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Date: Tue, 8 Sep 2020 07:44:16 +0200
From: Christoph Hellwig <hch@lst.de>
To: Alexey Kardashevskiy <aik@ozlabs.ru>
Subject: Re: [PATCH kernel] powerpc/dma: Fix dma_map_ops::get_required_mask
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Cc: Oliver O'Halloran <oohall@gmail.com>, linuxppc-dev@lists.ozlabs.org,
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On Tue, Sep 08, 2020 at 11:51:06AM +1000, Alexey Kardashevskiy wrote:
> What is dma_get_required_mask() for anyway? What "requires" what here?

Yes, it is a really odd API.  It comes from classic old PCI where
64-bit addressing required an additional bus cycle, and various devices
had different addressing schemes, with the smaller addresses beeing
more efficient.  So this allows the driver to request the "required"
addressing mode to address all memory.  "preferred" might be a better
name as we'll bounce buffer if it isn't met.  I also don't really see
why a driver would ever want to use it for a modern PCIe device.