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

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

On Thu, Jul 25, 2024 at 02:49:01PM +0300, Baruch Siach wrote:
> Hi Catalin,
> 
> On Tue, Jun 18 2024, Catalin Marinas wrote:
> > 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 won't argue with that assertion. My target system RAM happens to start
> at that the lower end of devices DMA zone, so I'm fine with skipping
> this patch.
> 
> Just curious. What is the inherent limitation that prevents Linux from
> supporting DMA zone with lower limit above RAM start?

It's the way the zone allocation fallback mechanism works. Let's say a
ZONE_DMA32 allocation fails, it falls back to ZONE_DMA and it's supposed
to be compatible with the GFP_DMA32 request. If you have some other zone
below ZONE_DMA, it should also be compatible with GFP_DMA allocations.

-- 
Catalin