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Date:   Sat, 8 Aug 2020 08:33:54 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Cc:     Christoph Hellwig <hch@lst.de>, amit.pundir@linaro.org,
        linux-kernel@vger.kernel.org,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>, rientjes@google.com,
        jeremy.linton@arm.com, linux-rpi-kernel@lists.infradead.org,
        iommu@lists.linux-foundation.org
Subject: Re: [PATCH v3 2/2] dma-pool: Only allocate from CMA when in same
 memory zone
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On Fri, Aug 07, 2020 at 10:50:19AM +0200, Nicolas Saenz Julienne wrote:
> 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.

The pool must be about a 32-bit physical address.  The offsets can be
different for every device..