Re: [GIT PULL] FSL QSPI device tree cleanup for 4.21

[Arnd Bergmann <arnd@xxxxxxxx>]

On Thu, Dec 20, 2018 at 7:23 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> On Thu, Dec 20, 2018 at 5:33 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > On Thu, Dec 20, 2018 at 5:23 PM Schrempf Frieder
> > > As the new driver is a SPI driver, the common rules in
> > > Documentation/devicetree/bindings/spi/spi-bus.txt apply and
> > > spi-[tx/rx]-bus-width are defined to default to 1 if not specified.
> > > So we can't set those properties in the driver if they are missing in
> > > the dtb as we would deviate from the standard, although it would make
> > > more sense for a *Quad*-SPI driver to default to 4.
> >
> > Right, I guess that makes sense.
> >
> > Now, you could change the binding for the fsl-quadspi driver to
> > require the bus-width properties, which would then just make
> > the old dtb files invalid, and you give us an excuse to override
> > the properties by adding the =4 ones, once that is documented
> > in the driver specific binding.
> >
> > Since it's only a performance difference, I suppose we can live
> > with the limitation and stay a little closer to the normal binding.
>
> Please note that a quad-capable SPI FLASH may be physically wired
> to a quad-capable SPI controller using only 2 wires, so defaulting to
> 4 may cause issues.  Defaulting to 2 may work, if the device is
> dual-capable.

Right, this would only work because the old driver used 4 wires
in the absence of the DT property, and the driver would warn
when it's not there for new dtb files.

      Arnd

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