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Date: Thu, 18 Jul 2019 13:36:33 +0200
From: Heiko Carstens <heiko.carstens@de.ibm.com>
Subject: Re: Is __dma_direct_alloc_pages broken on s390?
References: <20190718091700.353b3721@ezekiel.suse.cz>
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Sender: linux-s390-owner@vger.kernel.org
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To: Petr Tesarik <ptesarik@suse.cz>
Cc: linux-s390@vger.kernel.org, Halil Pasic <pasic@linux.ibm.com>

On Thu, Jul 18, 2019 at 09:17:00AM +0200, Petr Tesarik wrote:
> Hi all,
> 
> while looking into DMA allocation, I noticed that
> __dma_direct_optimal_gfp_mask() in kernel/dma/direct.c can probably be
> improved. It uses GFP_DMA if dev->coherent_dma_mask is less than
> DMA_BIT_MASK(ARCH_ZONE_DMA_BITS). There is no s390-specific definition
> of ARCH_ZONE_DMA_BITS. The default is 24 bits, but the DMA zone on s390
> is 31 bits. CCW subchannel devices set sch->dev.coherent_dma_mask to
> DMA_BIT_MASK(31), which is greater than DMA_BIT_MASK(24), so buffers
> are allocated from the Normal zone first.
> 
> Would it make sense to set ARCH_ZONE_BITS to 31 on s390, or did I miss
> something?

No, this seems to be broken. Halil, can you look into this and provide
a patch?