Re: [PATCH RFC v2 2/5] of: get dma area lower limit

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Apr 09, 2024 at 09:17:55AM +0300, Baruch Siach wrote:
> of_dma_get_max_cpu_address() returns the highest CPU address that
> devices can use for DMA. The implicit assumption is that all CPU
> addresses below that limit are suitable for DMA. However the
> 'dma-ranges' property this code uses also encodes a lower limit for DMA
> that is potentially non zero.
> 
> Rename to of_dma_get_cpu_limits(), and extend to retrieve also the lower
> limit for the same 'dma-ranges' property describing the high limit.

I don't understand the reason for the lower limit. The way the Linux
zones work is that ZONE_DMA always starts from the start of the RAM. It
doesn't matter whether it's 0 or not, you'd not allocate below the start
of RAM anyway. If you have a device that cannot use the bottom of the
RAM, it is pretty broken and not supported by Linux.

I think you added this limit before we tried to move away from
zone_dma_bits to a non-power-of-two limit (zone_dma_limit). With the
latter, we no longer need tricks with the lower limit,
of_dma_get_max_cpu_address() should capture the smallest upper CPU
address limit supported by all devices (and that's where ZONE_DMA should
end).

-- 
Catalin