Date:   Thu, 12 Apr 2018 16:10:06 +0200
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Sinan Kaya <okaya@...eaurora.org>
Cc:     Auger Eric <eric.auger@...hat.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Baptiste Reynal <b.reynal@...tualopensystems.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        KVM list <kvm@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] vfio: platform: Add generic DT reset support

On Thu, 2018-04-12 at 15:12 +0200, Geert Uytterhoeven wrote:
> Hi Sinan,
> 
> On Thu, Apr 12, 2018 at 2:36 PM, Sinan Kaya <okaya@...eaurora.org> wrote:
> > On 4/12/2018 7:49 AM, Auger Eric wrote:
> > > On 12/04/18 13:32, Geert Uytterhoeven wrote:
> > > > On Thu, Apr 12, 2018 at 12:31 PM, Auger Eric <eric.auger@...hat.com> wrote:
> > > > > On 11/04/18 11:15, Geert Uytterhoeven wrote:
> > > > > > Vfio-platform requires reset support, provided either by ACPI, or, on DT
> > > > > > platforms, by a device-specific reset driver matching against the
> > > > > > device's compatible value.
> > > > > > 
> > > > > > On many SoCs, devices are connected to an SoC-internal reset controller.
> > > > > > If the reset hierarchy is described in DT using "resets" properties,
> > > > > > such devices can be reset in a generic way through the reset controller
> > > > > > subsystem.  Hence add support for this, avoiding the need to write
> > > > > > device-specific reset drivers for each single device on affected SoCs.
> > > > > > 
> > > > > > Devices that do require a more complex reset procedure can still provide
> > > > > > a device-specific reset driver, as that takes precedence.
> > > > > > 
> > > > > > Note that this functionality depends on CONFIG_RESET_CONTROLLER=y, and
> > > > > > becomes a no-op (as in: "No reset function found for device") if reset
> > > > > > controller support is disabled.
> > > > > > 
> > > > > > Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
> > > > > > Reviewed-by: Philipp Zabel <p.zabel@...gutronix.de>
> > > > > > --- a/drivers/vfio/platform/vfio_platform_common.c
> > > > > > +++ b/drivers/vfio/platform/vfio_platform_common.c
> > > > > > @@ -127,8 +130,16 @@ static int vfio_platform_get_reset(struct vfio_platform_device *vdev)
> > > > > >               vdev->of_reset = vfio_platform_lookup_reset(vdev->compat,
> > > > > >                                                       &vdev->reset_module);
> > > > > >       }
> > > > > > +     if (vdev->of_reset)
> > > > > > +             return 0;
> > > > > > +
> > > > > > +     rstc = of_reset_control_get_exclusive(vdev->device->of_node, NULL);
> > > > > 
> > > > > Shouldn't we prefer the top level reset_control_get_exclusive()?
> > > > 
> > > > I guess that should work, too.
> > > > 
> > > > > To make sure about the exclusive/shared terminology, does
> > > > > get_reset_control_get_exclusive() check we have an exclusive wire
> > > > > between this device and the reset controller?
> > > > 
> > > > AFAIU, the "exclusive" means that only a single user can obtain access to
> > > > the reset, and it does not guarantee that we have an exclusive wire between
> > > > the device and the reset controller.
> > > > 
> > > > The latter depends on the SoC's reset topology. If a reset wire is shared
> > > > by multiple devices (e.g. resets shared by PWM or Display Unit devices on
> > > > R-Car SoCs), exporting a subset of these devices to a guest is a bad idea,
> > > > indeed.
> > > 
> > > So who's going to check this assigned device will not trigger a reset of
> > > other non assigned devices sharing the same reset controller?

If the reset control is requested as exclusive, any other driver
requesting the same reset control will fail (or this reset_control_get
will fail, whichever comes last).

> > I like the direction in general. I was hoping that you'd call it of_reset_control
> > rather than reset_control.
> 
> You mean vfio_platform_device.of_reset_control?
> If we switch to reset_control_get_exclusive(), that doesn't make much sense...
> 
> > Is there anything in the OF spec about what to expect from DT's reset?
> 
> Documentation/devicetree/bindings/reset/reset.txt says:
> 
> "A word on where to place reset signal consumers in device tree: It is possible
>  in hardware for a reset signal to affect multiple logically separate HW blocks
>  at once. In this case, it would be unwise to represent this reset signal in
>  the DT node of each affected HW block, since if activated, an unrelated block
>  may be reset. Instead, reset signals should be represented in the DT node
>  where it makes most sense to control it; this may be a bus node if all
>  children of the bus are affected by the reset signal, or an individual HW
>  block node for dedicated reset signals. The intent of this binding is to give
>  appropriate software access to the reset signals in order to manage the HW,
>  rather than to slavishly enumerate the reset signal that affects each HW
>  block."

This was written in 2012, and unfortunately the DT binding documentation
does not inform hardware development, and has not been updated since.

There's generally two kinds of reset uses:
- either to bring a device into a known state at a given point in time,
  which is often done using a timed assert/deassert sequence,
- or just to park a device while not in active use (must deassert any
  time before use, may or may not assert any time after use)

For the former case, the above paragraph makes a lot of sense, because
when it is necessary to reset a device that shares the reset line with
another, either choice between disturbing the other device, or not
being able to reset when necessary, is a bad one. The reset controller
framework supports those use cases via the reset_control_get_exclusive
function variants.

But for the latter case, there is absolutely no need to forbid sharing
reset lines among multiple devices, as deassertion/assertion can just be
handled reference counted, like clocks or power management. The reset
controller framework supports those use cases via the
reset_control_get_shared function variants.

The case we are talking about here is the first one.

> So according to the bindings, a specific reset should apply to a single
> device only.

Indeed sharing reset lines between peripherals has become unexpectedly
common, making it impractical to forbid shared resets in the device
tree.

> A quick check shows there are several violators:
> 
> $ for i in $(git grep -lw resets -- "*dts*"); do grep -w resets $i |
> sort | uniq -c | sed -e "s@^@$i:@g"; done | grep -v ":      1 "
> arch/arm/boot/dts/aspeed-g4.dtsi:     14 resets = <&syscon ASPEED_RESET_I2C>;
> arch/arm/boot/dts/aspeed-g5.dtsi:     14 resets = <&syscon ASPEED_RESET_I2C>;
> arch/arm/boot/dts/atlas7.dtsi:      2 resets = <&car 29>;
> arch/arm/boot/dts/gemini.dtsi:      2 resets = <&syscon GEMINI_RESET_IDE>;
> arch/arm/boot/dts/meson8.dtsi:      2 resets = <&reset RESET_USB_OTG>;
> arch/arm/boot/dts/meson8b.dtsi:      2 resets = <&reset RESET_USB_OTG>;
> arch/arm/boot/dts/r8a7743.dtsi:      7 resets = <&cpg 523>;
> arch/arm/boot/dts/r8a7743.dtsi:      2 resets = <&cpg 703>;
> arch/arm/boot/dts/r8a7743.dtsi:      2 resets = <&cpg 704>;
> arch/arm/boot/dts/r8a7745.dtsi:      7 resets = <&cpg 523>;
> arch/arm/boot/dts/r8a7745.dtsi:      2 resets = <&cpg 703>;
> arch/arm/boot/dts/r8a7745.dtsi:      2 resets = <&cpg 704>;
> arch/arm/boot/dts/r8a7790.dtsi:      3 resets = <&cpg 703>;
> arch/arm/boot/dts/r8a7790.dtsi:      2 resets = <&cpg 704>;
> arch/arm/boot/dts/r8a7791.dtsi:      2 resets = <&cpg 703>;
> arch/arm/boot/dts/r8a7791.dtsi:      2 resets = <&cpg 704>;
> arch/arm/boot/dts/r8a7794.dtsi:      2 resets = <&cpg 703>;
> arch/arm/boot/dts/r8a7794.dtsi:      2 resets = <&cpg 704>;
> arch/arm/boot/dts/stih410.dtsi:      2 resets = <&powerdown
> STIH407_USB2_PORT0_POWERDOWN>,
> arch/arm/boot/dts/stih410.dtsi:      2 resets = <&powerdown
> STIH407_USB2_PORT1_POWERDOWN>,
> arch/arm/boot/dts/stih410.dtsi:      2 resets = <&softreset
> STIH407_PICOPHY_SOFTRESET>,
> arch/arm/boot/dts/stih418.dtsi:      2 resets = <&powerdown
> STIH407_USB2_PORT0_POWERDOWN>,
> arch/arm/boot/dts/stih418.dtsi:      2 resets = <&powerdown
> STIH407_USB2_PORT1_POWERDOWN>,
> arch/arm/boot/dts/stih418.dtsi:      2 resets = <&softreset
> STIH407_PICOPHY_SOFTRESET>,
> arch/arm/boot/dts/sun9i-a80.dtsi:      2 resets = <&de_clocks RST_FE0>;
> arch/arm/boot/dts/sun9i-a80.dtsi:      2 resets = <&usb_clocks RST_USB0_HCI>;
> arch/arm/boot/dts/sun9i-a80.dtsi:      2 resets = <&usb_clocks RST_USB2_HCI>;
> arch/arm/boot/dts/sunxi-h3-h5.dtsi:      2 resets = <&ccu
> RST_BUS_EHCI0>, <&ccu RST_BUS_OHCI0>;
> arch/arm/boot/dts/sunxi-h3-h5.dtsi:      2 resets = <&ccu
> RST_BUS_EHCI1>, <&ccu RST_BUS_OHCI1>;
> arch/arm/boot/dts/sunxi-h3-h5.dtsi:      2 resets = <&ccu
> RST_BUS_EHCI2>, <&ccu RST_BUS_OHCI2>;
> arch/arm/boot/dts/sunxi-h3-h5.dtsi:      2 resets = <&ccu
> RST_BUS_EHCI3>, <&ccu RST_BUS_OHCI3>;
> arch/arm64/boot/dts/amlogic/meson-gxbb.dtsi:      2 resets = <&reset
> RESET_USB_OTG>;
> arch/arm64/boot/dts/nvidia/tegra186.dtsi:      2 resets = <&bpmp
> TEGRA186_RESET_DSI>;
> arch/arm64/boot/dts/renesas/r8a7795.dtsi:      3 resets = <&cpg 328>;
> arch/arm64/boot/dts/renesas/r8a7795.dtsi:      7 resets = <&cpg 523>;
> arch/arm64/boot/dts/renesas/r8a7795.dtsi:      3 resets = <&cpg 700>;
> arch/arm64/boot/dts/renesas/r8a7795.dtsi:      3 resets = <&cpg 701>;
> arch/arm64/boot/dts/renesas/r8a7795.dtsi:      3 resets = <&cpg 702>;
> arch/arm64/boot/dts/renesas/r8a7795.dtsi:      3 resets = <&cpg 703>;
> arch/arm64/boot/dts/renesas/r8a7796.dtsi:      3 resets = <&cpg 328>;
> arch/arm64/boot/dts/renesas/r8a7796.dtsi:      7 resets = <&cpg 523>;
> arch/arm64/boot/dts/renesas/r8a7796.dtsi:      3 resets = <&cpg 702>;
> arch/arm64/boot/dts/renesas/r8a7796.dtsi:      3 resets = <&cpg 703>;
> arch/arm64/boot/dts/renesas/r8a77965.dtsi:      3 resets = <&cpg 328>;
> arch/arm64/boot/dts/renesas/r8a77965.dtsi:      7 resets = <&cpg 523>;
> arch/arm64/boot/dts/renesas/r8a77965.dtsi:      2 resets = <&cpg 702>;
> arch/arm64/boot/dts/renesas/r8a77965.dtsi:      4 resets = <&cpg 703>;
> arch/arm64/boot/dts/renesas/r8a77995.dtsi:      4 resets = <&cpg 523>;
> arch/arm64/boot/dts/renesas/r8a77995.dtsi:      3 resets = <&cpg 703>;
> $
> 
> Perhaps we should start grouping devices sharing a reset signal in a
> "simple-bus" node?
> 
> Phillip: any comments?

For some of those it may be possible, but that is basically just a work-
around for reality not matching expectations. There may be other cases
where devices sharing a reset line are not even in the same parent node
because they are controlled via a different bus. In general, I don't
think it is feasible or desirable to force grouping of devices that
share the same reset line into a common parent node.

My suggestion would be to relax the language in the reset.txt DT
bindings doc.

regards
Philipp

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