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

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Date:   Fri, 14 Aug 2020 08:06:42 +0200
From:   Christoph Hellwig <hch@....de>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     amit.pundir@...aro.org, linux-kernel@...r.kernel.org,
        jeremy.linton@....com, iommu@...ts.linux-foundation.org,
        linux-rpi-kernel@...ts.infradead.org, rientjes@...gle.com,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v3 2/2] dma-pool: Only allocate from CMA when in same
 memory zone

On Sat, Aug 08, 2020 at 08:33:54AM +0200, Christoph Hellwig wrote:
> 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..

Do you plan to resend this one without the memblock_start_of_DRAM
thingy?