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Subject: Re: [PATCH] ARM: dts: chromecast: override bad bootloader memory info
To:     Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Cc:     linux-kernel@vger.kernel.org,
        Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        "moderated list:ARM/Synaptics Berlin SoC support" 
        <linux-arm-kernel@lists.infradead.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>
References: <20180514215645.17592-2-tommyhebb@gmail.com>
 <20180515102939.3ab8797c@xhacker.debian>
From:   Tom Hebb <tommyhebb@gmail.com>
Message-ID: <9d800fe2-6c59-1125-5d05-9ca0faa95150@gmail.com>
Date:   Tue, 15 May 2018 02:04:50 -0400
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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 {
>