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Subject: Re: [PATCH] of/fdt: Make sure no-map does not remove already reserved
 regions
From:   Jan Kiszka <jan.kiszka@siemens.com>
To:     Nicolas Boichat <drinkcat@chromium.org>,
        Rob Herring <robh+dt@kernel.org>,
        Andre Przywara <andre.przywara@arm.com>,
        Phil Elwell <phil@raspberrypi.org>
Cc:     Frank Rowand <frowand.list@gmail.com>, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Ian Campbell <ian.campbell@citrix.com>,
        Grant Likely <grant.likely@linaro.org>,
        Stephen Boyd <swboyd@chromium.org>
References: <20190703050827.173284-1-drinkcat@chromium.org>
 <12b02977-d038-8fc7-d61e-e694a6b90f7b@siemens.com>
Message-ID: <5154396c-fffd-8e9d-3e2e-860fff35e9fc@siemens.com>
Date:   Mon, 22 Mar 2021 19:05:12 +0100
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On 22.03.21 08:58, Jan Kiszka wrote:
> On 03.07.19 07:08, Nicolas Boichat wrote:
>> If the device tree is incorrectly configured, and attempts to
>> define a "no-map" reserved memory that overlaps with the kernel
>> data/code, the kernel would crash quickly after boot, with no
>> obvious clue about the nature of the issue.
>>
>> For example, this would happen if we have the kernel mapped at
>> these addresses (from /proc/iomem):
>> 40000000-41ffffff : System RAM
>>   40080000-40dfffff : Kernel code
>>   40e00000-411fffff : reserved
>>   41200000-413e0fff : Kernel data
>>
>> And we declare a no-map shared-dma-pool region at a fixed address
>> within that range:
>> mem_reserved: mem_region {
>> 	compatible = "shared-dma-pool";
>> 	reg = <0 0x40000000 0 0x01A00000>;
>> 	no-map;
>> };
>>
>> To fix this, when removing memory regions at early boot (which is
>> what "no-map" regions do), we need to make sure that the memory
>> is not already reserved. If we do, __reserved_mem_reserve_reg
>> will throw an error:
>> [    0.000000] OF: fdt: Reserved memory: failed to reserve memory
>>    for node 'mem_region': base 0x0000000040000000, size 26 MiB
>> and the code that will try to use the region should also fail,
>> later on.
>>
>> We do not do anything for non-"no-map" regions, as memblock
>> explicitly allows reserved regions to overlap, and the commit
>> that this fixes removed the check for that precise reason.
>>
>> Fixes: 094cb98179f19b7 ("of/fdt: memblock_reserve /memreserve/ regions in the case of partial overlap")
>> Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
>> ---
>>  drivers/of/fdt.c | 10 +++++++++-
>>  1 file changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
>> index cd17dc62a71980a..a1ded43fc332d0c 100644
>> --- a/drivers/of/fdt.c
>> +++ b/drivers/of/fdt.c
>> @@ -1138,8 +1138,16 @@ int __init __weak early_init_dt_mark_hotplug_memory_arch(u64 base, u64 size)
>>  int __init __weak early_init_dt_reserve_memory_arch(phys_addr_t base,
>>  					phys_addr_t size, bool nomap)
>>  {
>> -	if (nomap)
>> +	if (nomap) {
>> +		/*
>> +		 * If the memory is already reserved (by another region), we
>> +		 * should not allow it to be removed altogether.
>> +		 */
>> +		if (memblock_is_region_reserved(base, size))
>> +			return -EBUSY;
>> +
>>  		return memblock_remove(base, size);
>> +	}
>>  	return memblock_reserve(base, size);
>>  }
>>  
>>
> 
> Likely the wrong patch to blame but hopefully the right audience:
> 
> I'm trying to migrate my RPi4 setup to mainline, and this commit breaks 
> booting with TF-A (current master) in the loop. Error:
> 
> [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x410fd083]                                                                                                                                                                        
> [    0.000000] Linux version 5.10.24+ (jan@md1f2u6c) (aarch64-linux-gnu-gcc (GNU Toolchain for the A-profile Architecture 9.2-2019.12 (arm-9.10)) 9.2.1 20191025, GNU ld (GNU Toolchain for the A-profile Architecture 9.2-2019.12 (arm-9.10)1
> [    0.000000] Machine model: Raspberry Pi 4 Model B Rev 1.1                                                                                                                                                                                  
> [    0.000000] efi: UEFI not found.                                                                                                                                                                                                           
> [    0.000000] OF: fdt: Reserved memory: failed to reserve memory for node 'atf@0': base 0x0000000000000000, size 0 MiB                                                                                                                       
> 
> And then we hang later on when Linux does start to use that memory and 
> seems to trigger an exception.
> 
> Is there a bug in the upstream RPi4 DT?
> 

FWIW, this is triggering the conflict:

(arch/arm/boot/dts/bcm283x.dtsi)

/* firmware-provided startup stubs live here, where the secondary CPUs are
 * spinning.
 */
/memreserve/ 0x00000000 0x00001000;

I strongly suspect this is only needed in case of TF-A-free boot. With 
TF-A we have standard PCSI (my motivation to use TF-A in the first 
place) - and then this is in conflict with the firmware's reservation.

Do we need separate DTs for this use case? Or should TF-A account for 
this?

Jan

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