[PATCH] ARM: dts: chromecast: override bad bootloader memory info

[PATCH] ARM: dts: chromecast: override bad bootloader memory info

[Thomas Hebb]

On the Chromecast, the bootloader provides us with an ATAG_MEM of
start=0x01000000 and size=0x3eff8000. This is clearly incorrect, as the
range given encompasses nearly a GiB but the Chromecast only has 512MiB
of RAM! Additionally, this causes the kernel to be decompressed at
0x00008000, below the claimed beginning of RAM, and so the boot fails.

Since the existing ATAG parsing code runs before the kernel is even
decompressed and irrevocably patches the device tree, don't even try
to bypass it. Instead, use the "linux,usable-memory" property instead
of the "reg" property to define the real range. The ATAG code only
overwrites reg, but linux,usable-memory is checked first in the OF
driver, so the fact that reg gets changed makes no difference.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
---
 arch/arm/boot/dts/berlin2cd-google-chromecast.dts | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/arm/boot/dts/berlin2cd-google-chromecast.dts b/arch/arm/boot/dts/berlin2cd-google-chromecast.dts
index 20f31cdeaf38..54221f55bfa2 100644
--- a/arch/arm/boot/dts/berlin2cd-google-chromecast.dts
+++ b/arch/arm/boot/dts/berlin2cd-google-chromecast.dts
@@ -52,7 +52,17 @@
 
 	memory@0 {
 		device_type = "memory";
-		reg = <0x00000000 0x20000000>; /* 512 MB */
+
+		/*
+		 * We're using "linux,usable-memory" instead of "reg" here
+		 * because the (signed and encrypted) bootloader that shipped
+		 * with this device provides an incorrect memory range in
+		 * ATAG_MEM. Linux helpfully overrides the "reg" property with
+		 * data from the ATAG, so we can't specify the proper range
+		 * normally. Fortunately, this alternate property is checked
+		 * first by the OF driver, so we can (ab)use it instead.
+		 */
+		linux,usable-memory = <0x00000000 0x20000000>; /* 512 MB */
 	};
 
 	leds {
-- 
2.17.0

Re: [PATCH] ARM: dts: chromecast: override bad bootloader memory info

[Jisheng Zhang]

Hi,

On Mon, 14 May 2018 17:56:45 -0400 Thomas Hebb  wrote:

> On the Chromecast, the bootloader provides us with an ATAG_MEM of
> start=0x01000000 and size=0x3eff8000. This is clearly incorrect, as the
> range given encompasses nearly a GiB but the Chromecast only has 512MiB
> of RAM! Additionally, this causes the kernel to be decompressed at
> 0x00008000, below the claimed beginning of RAM, and so the boot fails.
> 
> Since the existing ATAG parsing code runs before the kernel is even
> decompressed and irrevocably patches the device tree, don't even try

This means you enabled ARM_ATAG_DTB_COMPAT. could we disable it instead?
The ATAG is useless when we provide dtb. And IIRC, the ATAG is provided due
to legacy history code.

Thanks

> to bypass it. Instead, use the "linux,usable-memory" property instead
> of the "reg" property to define the real range. The ATAG code only
> overwrites reg, but linux,usable-memory is checked first in the OF
> driver, so the fact that reg gets changed makes no difference.
> 
> Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
> ---
>  arch/arm/boot/dts/berlin2cd-google-chromecast.dts | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/boot/dts/berlin2cd-google-chromecast.dts b/arch/arm/boot/dts/berlin2cd-google-chromecast.dts
> index 20f31cdeaf38..54221f55bfa2 100644
> --- a/arch/arm/boot/dts/berlin2cd-google-chromecast.dts
> +++ b/arch/arm/boot/dts/berlin2cd-google-chromecast.dts
> @@ -52,7 +52,17 @@
>  
>  	memory@0 {
>  		device_type = "memory";
> -		reg = <0x00000000 0x20000000>; /* 512 MB */
> +
> +		/*
> +		 * We're using "linux,usable-memory" instead of "reg" here
> +		 * because the (signed and encrypted) bootloader that shipped
> +		 * with this device provides an incorrect memory range in
> +		 * ATAG_MEM. Linux helpfully overrides the "reg" property with
> +		 * data from the ATAG, so we can't specify the proper range
> +		 * normally. Fortunately, this alternate property is checked
> +		 * first by the OF driver, so we can (ab)use it instead.
> +		 */
> +		linux,usable-memory = <0x00000000 0x20000000>; /* 512 MB */
>  	};
>  
>  	leds {

Re: [PATCH] ARM: dts: chromecast: override bad bootloader memory info

[Tom Hebb]

Hi,

On 05/14/2018 10:29 PM, Jisheng Zhang wrote:
> Hi,
> 
> On Mon, 14 May 2018 17:56:45 -0400 Thomas Hebb  wrote:
> 
>> On the Chromecast, the bootloader provides us with an ATAG_MEM of
>> start=0x01000000 and size=0x3eff8000. This is clearly incorrect, as the
>> range given encompasses nearly a GiB but the Chromecast only has 512MiB
>> of RAM! Additionally, this causes the kernel to be decompressed at
>> 0x00008000, below the claimed beginning of RAM, and so the boot fails.
>>
>> Since the existing ATAG parsing code runs before the kernel is even
>> decompressed and irrevocably patches the device tree, don't even try
> 
> This means you enabled ARM_ATAG_DTB_COMPAT. could we disable it instead?
> The ATAG is useless when we provide dtb. And IIRC, the ATAG is provided due
> to legacy history code.
> 
> Thanks

Thanks for the quick review! It's true that compiling without
ARM_ATAG_DTB_COMPAT will prevent ATAG_MEM from getting parsed at all.
However, it will also prevent ATAG_CMDLINE from getting parsed, and the
command line from the Chromecast's bootloader does actually contain some
useful information--notably the mode in which the system was booted
(normal or recovery).

Userspace could conceivably want this information, so it's preferable
for the kernel to boot regardless of whether ARM_ATAG_DTB_COMPAT is
enabled. That's the intent of this patch.

I do agree in principle that ARM_ATAG_DTB_COMPAT should be disabled
whenever possible.

>> to bypass it. Instead, use the "linux,usable-memory" property instead
>> of the "reg" property to define the real range. The ATAG code only
>> overwrites reg, but linux,usable-memory is checked first in the OF
>> driver, so the fact that reg gets changed makes no difference.
>>
>> Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
>> ---
>>  arch/arm/boot/dts/berlin2cd-google-chromecast.dts | 12 +++++++++++-
>>  1 file changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm/boot/dts/berlin2cd-google-chromecast.dts b/arch/arm/boot/dts/berlin2cd-google-chromecast.dts
>> index 20f31cdeaf38..54221f55bfa2 100644
>> --- a/arch/arm/boot/dts/berlin2cd-google-chromecast.dts
>> +++ b/arch/arm/boot/dts/berlin2cd-google-chromecast.dts
>> @@ -52,7 +52,17 @@
>>  
>>  	memory@0 {
>>  		device_type = "memory";
>> -		reg = <0x00000000 0x20000000>; /* 512 MB */
>> +
>> +		/*
>> +		 * We're using "linux,usable-memory" instead of "reg" here
>> +		 * because the (signed and encrypted) bootloader that shipped
>> +		 * with this device provides an incorrect memory range in
>> +		 * ATAG_MEM. Linux helpfully overrides the "reg" property with
>> +		 * data from the ATAG, so we can't specify the proper range
>> +		 * normally. Fortunately, this alternate property is checked
>> +		 * first by the OF driver, so we can (ab)use it instead.
>> +		 */
>> +		linux,usable-memory = <0x00000000 0x20000000>; /* 512 MB */
>>  	};
>>  
>>  	leds {
>