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Date:   Thu, 21 Nov 2019 16:24:57 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
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        Andy Lutomirski <luto@kernel.org>,
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Subject: Re: [PATCH v2] dma-mapping: treat dev->bus_dma_mask as a DMA limit
Message-ID: <20191121152457.GA525@lst.de>
References: <20191121092646.8449-1-nsaenzjulienne@suse.de>
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On Thu, Nov 21, 2019 at 10:26:44AM +0100, Nicolas Saenz Julienne wrote:
> Using a mask to represent bus DMA constraints has a set of limitations.
> The biggest one being it can only hold a power of two (minus one). The
> DMA mapping code is already aware of this and treats dev->bus_dma_mask
> as a limit. This quirk is already used by some architectures although
> still rare.
> 
> With the introduction of the Raspberry Pi 4 we've found a new contender
> for the use of bus DMA limits, as its PCIe bus can only address the
> lower 3GB of memory (of a total of 4GB). This is impossible to represent
> with a mask. To make things worse the device-tree code rounds non power
> of two bus DMA limits to the next power of two, which is unacceptable in
> this case.
> 
> In the light of this, rename dev->bus_dma_mask to dev->bus_dma_limit all
> over the tree and treat it as such. Note that dev->bus_dma_limit should
> contain the higher accesible DMA address.
> 
> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>

I've tentatively added this patch to the dma-mapping tree based on
Robins principal approval of the last version.  That way tomorrows
linux-next run should still pick it up.