Re: [PATCH 1/2] ARM: dts: bcm2711: force CMA into first GB of memory

[Florian Fainelli <f.fainelli@gmail.com>]

On 11/4/19 9:51 AM, Florian Fainelli wrote:
> On 11/4/19 9:09 AM, Stefan Wahren wrote:
> 
> [snip]
> 
>>> +	reserved-memory {
>>> +		#address-cells = <2>;
>>> +		#size-cells = <1>;
>>> +		ranges;
>>> +
>>> +		/*
>>> +		 * arm64 reserves the CMA by default somewhere in ZONE_DMA32,
>>> +		 * that's not good enough for the Raspberry Pi 4 as some
>>> +		 * devices can only address the lower 1G of memory (ZONE_DMA).
>>> +		 */
>>> +		linux,cma {
>>> +			compatible = "shared-dma-pool";
>>> +			size = <0x2000000>; /* 32MB */
>>> +			alloc-ranges = <0x0 0x00000000 0x40000000>;
>>> +			reusable;
>>> +			linux,cma-default;
>>> +		};
>>> +	};
>>> +
>>
>> i think this is a SoC-specific issue not a board specifc one. Please
>> move this to bcm2711.dtsi
> 
> This sounds like a possibly fragile solution if someone changes
> CONFIG_CMA_SIZE_MBYTES to a value greater than 32MB no?
> 
> I know we don't want machine descriptors for ARM64 kernels, but since
> there is already a specific 2711 machine compatible string check, maybe
> you could use that as well for determining whether arm64_dma_phys_limit
> or arm64_dma32_phys_limit should be chosen?

This last sentence was referring to an earlier version of another patch
series, this is not being done right now, although ARCH_BCM2835 does
forcibly select ZONE_DMA. Nevermind then, I do not see a cleaner
solution right now either.
-- 
Florian