
>>>>>>> is now configured in DMA mode.
>>>>>>> UART4 (console UART) has to be kept in irq mode, as DMA support for
>>>>>>> console has been removed from the driver by commit e359b4411c28
>>>>>>> ("serial: stm32: fix threaded interrupt handling").
>>>>>>
>>>>>> Do I understand correctly that your first patch breaks consoles of
>>>>>> most/all boards, because they will briefly use DMA, which is refused
>>>>>> by the stm32-usart driver and then you add a patch for each board
>>>>>> to fix that breakage?
>>>>>
>>>>> We have two solutions and both have pro/drawbacks. The first one 
>>>>> (Erwan
>>>>> ones, can break the boot if the patch is taken "alone". Your 
>>>>> proposition
>>>>> avoids this breakage but deletes a non define property (which is a 
>>>>> bit
>>>>> weird). However I prefer to keep a functional behavior, and keep 
>>>>> Ahmad
>>>>> proposition. Ahmad, just one question, dt-bindings check doesn't
>>>>> complain about it ?
>>>>>
>>>>> Cheers
>>>>> Alex
>>>>>
>>>>>>
>>>>>> Such intermittent breakage makes bisection a hassle. 
>>>>>> /delete-property/
>>>>>> is a no-op when the property doesn't exist, so you could move the 
>>>>>> first
>>>>>> patch to the very end to avoid intermittent breakage.
>>>>>>
>>>>>> I also think that the driver's behavior is a bit harsh. I think 
>>>>>> it would
>>>>>> be better for the UART driver to print a warning and fall back to
>>>>>> PIO for console instead of outright refusing and rendering the 
>>>>>> system
>>>>>> silent. That's not mutually exclusive with your patch series here,
>>>>>> of course.
>>>>>>
>>>>>> Cheers,
>>>>>> Ahmad
>>>>>>
>>>>
>>>> The driver implementation will consider the request to probe the UART
>>>> console in DMA mode as an error (-ENODEV), and will fallback this 
>>>> UART probe
>>>> in irq mode.
>>>
>>>> Whatever the patch ordering, the boot will never be broken. The 
>>>> board dt
>>>> patches aim to get a "proper" implementation, but from functional
>>>> perspective the driver will manage a request to probe an UART 
>>>> console in DMA
>>>> mode as an error and fall it back in irq mode.
>>>
>>> I didn't debug this further yet, but my machine (with an out-of-tree
>>> dts) fails to boot 6.1-rc4 without removing the dma properties from the
>>> console UART. This is a bug isn't it? The same dts created a working
>>> setup with stm32mp157.dtsi from 5.15 + kernel 5.15.
>
> Hi Uwe,
>
> Could you confirm earlycon is enabled on your setup?
>
> Without earlycon, boot is ok, even with dma properties, at least on 
> stm32mp157c-dk2.
>
>>>
>>> I can debug this further, but maybe you know off-hand what the problem
>>> is?
>>
>> +CC Amelie, as this might be related to the DMA series that landed 
>> recently:
>>
>> $ git log --oneline v5.18..v6.0 -- drivers/dma/stm32*
>
> Hi Marek,
>
> We haven't yet investigated the issue, and if latest DMA updates could 
> explain why earlycon breaks the boot.
>
>
> +TO Valentin, as he's now in charge of UART driver.
> Valentin and I will investigate this issue.
>
> Regards,
> Amelie