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From:   Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Date:   Tue, 27 Aug 2019 23:15:37 +0200
Message-ID: <CAFBinCD1oKxYm8QD7XfZUWq_HC5A4GLMmLCnZrcRvpTxrKo30w@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] reset: Reset controller driver for Intel LGM SoC
To:     "Chuan Hua, Lei" <chuanhua.lei@linux.intel.com>
Cc:     eswara.kota@linux.intel.com, cheol.yong.kim@intel.com,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        p.zabel@pengutronix.de, qi-ming.wu@intel.com, robh@kernel.org,
        Hauke Mehrtens <hauke@hauke-m.de>
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Hi,

On Tue, Aug 27, 2019 at 4:23 AM Chuan Hua, Lei
<chuanhua.lei@linux.intel.com> wrote:
[...]
> >> 1. reset-lantiq.c use index instead of register offset + bit position.
> >> index reset is good for a small system (< 64). However, it will become very
> >> difficult to use if you have  > 100 reset. So we use register offset +
> >> bit position
> > reset-lantiq uses bit bit positions for specifying the reset line.
> > for example this is from OpenWrt's vr9.dtsi:
> >    reset0: reset-controller@10 {
> >      ...
> >      reg = <0x10 4>, <0x14 4>;
> >      #reset-cells = <2>;
> >    };
> >
> >    gphy0: gphy@20 {
> >      ...
> >      resets = <&reset0 31 30>, <&reset1 7 7>;
> >      reset-names = "gphy", "gphy2";
> >    };
> >
> > in my own words this means:
> > - all reset0 reset bits are at offset 0x10 (parent is RCU)
> > - all reset0 status bits are at offset 0x14 (parent is RCU)
> > - the first reset line uses reset bit 31 and status bit 30
> > - the second reset line uses reset bit 7 and status bit 7
> > - there can be multiple reset-controller instances, each taking the
> > reset and status offsets (OpenWrt's vr9.dtsi specifies the second RCU
> > reset controller "reset1" with reset offset 0x48 and status offset
> > 0x24)
>
> in reset-lantiq.c, we split each reset request /status pair into one
> reset controller.
>
> Each reset controller handles up to 32 resets. It will create up to 9
> even more
> reset controllers in the new SoCs. In reality, there is only one RCU
> controller for all
> SoCs. These designs worked but did not follow what hardware implemented.
>
> After checking the existing code and referring to other implementation,
> we decided to
> use register offset + bit position method. It can support all SoCs with
> this methods
> without code change(device tree change only).
maybe I have a different interpretation of what "RCU" does.
let me explain it in my own words based on my knowledge about VRX200:
- in my own words it is a multi function device with the following
functionality:
- it contains two reset controllers (reset at 0x10, status 0x14 and
reset at 0x48, status at 0x24)
- it contains two USB2 PHYs (PHY registers at 0x18, ANA cfg at 0x38
and PHY registers at 0x34, ANA cfg at 0x3c)
- it contains the configuration for the two GPHY IP blocks (at 0x20 and 0x68)
- it contains endianness configuration registers (for PCI, PCIe, ...)
- it contains the watchdog boot status (whether the SoC was previously
reset by the WDT)
- maybe more, but I don't know anything else about it

we tried our best to document this in
Documentation/devicetree/bindings/mips/lantiq/rcu.txt

I'm not sure about the details of the RCU on the LGM SoCs:
if it contains more than just reset controllers then please let Rob
Herring (dt-bindings maintainer) know about this.
we may only have one chance to do it right, if we start with a
"broken" binding then devices with incompatible bootloaders etc. may
have already shipped
(in general: that is why the devicetree maintainers want to have all
device properties documented in the binding, even if the driver does
not support all of them yet)

> >> 2. reset-lantiq.c does not support device restart which is part of the
> >> reset in
> >> old lantiq SoC. It moved this part into arch/mips/lantiq directory.
> > it was moved to the .dts instead of the arch code. again from
> > OpenWrt's vr9.dtsi [0]:
> >    reboot {
> >      compatible = "syscon-reboot";
> >      regmap = <&rcu0>;
> >      offset = <0x10>;
> >      mask = <0xe0000000>;
> >    };
> >
> > this sets the reset0 reset bits 31, 30 and 29 at reboot
> ok. but not sure why we need to reset bit 31 and 29. global softwre
> reset is bit 30.
I don't know either. depending on what the LGM SoCs need you can
change the "mask" property to the value that fits that SoC best

[...]
> > - other reset lines only support reset pulses. the _reset function
> > should be used in this case
> > - the _reset function should only assert the reset line, then wait
> > until the hardware automatically de-asserts it (without any further
> > write)
> Yes, this is called hardware reset. We can't control reset duration.
> > is this the same for all, old and new SoCs?
>
> New SoCs have removed support for hardware reset after software's feedback.
>
> Old SoCs such as VRX200/ARX300 has both software/hardware resets
nice, it's good to see teamwork between hardware and software teams!

[...]
> >> 4. Code not optimized and intel internal review not assessed.
> > insights from you (like the issue with the reset callback) are very
> > valuable - this shows that we should focus on having one driver.
> >
> >> Based on the above findings, I would suggest reset-lantiq.c to move to
> >> reset-intel-syscon.c
> > my concern with having two separate drivers is that it will be hard to
> > migrate from reset-lantiq to the "optimized" reset-intel-syscon
> > driver.
> > I don't have access to the datasheets for the any Lantiq/Intel SoC
> > (VRX200 and even older).
> > so debugging issues after switching from one driver to another is
> > tedious because I cannot tell which part of the driver is causing a
> > problem (it's either "all code from driver A" vs "all code from driver
> > B", meaning it's hard to narrow it down).
> > with separate commits/patches that are improving the reset-lantiq
> > driver I can do git bisect to find the cause of a problem on the older
> > SoCs (VRX200 for example)
>
> Our internal version supports XRX350/XRX500/PRX300(MIPS based) and
> latest Lighting Mountain(X86 based). Migration to reset-intel-syscon.c
> should be straight forward.
what about the _reset callback on the XRX350/XRX500/PRX300 SoCs - do
they only use level resets (_assert and _deassert) or are some reset
lines using reset pulses (_reset)?

when we wanted to switch from reset-lantiq.c to reset-intel-syscon.c
we still had to add support for the _reset callback as this is missing
in reset-intel-syscon.c currently


Martin