linux-kernel - Re: [PATCH] pci: acpi: Generic function for setting up PCI device DMA coherency

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Date:	Mon, 24 Aug 2015 13:14:59 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
Cc:	Rafael Wysocki <rjw@...ysocki.net>, Len Brown <lenb@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Hanjun Guo <hanjun.guo@...aro.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	Rob Herring <robh+dt@...nel.org>,
	Murali Karicheri <m-karicheri2@...com>,
	Jeremy Linton <jeremy.linton@....com>
Subject: Re: [PATCH] pci: acpi: Generic function for setting up PCI device DMA coherency

On Mon, Aug 24, 2015 at 12:09 PM, Suravee Suthikulpanit
<Suravee.Suthikulpanit@....com> wrote:

>>> commit 84cfb2213cd400fef227ec0d7829ec4e12895da9
>>> Author: Bjorn Helgaas <bhelgaas@...gle.com>
>>> Date:   Thu Aug 13 19:49:52 2015 -0500
>>>
>>>      ACPI / scan: Rename acpi_check_dma() to acpi_dma_is_coherent()
>>>
>>>      The name "acpi_check_dma()" doesn't give any much indication about
>>> what
>>>      exactly it checks.  The function also returns information both as a
>>> normal
>>>      return value and as the "bool *coherent" return parameter.  But
>>> "*coherent"
>>>      doesn't actually give any extra information: it is unchanged when
>>> returning
>>>      false and set to true when returning true.
>>>
>>>      Rename acpi_check_dma() to acpi_dma_is_coherent() so the callers
>>> read more
>>>      naturally.  Drop the return parameter and just use the function
>>> return
>>>      value.
>>>
>>>      Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
>
>
> This was because, at one point, we wanted to be able to differentiate
> between the case _CCA=0 and missing _CCA in ARM64, where we would support
> DMA (using arch-specific cache maintenance) if _CCA=0, and disable DMA when
> missing _CCA on ARM64.
>
> It seems like the logic is now required (please see
> https://www.mail-archive.com/linux-usb@vger.kernel.org/msg62735.html). So,
> we would need the true/false return, and the coherent variable to be able to
> differentiate between the two cases.
>
> Please let me know what you think.

It's hard for me to comment without seeing the actual patches.  I
think returning two values (_CCA-seen and coherent) is a confusing
interface.
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