linux-kernel - Re: [PATCH v2 04/16] spi: dt-bindings: dw-apb-ssi: update spi-{r,t}x-bus-width

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Message-ID: <20220628152601.GA465684-robh@kernel.org>
Date:   Tue, 28 Jun 2022 09:26:01 -0600
From:   Rob Herring <robh@...nel.org>
To:     Serge Semin <fancer.lancer@...il.com>
Cc:     Conor Dooley <mail@...chuod.ie>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        Vinod Koul <vkoul@...nel.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Palmer Dabbelt <palmer@...osinc.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Niklas Cassel <niklas.cassel@....com>,
        Dillon Min <dillon.minfei@...il.com>,
        Heng Sia <jee.heng.sia@...el.com>,
        Jose Abreu <joabreu@...opsys.com>,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, dmaengine@...r.kernel.org,
        alsa-devel@...a-project.org, linux-spi@...r.kernel.org,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v2 04/16] spi: dt-bindings: dw-apb-ssi: update
 spi-{r,t}x-bus-width

On Mon, Jun 27, 2022 at 11:21:49PM +0300, Serge Semin wrote:
> On Mon, Jun 27, 2022 at 08:39:52PM +0100, Conor Dooley wrote:
> > From: Conor Dooley <conor.dooley@...rochip.com>
> > 
> > Most users of dw-apb-ssi use spi-{r,t}x-bus-width of 1, however the
> > Canaan k210 is wired up for a width of 4.
> > Quoting Serge:
> > The modern DW APB SSI controllers of v.4.* and newer also support the
> > enhanced SPI Modes too (Dual, Quad and Octal). Since the IP-core
> > version is auto-detected at run-time there is no way to create a
> > DT-schema correctly constraining the Rx/Tx SPI bus widths.
> > /endquote
> > 
> > As such, drop the restriction on only supporting a bus width of 1.
> > 
> > Link: https://lore.kernel.org/all/20220620205654.g7fyipwytbww5757@mobilestation/
> > Signed-off-by: Conor Dooley <conor.dooley@...rochip.com>
> > ---
> > Serge, I dropped your R-b when I swapped to the default
> > property since it changed the enum.
> > ---
> >  Documentation/devicetree/bindings/spi/snps,dw-apb-ssi.yaml | 6 ------
> >  1 file changed, 6 deletions(-)
> > 
> > diff --git a/Documentation/devicetree/bindings/spi/snps,dw-apb-ssi.yaml b/Documentation/devicetree/bindings/spi/snps,dw-apb-ssi.yaml
> > index e25d44c218f2..0a43d6e0ef91 100644
> > --- a/Documentation/devicetree/bindings/spi/snps,dw-apb-ssi.yaml
> > +++ b/Documentation/devicetree/bindings/spi/snps,dw-apb-ssi.yaml
> > @@ -143,12 +143,6 @@ patternProperties:
> >          minimum: 0
> >          maximum: 3
> >  
> 
> > -      spi-rx-bus-width:
> > -        const: 1
> > -
> > -      spi-tx-bus-width:
> > -        const: 1
> > -
> 
> My comment was:
> > > > You can just use a more relaxed constraint "enum: [1 2 4 8]" here
> > >
> > > 8 too? sure.
> Then Rob said:
> > Then no constraints needed because the common definition already has
> > this presumably.
> 
> IMO preserving the device-specific constraints even if they match the
> generic ones has some maintainability benefits. What if you get to
> discover a new HW which supports Hexal mode? 

x16? Wouldn't we be back to parallel NOR and the problems with parallel 
buses?

> Then you would have
> needed to update the common schema constraints. But that would have
> caused permitting the unsupported bus-mode for all the schemas, which
> isn't correct. So as I see it the explicit bus-width enumeration would
> be ok to have here. But I'll leave it for Rob to make a final
> decision.

Assuming a new width does appear, it's just a matter of time before the 
DW block has a new rev supporting it too, so there's 2 places to update. 
Also, a given platform may pinout less than the block supports, so you 
can't ever be 100% sure an out of range value is in a DT.

But either way is okay with me. If you do keep constraints, you only 
need 'maximum: 8'.

Acked-by: Rob Herring <robh@...nel.org>

Rob