linux-kernel - Re: [PATCH v2 01/11] dt-bindings: spi: allow expressing DTR capability

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Date:   Thu, 27 Feb 2020 18:06:58 +0100
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Mark Rutland <mark.rutland@....com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Vignesh Raghavendra <vigneshr@...com>,
        Tudor Ambarus <tudor.ambarus@...rochip.com>,
        Richard Weinberger <richard@....at>,
        Sekhar Nori <nsekhar@...com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        MTD Maling List <linux-mtd@...ts.infradead.org>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Pratyush Yadav <p.yadav@...com>
Subject: Re: [PATCH v2 01/11] dt-bindings: spi: allow expressing DTR
 capability

On Thu, 27 Feb 2020 16:44:25 +0000
Mark Brown <broonie@...nel.org> wrote:

> On Thu, Feb 27, 2020 at 05:40:31PM +0100, Geert Uytterhoeven wrote:
> > On Thu, Feb 27, 2020 at 5:28 PM Mark Brown <broonie@...nel.org> wrote:  
> 
> > > It's what we do for other properties, and if this is anything like the
> > > other things adding extra wiring you can't assume that the ability to
> > > use the feature for TX implies RX.  
> 
> > Double Transfer Rate uses the same wire.  
> 
> But is it still on either the TX or RX signals?

There's no separate RX/TX pins when using xD-xD-xD modes (pins switch
from RX to TX) and I doubt DTR will ever be used on single SPI.

> 
> > But as you sample at both the rising and the falling edges of the clock, this
> > makes the cpha setting meaningless for such transfers, I think ;-)  
> 
> Might affect what the first bit is possibly?
> 
> > However, as the future may bring us QDR, perhaps this should not be a
> > boolean flag, but an integer value?
> > Cfr. spi-tx-bus-width vs. the original spi-tx-dual/spi-tx-quad proposal.  
> 
> > What would be a good name (as we only need one)? spi-data-phases?  
> 
> Sounds reasonable, apart from the increasingly vague connection with
> something that's recognizably SPI :P

Or maybe we should refrain from adding a new flag and wait a bit to see
if this DTR mode is actually used for regular SPI transfers (AKA not
spi-mem) :-).