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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Wed, 12 Sep 2018 11:43:53 +0200
Message-ID: <CAMuHMdWefU0pw=c9TKOa+wGHchpg8JRfE0OnC4+wKDt-qrbH1A@mail.gmail.com>
Subject: Re: [PATCH v12 0/6] Driver for at91 usart in spi mode
To:     Lee Jones <lee.jones@linaro.org>
Cc:     Alexandre Belloni <alexandre.belloni@bootlin.com>,
        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>,
        Arnd Bergmann <arnd@arndb.de>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>,
        linux-spi <linux-spi@vger.kernel.org>
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Hi Lee,

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).

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds