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Date:   Wed, 12 Sep 2018 12:43:52 +0100
From:   Lee Jones <lee.jones@linaro.org>
To:     Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc:     Geert Uytterhoeven <geert@linux-m68k.org>,
        radu_nicolae.pirea@upb.ro, Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Nicolas Ferre <nicolas.ferre@microchip.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Mark Brown <broonie@kernel.org>, Jiri Slaby <jslaby@suse.com>,
        Richard Genoud <richard.genoud@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH v12 0/6] Driver for at91 usart in spi mode
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On Wed, 12 Sep 2018, Alexandre Belloni wrote:

> On 12/09/2018 11:54:07+0100, Lee Jones wrote:
> > On Wed, 12 Sep 2018, Geert Uytterhoeven wrote:
> > > On Wed, Sep 12, 2018 at 10:41 AM Lee Jones <lee.jones@linaro.org> wrote:
> > > > On Wed, 12 Sep 2018, Alexandre Belloni wrote:
> > > > > On 11/09/2018 23:54:40+0100, Lee Jones wrote:
> > > > > > > > http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-6438-32-bit-ARM926-Embedded-Microprocessor-SAM9G45_Datasheet.pdf
> > > > > > > >
> > > > > > > > USART doc starting p572, registers p621.
> > > > > >
> > > > > > After looking at the datasheet, I don't see any reason why one of the
> > > > > > two drivers can't be selected using different compatible strings.
> > > > >
> > > > > Because there is only one IP and we don't use the device tree to selecet
> > > > > linux specific drivers.
> > > >
> > > > We do it all the time.  There are loads of MFDs (def: same IP, with
> > > > different functions) which have separate compatibles for their various
> > > > functions.  If you wish this IP to operate as an SPI controller, it
> > > > should have an SPI compatible, if you wish it to operate as a U(S)ART,
> > > > then it should have a UART compatible.  It's what we do for most of
> > > > the other MFDs in the kernel.
> > > 
> > > There is a big difference: MFD functions are(more or less) independent
> > > functions, which can be used at the same time. It makes perfect sense for a
> > > single IP block that has both SPI and UART interfaces, that can be used at
> > > the same time.
> > > 
> > > In this case, there is a single piece of hardware that can perform
> > > different functions, but not at the same time. Performing a different
> > > function means configuring the hardware for that function, hence using a
> > > different driver (from a different subsystem).
> > 
> > Yes, I can see that PoV.
> > 
> > But ... we can't have it both ways.  *Either* it's a true MFD, in
> > which case it can/should have 2 separate compatible strings which can
> > be specified directly from the DT.  *Or* it's not an MFD.  In the
> > latter case, which I think we're all agreeing on (else we'd have 2
> > compatible strings), MFD is not the place to handle this (my original
> > point).
> > 
> 
> If that is what bothers you, then let's move it out of mfd.

As I've already mentioned.  I don't just want it moved out of MFD and
shoved somewhere else.  My aim is to fix this properly.

> > So ... this is a USART device which can do SPI, right?
> > 
> > My current thinking is that; as this is a USART device first &
> > foremost, the USART should be probed in the first instance regardless,
> > then if SPI mode is specified it (the USART driver) registers the SPI
> > platform driver (as MFD does currently) and exits gracefully, allowing
> > the SPI driver to take over.
> > 
> > Spanner in the works: is it physically possible to change the mode at
> > run-time?  :s
> 
> Yes it is possible but on Linux that will not happen without probing
> the drivers again.

Not sure I understand what you mean.

I'm suggesting that you use the same platform_* interfaces MFD uses to
register the SPI driver if SPI mode has been selected.  Only do so
from the appropriate driver i.e. USART.

> I think DT overlays will be the only possible use
> case because on SPI, you'd still have to provide nodes for the connected
> SPI devices.

Since SPI is a function of the USART you should describe is as such
via a child node.

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
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