Provide more information about RAM from the bootloader

[konradybcio]

Qualcomm currently does

/memory[@somewhere]/ddr_device_type =

• 5 - LPDDR3 # 3X never existed, 3E (3 overclocked) - did
• 6 - LPDDR4
• 7 - LPDDR4X
• 8 - LPDDR5
• 9 - LPDDR5X

with 1, 2, 3, 4 presumably going to LPDDR(1 / 2_S2 / 2_S4 / 2N), though no sources prove that

Introducing a common property to propagate that information to the operating system seems rather useful. In this Qualcomm-specific example, many on-SoC IP blocks need different settings based on the DDR type.

[krzk]

I think this duplicates a bit Documentation/devicetree/bindings/memory-controllers/ddr/ in Linux kernel. We already describe memory under memory controllers. I guess the top level memory is only for parsing size/location for FW/OS.

[lumag]

@krzk Generally I'd agree with you. The problem is:

• This property has unfortunately already been used by the existing Qualcomm bootloaders,
• Several bits in the GPU and display drivers should take DDR type into account when programming registers

I hate to ask this, but is there a chance to receive a pardon for this property, provided it is limited to Qualcomm platorms?

[robherring]

Go look at 'lshw'. It already supports some similar properties in the memory nodes dating back to PowerMac days.

[lumag]

@robherring I'm not sure if you comment is 'pro' or 'contra', could you please be more specific?

[robherring]

I'm saying if what's supported by lshw works for you, then I'd be open to supporting it.

However, note that with UEFI boot, there are no memory nodes...

[lumag]

@robherring no, they are not supported by lshw.

We are currently in the following position: the existing bootloader writes this DT property. While we potentially can try persuading Qualcomm to use correct DT entry (no guarantees on the result though), existing devices are already ddr_device_type. The bootloader is cryptographically signed by the device vendor, so updating old them on existing hardware is close to being impossible.

Parsing this DDR memory type becomes important for the display and GPU drivers to properly setup the hardware. Thus we are asking for an "exception": we'd like to document the mentioned /memory@[something]/ddr_device_type for Qualcomm devices and use this DT property.