linux-kernel - Re: [PATCH 02/11] dt-bindings: serial: snps-dw-apb-uart: Relax dma-names order constraint

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Date:   Mon, 20 Mar 2023 11:01:37 -0500
From:   Rob Herring <robh@...nel.org>
To:     Cristian Ciocaltea <cristian.ciocaltea@...labora.com>
Cc:     Conor Dooley <conor@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Cristian Marussi <cristian.marussi@....com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Nicolas Frattaroli <frattaroli.nicolas@...il.com>,
        Heiko Stuebner <heiko@...ech.de>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Daniel Drake <drake@...lessm.com>,
        Katsuhiro Suzuki <katsuhiro@...suster.net>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        alsa-devel@...a-project.org, linux-rockchip@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, kernel@...labora.com
Subject: Re: [PATCH 02/11] dt-bindings: serial: snps-dw-apb-uart: Relax
 dma-names order constraint

On Fri, Mar 17, 2023 at 07:43:53PM +0200, Cristian Ciocaltea wrote:
> On 3/17/23 18:26, Conor Dooley wrote:
> > On Fri, Mar 17, 2023 at 04:54:47PM +0100, Krzysztof Kozlowski wrote:
> > > On 17/03/2023 11:21, Cristian Ciocaltea wrote:
> > > > On 3/17/23 10:31, Krzysztof Kozlowski wrote:
> > > > > On 15/03/2023 12:47, Cristian Ciocaltea wrote:
> > > > > > Commit 370f696e4474 ("dt-bindings: serial: snps-dw-apb-uart: add dma &
> > > > > > dma-names properties") documented dma-names property to handle Allwiner
> > > > > > D1 dtbs_check warnings, but relies on a strict rx->tx ordering, which is
> > > > > > the reverse of what a different board expects:
> > > > > > 
> > > > > >     rk3326-odroid-go2.dtb: serial@...30000: dma-names:0: 'rx' was expected
> > > > > > 
> > > > > > A quick and incomplete check shows the inconsistency is present in many
> > > > > > other DT files:
> > > > > 
> > > > > Why not fixing the DTS? The properties should have fixed order.
> > > > 
> > > > I was initially concerned about the risk of a potential ABI breakage,
> > > > but I think that's not really a problem since dma-names is not directly
> > > > accessed in the driver and DT Kernel API doesn't rely on a particular order.
> > > > 
> > > > If there are no objections, I would switch the order in the binding to
> > > > tx->rx, since that's what most of the DTS use, and fix the remaining ones.
> > > 
> > > Since we added the order recently, I rather assume it is the correct or
> > > preferred one.
> > 
> > IIRC I checked around the other serial bindings & there was not a
> > consistent order that all serial bindings used, so I picked the order that
> > was used across the various allwinner boards that do use dma-names.
> 
> Thanks for clarifying this, Conor! Would it be fine to switch to tx->rx
> order as it requires less changes to fix the inconsistencies?
> 
> > Before changing dts files, it's probably a good idea to make sure that
> > the dma-names are not used somewhere outside of Linux.
> 
> Right, that means we cannot exclude the ABI breakage concern. Not sure how
> easy would be to actually verify this. Hence I wonder if there is really no
> chance to allow the flexible order in the binding..

If it changes and someone complains, then yes we'll allow flexible 
order.

Rob