[PATCH 0/2] MIPS: BMIPS: dts: update device nodes for bcm7xxx platforms

[PATCH 0/2] MIPS: BMIPS: dts: update device nodes for bcm7xxx platforms

[Jaedon Shin]

Hi all,

This patch series contain changing device nodes of the bcm7xxx platforms.

Jaedon Shin (2):
  MIPS: BMIPS: dts: remove unsupported entry for bcm7362
  MIPS: BMIPS: dts: add uart device nodes to bcm7xxx platforms

 arch/mips/boot/dts/brcm/bcm7346.dtsi      | 26 ++++++++++++++++++++++++++
 arch/mips/boot/dts/brcm/bcm7358.dtsi      | 26 ++++++++++++++++++++++++++
 arch/mips/boot/dts/brcm/bcm7360.dtsi      | 26 ++++++++++++++++++++++++++
 arch/mips/boot/dts/brcm/bcm7362.dtsi      | 26 ++++++++++++++++++++++++++
 arch/mips/boot/dts/brcm/bcm97346dbsmb.dts |  8 ++++++++
 arch/mips/boot/dts/brcm/bcm97358svmb.dts  |  8 ++++++++
 arch/mips/boot/dts/brcm/bcm97360svmb.dts  |  8 ++++++++
 arch/mips/boot/dts/brcm/bcm97362svmb.dts  | 10 +++++++++-
 8 files changed, 137 insertions(+), 1 deletion(-)

-- 
2.4.0

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[PATCH 1/2] MIPS: BMIPS: dts: remove unsupported entry for bcm7362

[Jaedon Shin]

Remove unsupported memory entry for the bcm7362 platform. The BMIPS4380
processor only supports ZONE_NORMAL is not available for HIGHMEM.

Signed-off-by: Jaedon Shin <jaedon.shin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
---
 arch/mips/boot/dts/brcm/bcm97362svmb.dts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/mips/boot/dts/brcm/bcm97362svmb.dts b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
index b7b88e5dc9e7..ab8b01fa7dcf 100644
--- a/arch/mips/boot/dts/brcm/bcm97362svmb.dts
+++ b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
@@ -8,7 +8,7 @@
 
 	memory@0 {
 		device_type = "memory";
-		reg = <0x00000000 0x10000000>, <0x20000000 0x30000000>;
+		reg = <0x00000000 0x10000000>;
 	};
 
 	chosen {
-- 
2.4.0

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[PATCH 2/2] MIPS: BMIPS: dts: add uart device nodes to bcm7xxx platforms

[Jaedon Shin]

Add two uart device nodes known as the uart1 and uart2 for the bcm7xxx
platforms.

Signed-off-by: Jaedon Shin <jaedon.shin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
---
 arch/mips/boot/dts/brcm/bcm7346.dtsi      | 26 ++++++++++++++++++++++++++
 arch/mips/boot/dts/brcm/bcm7358.dtsi      | 26 ++++++++++++++++++++++++++
 arch/mips/boot/dts/brcm/bcm7360.dtsi      | 26 ++++++++++++++++++++++++++
 arch/mips/boot/dts/brcm/bcm7362.dtsi      | 26 ++++++++++++++++++++++++++
 arch/mips/boot/dts/brcm/bcm97346dbsmb.dts |  8 ++++++++
 arch/mips/boot/dts/brcm/bcm97358svmb.dts  |  8 ++++++++
 arch/mips/boot/dts/brcm/bcm97360svmb.dts  |  8 ++++++++
 arch/mips/boot/dts/brcm/bcm97362svmb.dts  |  8 ++++++++
 8 files changed, 136 insertions(+)

diff --git a/arch/mips/boot/dts/brcm/bcm7346.dtsi b/arch/mips/boot/dts/brcm/bcm7346.dtsi
index 1f30728a3177..d817bb46b934 100644
--- a/arch/mips/boot/dts/brcm/bcm7346.dtsi
+++ b/arch/mips/boot/dts/brcm/bcm7346.dtsi
@@ -24,6 +24,8 @@
 
 	aliases {
 		uart0 = &uart0;
+		uart1 = &uart1;
+		uart2 = &uart2;
 	};
 
 	cpu_intc: cpu_intc {
@@ -118,6 +120,30 @@
 			status = "disabled";
 		};
 
+		uart1: serial@406940 {
+			compatible = "ns16550a";
+			reg = <0x406940 0x20>;
+			reg-io-width = <0x4>;
+			reg-shift = <0x2>;
+			native-endian;
+			interrupt-parent = <&periph_intc>;
+			interrupts = <65>;
+			clocks = <&uart_clk>;
+			status = "disabled";
+		};
+
+		uart2: serial@406980 {
+			compatible = "ns16550a";
+			reg = <0x406980 0x20>;
+			reg-io-width = <0x4>;
+			reg-shift = <0x2>;
+			native-endian;
+			interrupt-parent = <&periph_intc>;
+			interrupts = <66>;
+			clocks = <&uart_clk>;
+			status = "disabled";
+		};
+
 		enet0: ethernet@430000 {
 			phy-mode = "internal";
 			phy-handle = <&phy1>;
diff --git a/arch/mips/boot/dts/brcm/bcm7358.dtsi b/arch/mips/boot/dts/brcm/bcm7358.dtsi
index 2c2aa9368f76..277a90adc1a7 100644
--- a/arch/mips/boot/dts/brcm/bcm7358.dtsi
+++ b/arch/mips/boot/dts/brcm/bcm7358.dtsi
@@ -18,6 +18,8 @@
 
 	aliases {
 		uart0 = &uart0;
+		uart1 = &uart1;
+		uart2 = &uart2;
 	};
 
 	cpu_intc: cpu_intc {
@@ -112,6 +114,30 @@
 			status = "disabled";
 		};
 
+		uart1: serial@406840 {
+			compatible = "ns16550a";
+			reg = <0x406840 0x20>;
+			reg-io-width = <0x4>;
+			reg-shift = <0x2>;
+			native-endian;
+			interrupt-parent = <&periph_intc>;
+			interrupts = <62>;
+			clocks = <&uart_clk>;
+			status = "disabled";
+		};
+
+		uart2: serial@406880 {
+			compatible = "ns16550a";
+			reg = <0x406880 0x20>;
+			reg-io-width = <0x4>;
+			reg-shift = <0x2>;
+			native-endian;
+			interrupt-parent = <&periph_intc>;
+			interrupts = <63>;
+			clocks = <&uart_clk>;
+			status = "disabled";
+		};
+
 		enet0: ethernet@430000 {
 			phy-mode = "internal";
 			phy-handle = <&phy1>;
diff --git a/arch/mips/boot/dts/brcm/bcm7360.dtsi b/arch/mips/boot/dts/brcm/bcm7360.dtsi
index f23b0aed276f..9e1e571ba346 100644
--- a/arch/mips/boot/dts/brcm/bcm7360.dtsi
+++ b/arch/mips/boot/dts/brcm/bcm7360.dtsi
@@ -18,6 +18,8 @@
 
 	aliases {
 		uart0 = &uart0;
+		uart1 = &uart1;
+		uart2 = &uart2;
 	};
 
 	cpu_intc: cpu_intc {
@@ -112,6 +114,30 @@
 			status = "disabled";
 		};
 
+		uart1: serial@406840 {
+			compatible = "ns16550a";
+			reg = <0x406840 0x20>;
+			reg-io-width = <0x4>;
+			reg-shift = <0x2>;
+			native-endian;
+			interrupt-parent = <&periph_intc>;
+			interrupts = <62>;
+			clocks = <&uart_clk>;
+			status = "disabled";
+		};
+
+		uart2: serial@406880 {
+			compatible = "ns16550a";
+			reg = <0x406880 0x20>;
+			reg-io-width = <0x4>;
+			reg-shift = <0x2>;
+			native-endian;
+			interrupt-parent = <&periph_intc>;
+			interrupts = <63>;
+			clocks = <&uart_clk>;
+			status = "disabled";
+		};
+
 		enet0: ethernet@430000 {
 			phy-mode = "internal";
 			phy-handle = <&phy1>;
diff --git a/arch/mips/boot/dts/brcm/bcm7362.dtsi b/arch/mips/boot/dts/brcm/bcm7362.dtsi
index da99db665bbc..6e65db86fc61 100644
--- a/arch/mips/boot/dts/brcm/bcm7362.dtsi
+++ b/arch/mips/boot/dts/brcm/bcm7362.dtsi
@@ -24,6 +24,8 @@
 
 	aliases {
 		uart0 = &uart0;
+		uart1 = &uart1;
+		uart2 = &uart2;
 	};
 
 	cpu_intc: cpu_intc {
@@ -118,6 +120,30 @@
 			status = "disabled";
 		};
 
+		uart1: serial@406840 {
+			compatible = "ns16550a";
+			reg = <0x406840 0x20>;
+			reg-io-width = <0x4>;
+			reg-shift = <0x2>;
+			native-endian;
+			interrupt-parent = <&periph_intc>;
+			interrupts = <62>;
+			clocks = <&uart_clk>;
+			status = "disabled";
+		};
+
+		uart2: serial@406880 {
+			compatible = "ns16550a";
+			reg = <0x406880 0x20>;
+			reg-io-width = <0x4>;
+			reg-shift = <0x2>;
+			native-endian;
+			interrupt-parent = <&periph_intc>;
+			interrupts = <63>;
+			clocks = <&uart_clk>;
+			status = "disabled";
+		};
+
 		enet0: ethernet@430000 {
 			phy-mode = "internal";
 			phy-handle = <&phy1>;
diff --git a/arch/mips/boot/dts/brcm/bcm97346dbsmb.dts b/arch/mips/boot/dts/brcm/bcm97346dbsmb.dts
index 70f196d89d26..3fe0445b9d37 100644
--- a/arch/mips/boot/dts/brcm/bcm97346dbsmb.dts
+++ b/arch/mips/boot/dts/brcm/bcm97346dbsmb.dts
@@ -21,6 +21,14 @@
 	status = "okay";
 };
 
+&uart1 {
+	status = "okay";
+};
+
+&uart2 {
+	status = "okay";
+};
+
 &enet0 {
 	status = "okay";
 };
diff --git a/arch/mips/boot/dts/brcm/bcm97358svmb.dts b/arch/mips/boot/dts/brcm/bcm97358svmb.dts
index d18e6d947739..a8dc01e30313 100644
--- a/arch/mips/boot/dts/brcm/bcm97358svmb.dts
+++ b/arch/mips/boot/dts/brcm/bcm97358svmb.dts
@@ -21,6 +21,14 @@
 	status = "okay";
 };
 
+&uart1 {
+	status = "okay";
+};
+
+&uart2 {
+	status = "okay";
+};
+
 &enet0 {
 	status = "okay";
 };
diff --git a/arch/mips/boot/dts/brcm/bcm97360svmb.dts b/arch/mips/boot/dts/brcm/bcm97360svmb.dts
index 4fe515500102..eee8b0e32681 100644
--- a/arch/mips/boot/dts/brcm/bcm97360svmb.dts
+++ b/arch/mips/boot/dts/brcm/bcm97360svmb.dts
@@ -21,6 +21,14 @@
 	status = "okay";
 };
 
+&uart1 {
+	status = "okay";
+};
+
+&uart2 {
+	status = "okay";
+};
+
 &enet0 {
 	status = "okay";
 };
diff --git a/arch/mips/boot/dts/brcm/bcm97362svmb.dts b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
index ab8b01fa7dcf..dd408413e922 100644
--- a/arch/mips/boot/dts/brcm/bcm97362svmb.dts
+++ b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
@@ -21,6 +21,14 @@
 	status = "okay";
 };
 
+&uart1 {
+	status = "okay";
+};
+
+&uart2 {
+	status = "okay";
+};
+
 &enet0 {
 	status = "okay";
 };
-- 
2.4.0

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Re: [PATCH 1/2] MIPS: BMIPS: dts: remove unsupported entry for bcm7362

[Kevin Cernekee]

On Fri, May 8, 2015 at 5:59 AM, Jaedon Shin <jaedon.shin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> Remove unsupported memory entry for the bcm7362 platform. The BMIPS4380
> processor only supports ZONE_NORMAL is not available for HIGHMEM.
>
> Signed-off-by: Jaedon Shin <jaedon.shin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> ---
>  arch/mips/boot/dts/brcm/bcm97362svmb.dts | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/mips/boot/dts/brcm/bcm97362svmb.dts b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
> index b7b88e5dc9e7..ab8b01fa7dcf 100644
> --- a/arch/mips/boot/dts/brcm/bcm97362svmb.dts
> +++ b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
> @@ -8,7 +8,7 @@
>
>         memory@0 {
>                 device_type = "memory";
> -               reg = <0x00000000 0x10000000>, <0x20000000 0x30000000>;
> +               reg = <0x00000000 0x10000000>;

Hmm, this is more of a kernel limitation than a hardware limitation,
though.  The board physically has 1GB of memory, right?  It is best if
the DT entry reflects the actual hardware configuration.

The Broadcom kernels enable the CPU's special "XKS01" feature to put
1GB of memory in ZONE_NORMAL:

https://github.com/Broadcom/stblinux-3.3/tree/master/linux

Maybe this would be worth adding to mainline.
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Re: [PATCH 1/2] MIPS: BMIPS: dts: remove unsupported entry for bcm7362

[Arnd Bergmann]

On Friday 08 May 2015 07:40:44 Kevin Cernekee wrote:
> On Fri, May 8, 2015 at 5:59 AM, Jaedon Shin <jaedon.shin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > Remove unsupported memory entry for the bcm7362 platform. The BMIPS4380
> > processor only supports ZONE_NORMAL is not available for HIGHMEM.
> >
> > Signed-off-by: Jaedon Shin <jaedon.shin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> > ---
> >  arch/mips/boot/dts/brcm/bcm97362svmb.dts | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/mips/boot/dts/brcm/bcm97362svmb.dts b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
> > index b7b88e5dc9e7..ab8b01fa7dcf 100644
> > --- a/arch/mips/boot/dts/brcm/bcm97362svmb.dts
> > +++ b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
> > @@ -8,7 +8,7 @@
> >
> >         memory@0 {
> >                 device_type = "memory";
> > -               reg = <0x00000000 0x10000000>, <0x20000000 0x30000000>;
> > +               reg = <0x00000000 0x10000000>;
> 
> Hmm, this is more of a kernel limitation than a hardware limitation,
> though.  The board physically has 1GB of memory, right?  It is best if
> the DT entry reflects the actual hardware configuration.
> 
> The Broadcom kernels enable the CPU's special "XKS01" feature to put
> 1GB of memory in ZONE_NORMAL:
> 
> https://github.com/Broadcom/stblinux-3.3/tree/master/linux
> 

What exactly is the kernel limitation here?

	Arnd
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Re: [PATCH 1/2] MIPS: BMIPS: dts: remove unsupported entry for bcm7362

[Jaedon Shin]

> On May 8, 2015, at 11:40 PM, Kevin Cernekee <cernekee-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> 
> On Fri, May 8, 2015 at 5:59 AM, Jaedon Shin <jaedon.shin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> Remove unsupported memory entry for the bcm7362 platform. The BMIPS4380
>> processor only supports ZONE_NORMAL is not available for HIGHMEM.
>> 
>> Signed-off-by: Jaedon Shin <jaedon.shin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
>> ---
>> arch/mips/boot/dts/brcm/bcm97362svmb.dts | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/arch/mips/boot/dts/brcm/bcm97362svmb.dts b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
>> index b7b88e5dc9e7..ab8b01fa7dcf 100644
>> --- a/arch/mips/boot/dts/brcm/bcm97362svmb.dts
>> +++ b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
>> @@ -8,7 +8,7 @@
>> 
>>        memory@0 {
>>                device_type = "memory";
>> -               reg = <0x00000000 0x10000000>, <0x20000000 0x30000000>;
>> +               reg = <0x00000000 0x10000000>;
> 
> Hmm, this is more of a kernel limitation than a hardware limitation,
> though.  The board physically has 1GB of memory, right?  It is best if
> the DT entry reflects the actual hardware configuration.
> 
> The Broadcom kernels enable the CPU's special "XKS01" feature to put
> 1GB of memory in ZONE_NORMAL:
> 
> https://github.com/Broadcom/stblinux-3.3/tree/master/linux
> 
> Maybe this would be worth adding to mainline.

Yes, It is right to have 1GB of memory.

In order to add XKS01 ioremap, keg, tlb, dma, etc. change is required.
However, there is no assurance they'll take those changes to the mainline.

Anyway, I also hope it added.--
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Re: [PATCH 1/2] MIPS: BMIPS: dts: remove unsupported entry for bcm7362

[Kevin Cernekee]

On Fri, May 8, 2015 at 8:32 AM, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
> On Friday 08 May 2015 07:40:44 Kevin Cernekee wrote:
>> On Fri, May 8, 2015 at 5:59 AM, Jaedon Shin <jaedon.shin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> > Remove unsupported memory entry for the bcm7362 platform. The BMIPS4380
>> > processor only supports ZONE_NORMAL is not available for HIGHMEM.
>> >
>> > Signed-off-by: Jaedon Shin <jaedon.shin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
>> > ---
>> >  arch/mips/boot/dts/brcm/bcm97362svmb.dts | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/arch/mips/boot/dts/brcm/bcm97362svmb.dts b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
>> > index b7b88e5dc9e7..ab8b01fa7dcf 100644
>> > --- a/arch/mips/boot/dts/brcm/bcm97362svmb.dts
>> > +++ b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
>> > @@ -8,7 +8,7 @@
>> >
>> >         memory@0 {
>> >                 device_type = "memory";
>> > -               reg = <0x00000000 0x10000000>, <0x20000000 0x30000000>;
>> > +               reg = <0x00000000 0x10000000>;
>>
>> Hmm, this is more of a kernel limitation than a hardware limitation,
>> though.  The board physically has 1GB of memory, right?  It is best if
>> the DT entry reflects the actual hardware configuration.
>>
>> The Broadcom kernels enable the CPU's special "XKS01" feature to put
>> 1GB of memory in ZONE_NORMAL:
>>
>> https://github.com/Broadcom/stblinux-3.3/tree/master/linux
>>
>
> What exactly is the kernel limitation here?

If we can't enable HIGHMEM, e.g. because the MIPS CPU has D$ aliases,
then Linux is supposed to ignore any RAM above the highmem/lowmem
boundary.

There is code in paging_init() that tries to do this.  Several years
ago it used to work, but the last time I tried it (~Oct 2014) it was
broken due to some other changes in MIPS early memory init, so Linux
hangs on boot unless you take the excess RAM out of DT.  Jaedon may be
running into the same issue.
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Re: [PATCH 1/2] MIPS: BMIPS: dts: remove unsupported entry for bcm7362

[Arnd Bergmann]

On Friday 08 May 2015 09:40:03 Kevin Cernekee wrote:
> On Fri, May 8, 2015 at 8:32 AM, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
> > On Friday 08 May 2015 07:40:44 Kevin Cernekee wrote:
> >> On Fri, May 8, 2015 at 5:59 AM, Jaedon Shin <jaedon.shin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >> > Remove unsupported memory entry for the bcm7362 platform. The BMIPS4380
> >> > processor only supports ZONE_NORMAL is not available for HIGHMEM.
> >> >
> >> > Signed-off-by: Jaedon Shin <jaedon.shin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> >> > ---
> >> >  arch/mips/boot/dts/brcm/bcm97362svmb.dts | 2 +-
> >> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >> >
> >> > diff --git a/arch/mips/boot/dts/brcm/bcm97362svmb.dts b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
> >> > index b7b88e5dc9e7..ab8b01fa7dcf 100644
> >> > --- a/arch/mips/boot/dts/brcm/bcm97362svmb.dts
> >> > +++ b/arch/mips/boot/dts/brcm/bcm97362svmb.dts
> >> > @@ -8,7 +8,7 @@
> >> >
> >> >         memory@0 {
> >> >                 device_type = "memory";
> >> > -               reg = <0x00000000 0x10000000>, <0x20000000 0x30000000>;
> >> > +               reg = <0x00000000 0x10000000>;
> >>
> >> Hmm, this is more of a kernel limitation than a hardware limitation,
> >> though.  The board physically has 1GB of memory, right?  It is best if
> >> the DT entry reflects the actual hardware configuration.
> >>
> >> The Broadcom kernels enable the CPU's special "XKS01" feature to put
> >> 1GB of memory in ZONE_NORMAL:
> >>
> >> https://github.com/Broadcom/stblinux-3.3/tree/master/linux
> >>
> >
> > What exactly is the kernel limitation here?
> 
> If we can't enable HIGHMEM, e.g. because the MIPS CPU has D$ aliases,
> then Linux is supposed to ignore any RAM above the highmem/lowmem
> boundary.
> 
> There is code in paging_init() that tries to do this.  Several years
> ago it used to work, but the last time I tried it (~Oct 2014) it was
> broken due to some other changes in MIPS early memory init, so Linux
> hangs on boot unless you take the excess RAM out of DT.  Jaedon may be
> running into the same issue.

Ok, I see. Could you avoid the problem by not requiring highmem?

We have some hacks on arch/arm/mach-realview to provide a nonlinear
virt_to_phys() function, which ends up moving all RAM underneath the
lowmem limit.

	Arnd
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Re: [PATCH 1/2] MIPS: BMIPS: dts: remove unsupported entry for bcm7362

[Kevin Cernekee]

On Fri, May 8, 2015 at 11:33 AM, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
>> > What exactly is the kernel limitation here?
>>
>> If we can't enable HIGHMEM, e.g. because the MIPS CPU has D$ aliases,
>> then Linux is supposed to ignore any RAM above the highmem/lowmem
>> boundary.
>>
>> There is code in paging_init() that tries to do this.  Several years
>> ago it used to work, but the last time I tried it (~Oct 2014) it was
>> broken due to some other changes in MIPS early memory init, so Linux
>> hangs on boot unless you take the excess RAM out of DT.  Jaedon may be
>> running into the same issue.
>
> Ok, I see. Could you avoid the problem by not requiring highmem?

I think the problem goes away if you disable CONFIG_HIGHMEM.

> We have some hacks on arch/arm/mach-realview to provide a nonlinear
> virt_to_phys() function, which ends up moving all RAM underneath the
> lowmem limit.

On this particular chip we can get a linear 1:1 virtual:physical
mapping for the first 1GB of system memory using the XKS01 hardware
feature and a bit of software support.  As Jaedon pointed out, a
couple of other things need to be tweaked in the arch/mips code:

 - Uncached/cached address conversions, as kseg1 is moved and reduced
 - Enabling XKS01 on each CPU thread on SMP boot, PM resume, or hot unplug
 - Setting aside a special VA range for uncached ioremaps, similar to
ARM, instead of assuming all kseg0 addresses have a corresponding
kseg1 address
 - Adding a wired TLB entry for PA 4000_0000 - 4fff_ffff

Without using XKS01, we'd need to use a nonlinear VA:PA mapping and
repurpose a large chunk of vmalloc space to map the upper 768MB of
system memory.  This would also require wired TLB entries and the
special ioremap range.  I never tried to make this work because the
XKS01 solution was cleaner.

Going beyond 1GB of system memory on this CPU would require HIGHMEM.
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