linux-kernel - Re: [PATCH v3 2/2] dma-pool: Only allocate from CMA when in same memory zone

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Date:   Fri, 07 Aug 2020 10:50:19 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     amit.pundir@...aro.org, linux-kernel@...r.kernel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>, rientjes@...gle.com,
        jeremy.linton@....com, linux-rpi-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v3 2/2] dma-pool: Only allocate from CMA when in same
 memory zone

On Fri, 2020-08-07 at 07:21 +0200, Christoph Hellwig wrote:
> On Thu, Aug 06, 2020 at 08:47:55PM +0200, Nicolas Saenz Julienne wrote:
> > There is no guarantee to CMA's placement, so allocating a zone specific
> > atomic pool from CMA might return memory from a completely different
> > memory zone. To get around this double check CMA's placement before
> > allocating from it.
> 
> As the builtbot pointed out, memblock_start_of_DRAM can't be used from
> non-__init code.  But lookig at it I think throwing that in
> is bogus anyway, as cma_get_base returns a proper physical address
> already.

It does indeed, but I'm comparing CMA's base with bitmasks that don't take into
account where the memory starts. Say memory starts at 0x80000000, and CMA falls
into ZONE_DMA [0x80000000 0xC0000000], if you want to compare it with
DMA_BIT_MASK(zone_dma_bits) you're forced to unify the memory bases.

That said, I now realize that this doesn't work for ZONE_DMA32 which has a hard
limit on 32bit addresses reglardless of the memory base.

That said I still need to call memblock_start_of_DRAM() any suggestions WRT
that? I could save the value in dma_atomic_pool_init(), which is __init code.

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