linux-kernel - Re: [PATCH v2 1/3] arm64: imx8mq: add imx8mq iomux-gpr field defines

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Message-ID: <1566468231.3653.8.camel@pengutronix.de>
Date:   Thu, 22 Aug 2019 12:03:51 +0200
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Guido Günther <agx@...xcpu.org>,
        Arnd Bergmann <arnd@...db.de>
Cc:     Mark Rutland <mark.rutland@....com>,
        DTML <devicetree@...r.kernel.org>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Daniel Vetter <daniel@...ll.ch>,
        Neil Armstrong <narmstrong@...libre.com>,
        David Airlie <airlied@...ux.ie>,
        Fabio Estevam <festevam@...il.com>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Jonas Karlman <jonas@...boo.se>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Rob Herring <robh+dt@...nel.org>,
        NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Robert Chiras <robert.chiras@....com>,
        Shawn Guo <shawnguo@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v2 1/3] arm64: imx8mq: add imx8mq iomux-gpr field defines

On Wed, 2019-08-21 at 19:42 +0200, Guido Günther wrote:
> Hi,
> On Tue, Aug 13, 2019 at 01:07:52PM +0200, Arnd Bergmann wrote:
> > On Tue, Aug 13, 2019 at 12:10 PM Guido Günther <agx@...xcpu.org> wrote:
> > > On Tue, Aug 13, 2019 at 10:08:44AM +0200, Arnd Bergmann wrote:
> > > > On Fri, Aug 9, 2019 at 6:24 PM Guido Günther <agx@...xcpu.org> wrote:
> > > > > 
> > > > > This adds all the gpr registers and the define needed for selecting
> > > > > the input source in the imx-nwl drm bridge.
> > > > > 
> > > > > Signed-off-by: Guido Günther <agx@...xcpu.org>
> > > > > +
> > > > > +#define IOMUXC_GPR0    0x00
> > > > > +#define IOMUXC_GPR1    0x04
> > > > > +#define IOMUXC_GPR2    0x08
> > > > > +#define IOMUXC_GPR3    0x0c
> > > > > +#define IOMUXC_GPR4    0x10
> > > > > +#define IOMUXC_GPR5    0x14
> > > > > +#define IOMUXC_GPR6    0x18
> > > > > +#define IOMUXC_GPR7    0x1c
> > > > 
> > > > (more of the same)
> > > > 
> > > > huh?
> > > 
> > > These are the names from the imx8MQ reference manual (general purpose
> > > registers, they lump together all sorts of things), it's the same on
> > > imx6/imx7):
> > > 
> > >     https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/mfd/syscon/imx6q-iomuxc-gpr.h
> > >     https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/mfd/syscon/imx7-iomuxc-gpr.h
> > > 
> > > > > +/* i.MX8Mq iomux gpr register field defines */
> > > > > +#define IMX8MQ_GPR13_MIPI_MUX_SEL              BIT(2)
> > > > 
> > > > I think this define should probably be local to the pinctrl driver, to
> > > > ensure that no other drivers fiddle with the registers manually.
> > > 
> > > The purpose of these bits is for a driver to fiddle with them to select
> > > the input source. Similar on imx7 it's already used for e.g. the phy
> > > refclk in the pci controller:
> > > 
> > >     https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/controller/dwc/pci-imx6.c#n638
> > 
> > That one should likely use either the clk interface or the phy
> > interface instead.
> > 
> > > The GPRs are not about pad configuration but gather all sorts of things
> > > (section 8.2.4 of the imx8mq reference manual): pcie setup, dsi related
> > > bits so I don't think this should be done via a pinctrl
> > > driver. Should we handle that differently than on imx6/7?
> > 
> > It would be nice to fix the existing code as well, but for the moment,
> > I only think we should not add more of that.
> > 
> > Generally speaking, we can use syscon to do random things that don't
> > have a subsystem of their own, but we should not use it to do things
> > that have an existing driver framework like pinctrl, clock, reset, phy
> > etc.
> 
> Since it's not an external pin i opted to use MUX_MMIO instead which
> seems like a good fit here. Does that make sense?

Yes, I agree. The i.MX6 display subsystem predates the mux framework,
otherwise I would have used it for the LVDS and HDMI muxes in the first
place. We should probably switch imx-drm over as well, in a backwards
compatible fashion. The &mux definitions are already there in
arch/arm/boot/dts/imx6q.dtsi.

regards
Philipp