linux-kernel - Re: [PATCH] spi: dt-bindings: samsung: make dma properties not required

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Message-ID: <CAPLW+4kjXK=EWx__h0bX0rJMrL33E=t4YDzSOfObmvtG9aS+jg@mail.gmail.com>
Date: Sat, 2 Mar 2024 10:23:16 -0600
From: Sam Protsenko <semen.protsenko@...aro.org>
To: Tudor Ambarus <tudor.ambarus@...aro.org>
Cc: Mark Brown <broonie@...nel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, andi.shyti@...nel.org, 
	robh+dt@...nel.org, conor+dt@...nel.org, linux-spi@...r.kernel.org, 
	linux-samsung-soc@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, andre.draszik@...aro.org, 
	peter.griffin@...aro.org, willmcvicker@...gle.com, kernel-team@...roid.com
Subject: Re: [PATCH] spi: dt-bindings: samsung: make dma properties not required

On Sat, Mar 2, 2024 at 3:36 AM Tudor Ambarus <tudor.ambarus@...aro.org> wrote:
>
>
>
> On 01.03.2024 22:42, Mark Brown wrote:
> > On Fri, Mar 01, 2024 at 01:28:35PM -0600, Sam Protsenko wrote:
> >> On Fri, Mar 1, 2024 at 5:55 AM Tudor Ambarus <tudor.ambarus@...aro.org> wrote:
> >
> >>> Since the addition of the driver in 2009, the driver selects between DMA
> >>> and polling mode depending on the transfer length - DMA mode for
> >>> transfers bigger than the FIFO depth, polling mode otherwise. All
> >>> versions of the IP support polling mode, make the dma properties not
> >>> required.
> >
> >> AFAIU, the device tree has nothing to do with drivers, it's about
> >> hardware description. Does making DMA properties not required here
>
> correct
>
> >> mean that there are some HW out there which doesn't integrate DMA in
>
> no, to me it means that the IP can work without DMA, only in PIO mode,
> regardless if DMA is integrated or not. Not required means that the
> property is not mandatory, which is what I'm trying to achieve here.
>
> >> SPI blocks? Even if this change is ok (I'm not sure), the
> >> argumentation doesn't look sound to me.
>
> switching to PIO mode in the driver for sizes smaller than FIFO depths
> in the driver guarantees that all existing compatibles support PIO mode.
>
> Are you saying that if there is a physical line between an IP and DMA
> controller, then the DMA properties must always be specified in dt? I
> thought they can be marked as optional in this case, and that's what I
> did with this patch.
>

No, I would wait for maintainers to clarify on that bit. Change itself
can be ok. But the commit message shouldn't mention the driver,
because the driver uses (depends on) device tree, not vice versa. The
device tree can be used in other projects as well (like U-Boot and
OP-TEE), so it should be designed to be universal and not depend on
kernel drivers. The commit message should be based on particular HW
layout features and how the patch makes the bindings describe that HW
better. It shouldn't rely on driver implementations.

Also, it may be beneficial for reviewers/maintainers if you mention
briefly (either in the commit message, patch #0, or under the "---"
stanza) what exactly problem are you trying to solve in your case with
this patch.

> >
> > I do remember there being some SoC which shipped a SPI controller in
> > that configuration for some reason.  Possibly one of the OEM ones rather
> > than one in a Samsung SoC?
>
> with DMA you mean?
>
> Thanks,
> ta