Date:	Thu, 21 May 2015 09:40:01 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	Andrew Bresticker <abrestic@...omium.org>,
	Jon Hunter <jonathanh@...dia.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Alexandre Courbot <gnurou@...il.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Samuel Ortiz <sameo@...ux.intel.com>
Subject: Re: [PATCH v8 4/9] mfd: Add binding document for NVIDIA Tegra XUSB

On Wed, 20 May 2015, Thierry Reding wrote:
> On Wed, May 20, 2015 at 07:35:51AM +0100, Lee Jones wrote:
> > On Tue, 19 May 2015, Andrew Bresticker wrote:
> > > On Thu, May 14, 2015 at 10:38 AM, Andrew Bresticker
> > > <abrestic@...omium.org> wrote:
> > > > On Thu, May 14, 2015 at 12:40 AM, Lee Jones <lee.jones@...aro.org> wrote:
> > > >> On Thu, 14 May 2015, Jon Hunter wrote:
> > > >>> On 13/05/15 15:39, Lee Jones wrote:
> > > >>> > On Mon, 04 May 2015, Andrew Bresticker wrote:
> > > >>> >
> > > >>> >> Add a binding document for the XUSB host complex on NVIDIA Tegra124
> > > >>> >> and later SoCs.  The XUSB host complex includes a mailbox for
> > > >>> >> communication with the XUSB micro-controller and an xHCI host-controller.
> > > >>> >>
> > > >>> >> Signed-off-by: Andrew Bresticker <abrestic@...omium.org>
> > > >>> >> Cc: Rob Herring <robh+dt@...nel.org>
> > > >>> >> Cc: Pawel Moll <pawel.moll@....com>
> > > >>> >> Cc: Mark Rutland <mark.rutland@....com>
> > > >>> >> Cc: Ian Campbell <ijc+devicetree@...lion.org.uk>
> > > >>> >> Cc: Kumar Gala <galak@...eaurora.org>
> > > >>> >> Cc: Samuel Ortiz <sameo@...ux.intel.com>
> > > >>> >> Cc: Lee Jones <lee.jones@...aro.org>
> > > >>> >> ---
> > > >>> >> Changes from v7:
> > > >>> >>  - Move non-shared resources into child nodes.
> > > >>> >> New for v7.
> > > >>> >> ---
> > > >>> >>  .../bindings/mfd/nvidia,tegra124-xusb.txt          | 37 ++++++++++++++++++++++
> > > >>> >>  1 file changed, 37 insertions(+)
> > > >>> >>  create mode 100644 Documentation/devicetree/bindings/mfd/nvidia,tegra124-xusb.txt
> > > >>> >>
> > > >>> >> diff --git a/Documentation/devicetree/bindings/mfd/nvidia,tegra124-xusb.txt b/Documentation/devicetree/bindings/mfd/nvidia,tegra124-xusb.txt
> > > >>> >> new file mode 100644
> > > >>> >> index 0000000..bc50110
> > > >>> >> --- /dev/null
> > > >>> >> +++ b/Documentation/devicetree/bindings/mfd/nvidia,tegra124-xusb.txt
> > > >>> >> @@ -0,0 +1,37 @@
> > > >>> >> +NVIDIA Tegra XUSB host copmlex
> > > >>> >> +==============================
> > > >>> >> +
> > > >>> >> +The XUSB host complex on Tegra124 and later SoCs contains an xHCI host
> > > >>> >> +controller and a mailbox for communication with the XUSB micro-controller.
> > > >>> >> +
> > > >>> >> +Required properties:
> > > >>> >> +--------------------
> > > >>> >> + - compatible: For Tegra124, must contain "nvidia,tegra124-xusb".
> > > >>> >> +   Otherwise, must contain '"nvidia,<chip>-xusb", "nvidia,tegra124-xusb"'
> > > >>> >> +   where <chip> is tegra132.
> > > >>> >> + - reg: Must contain the base and length of the XUSB FPCI registers.
> > > >>> >> + - ranges: Bus address mapping for the XUSB block.  Can be empty since the
> > > >>> >> +   mapping is 1:1.
> > > >>> >> + - #address-cells: Must be 2.
> > > >>> >> + - #size-cells: Must be 2.
> > > >>> >> +
> > > >>> >> +Example:
> > > >>> >> +--------
> > > >>> >> +  usb@0,70098000 {
> > > >>> >> +          compatible = "nvidia,tegra124-xusb";
> > > >>> >> +          reg = <0x0 0x70098000 0x0 0x1000>;
> > > >>> >> +          ranges;
> > > >>> >> +
> > > >>> >> +          #address-cells = <2>;
> > > >>> >> +          #size-cells = <2>;
> > > >>> >> +
> > > >>> >> +          usb-host@0,70090000 {
> > > >>> >> +                  compatible = "nvidia,tegra124-xhci";
> > > >>> >> +                  ...
> > > >>> >> +          };
> > > >>> >> +
> > > >>> >> +          mailbox {
> > > >>> >> +                  compatible = "nvidia,tegra124-xusb-mbox";
> > > >>> >> +                  ...
> > > >>> >> +          };
> > > >>> >
> > > >>> > This doesn't appear to be a proper MFD.  I would have the USB and
> > > >>> > Mailbox devices probe seperately and use a phandle to point the USB
> > > >>> > device to its Mailbox.
> > > >>> >
> > > >>> > usb@xyz {
> > > >>> >     mboxes = <&xusb-mailbox, [chan]>;
> > > >>> > };
> > > >>> >
> > > >>>
> > > >>> I am assuming that Andrew had laid it out like this to reflect the hw
> > > >>> structure. The mailbox and xhci controller are part of the xusb
> > > >>> sub-system and hence appear as child nodes. My understanding is that for
> > > >>> device-tree we want the device-tree structure to reflect the actual hw.
> > > >>> Is this not the case?
> > > >>
> > > >> Yes, the DT files should reflect h/w.  I have requested to see what
> > > >> the memory map looks like, so I might provide a more appropriate
> > > >> solution to accepting a pretty pointless MFD.
> > > >
> > > > FWIW, the address map for XUSB looks like this:
> > > >
> > > > XUSB_HOST: 0x70090000 - 0x7009a000
> > > >     xHCI registers: 0x70090000 - 0x70098000
> > > >     FPCI configuration registers: 0x70098000 - 0x70099000
> > > >     IPFS configuration registers: 0x70099000 - 0x7009a000
> > > >
> > > >> Two solutions spring to mind.  You can either call
> > > >> of_platform_populate() from the USB driver, as some already do:
> > > >>
> > > >>   drivers/usb/dwc3/dwc3-exynos.c:
> > > >>     ret = of_platform_populate(node, NULL, NULL, dev);
> > > >>   drivers/usb/dwc3/dwc3-keystone.c:
> > > >>     error = of_platform_populate(node, NULL, NULL, dev);
> > > >>   drivers/usb/dwc3/dwc3-omap.c:
> > > >>     ret = of_platform_populate(node, NULL, NULL, dev);
> > > >>   drivers/usb/dwc3/dwc3-qcom.c:
> > > >>     ret = of_platform_populate(node, NULL, NULL, qdwc->dev);
> > > >>   drivers/usb/dwc3/dwc3-st.c:
> > > >>     ret = of_platform_populate(node, NULL, NULL, dev);
> > > >>   drivers/usb/musb/musb_am335x.c:
> > > >>     ret = of_platform_populate(pdev->dev.of_node, NULL, NULL, &pdev->dev);
> > > >
> > > > This still requires a small, separate driver to setup the regmap and
> > > > do of_platform_populate().  The only difference is it lives in
> > > > drivers/usb/ instead of drivers/mfd/.
> > > >
> > > >> Or use the "simple-mfd", which is currently in -next:
> > > >>
> > > >>   git show next/master:Documentation/devicetree/bindings/mfd/mfd.txt
> > > >
> > > > I'm not too opposed to this, but Thierry was when I brought this up
> > > > before.  The issue here is that if we ever have to do something
> > > > besides setting up a regmap in the MFD, we'd have to change the
> > > > binding and break DT backwards-compatibility.
> > > 
> > > Any thoughts on this?  A minimal MFD seems to be the best way to
> > > future-proof this binding/driver should it need to be extended in the
> > > future.  If this is a firm NAK from you however, I'll need to let
> > > Jassi now so that he can un-queue the mailbox patches for 4.2....
> > 
> > I was waiting to hear Thierry's thoughts.  However, I am unconvinced
> > that you need an MFD driver for this and refuse to take a shell (read
> > "pointless") one on an "if we ever ..." clause.
> > 
> > Will you break backwards capability though?  I'm not sure you will.
> > Old DTBs will still use 'simple-mfd' and probe the devices in the
> > normal way.  *If* you introduce an MFD driver at a later date then the
> > old DTB will miss out the *new* functionality, which is expected and
> > accepted.
> 
> I'm a little confused by the simple-mfd approach. The only code I see in
> linux-next for this is a single line that adds the "simple-mfd" string
> to the OF device ID table in drivers/of/platform.c. As far as I can tell
> this will merely cause child devices to be created. There won't be a
> shared regmap and resources won't be set up properly either. Having a
> proper MFD driver seems to be the only way to achieve what we need.
> 
> The reason why every other simple-mfd users seems to get away with this
> is because they also match on syscon and that sets up a regmap of its
> own and the child device drivers use the syscon API to get at it. So I
> don't see how we can make use of simple-mfd to achieve what we need,
> unless we essentially copy what syscon does (but do proper resource
> management while at it).

If you have shared resources and your device isn't classed as a syscon
device then yes, simple-mfd probably isn't suitable for this use-case.
You might need to go into more detail with regards to "proper resource
management", as I'm not entirely sure I agree.

Still, this doesn't change the fact that, from what I've seen, I still
don't think you need a dedicated MFD driver.

What do you think of:

usb-host@0,70090000 {
	compatible = "nvidia,tegra124-xhci";
	reg = <0x0 0x70090000 0x0 0x80CF>,
	      <0x0 0x70098800 0x0 0x0800>,
	      <0x0 0x70099000 0x0 0x1000>;

	/* Something from the datasheet */
	reg-names = "xhci-before-mbox", "xhci-after-mbox", "ipfs";

	interrupts = <GIC_SPI 39 IRQ_TYPE_LEVEL_HIGH>;
	ranges;

	xusb_mbox: mailbox {
		compatible = "nvidia,tegra124-xusb-mbox";
		reg = <0x0 0x700980e0 0x0 0x13>,
		      <0x0 0x70098428 0x0 0x03>;

		/* Something from the datasheet */
		reg-names = "mbox-one", "mbox-two";
		interrupts = <GIC_SPI 40 IRQ_TYPE_LEVEL_HIGH>;
	};
};

Then hvae the XHCI driver call of_platform_populate() as I proposed
above?

> There is also the matter of clocks, resets, power supplies, etc. which
> simple-mfd can't take into account in its current form. From a hardware
> point of view, (some of) the clocks and resets are shared by the XHCI
> and the mailbox blocks, so the device tree node would have to take that
> into account. And a driver would also have to know which clocks, resets
> and power supplies to probe and the order in which they need to be
> enabled. simple-mfd doesn't provide any of that currently, so we'd
> likely need to hack around that in all sorts of weird ways in the child
> drivers. It makes much more sense for a top-level MFD driver to set up
> the shared hardware resources and then instantiate the child devices and
> let the drivers for those only care about the child-specific resources.
> 
> A catch-all driver will inevitably lead to implementing a midlayer with
> potentially all sorts of quirks to make it work with the various devices
> out there.
> 
> A much better implementation, in my opinion, would be to make simple-mfd
> a subclassable object and then have drivers use a helper library to call
> code that is common for simple-mfd kinds of devices. Something like this
> for example:
> 
> 	struct tegra_xusb {
> 		...
> 		struct mfd_simple mfd;
> 		...
> 	};
> 
> 	static int tegra_xusb_probe(struct platform_device *pdev)
> 	{
> 		struct tegra_xusb *xusb;
> 		...
> 		err = mfd_simple_register(&xusb->mfd);
> 		...
> 	}
> 
> Now all these drivers reuse all the code provided by the mfd_simple
> helper, which will instantiate the children, but it is also very easy to
> tie in the platform-specific glue (clocks, resets, regulators, ...) via
> the device-specific drivers.

I'm not keen on creating a not-so-simple-mfd driver.  Let's work with
what we've got for the time being.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
Follow Linaro: Facebook | Twitter | Blog
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