Date:   Tue, 8 Nov 2022 16:28:52 +0100
From:   Marek Vasut <marex@...x.de>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Erwan LE RAY <erwan.leray@...s.st.com>
Cc:     Alexandre TORGUE <alexandre.torgue@...s.st.com>,
        Ahmad Fatoum <a.fatoum@...gutronix.de>,
        devicetree@...r.kernel.org,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        linux-kernel@...r.kernel.org,
        Marcin Sloniewski <marcin.sloniewski@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Jagan Teki <jagan@...rulasolutions.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org,
        Amelie DELAUNAY <amelie.delaunay@...com>
Subject: Re: [PATCH 00/16] STM32 configure UART nodes for DMA

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.
> 
> 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*

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