From: Arnd Bergmann <arnd@arndb.de>
To: "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@amd.com>
Cc: "linaro-acpi@lists.linaro.org" <linaro-acpi@lists.linaro.org>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
"catalin.marinas@arm.com" <catalin.marinas@arm.com>,
"rjw@rjwysocki.net" <rjw@rjwysocki.net>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"will.deacon@arm.com" <will.deacon@arm.com>,
"linux-acpi@vger.kernel.org" <linux-acpi@vger.kernel.org>,
"lenb@kernel.org" <lenb@kernel.org>,
Charles Garcia-Tobin <Charles.Garcia-Tobin@arm.com>
Subject: Re: [Linaro-acpi] [PATCH 2/2] ACPI / scan: Parse _CCA and setup device coherency
Date: Wed, 29 Apr 2015 17:54:02 +0200 [thread overview]
Message-ID: <2817500.sgztgK2NzB@wuerfel> (raw)
In-Reply-To: <D1665A4F.19E17%suravee.suthikulpanit@amd.com>
On Wednesday 29 April 2015 14:57:10 Suthikulpanit, Suravee wrote:
> Otherwise, it would seem inconsistent with what states in the ACPI spec:
>
> CCA objects are only relevant for devices that can access CPU-visible
> memory,
> such as devices that are DMA capable. On ARM based systems, the _CCA
> object
> must be supplied all such devices. On Intel platforms, if the _CCA
> object is
> not supplied, the OSPM will assume the devices are hardware cache
> coherent.
>
> From the statement above, I interpreted as if it is not present, it would
> be non-coherent.
>
My guess is that this section was included for Windows Phone, which runs
on embedded SoCs that usually have noncoherent DMA in a particular way.
Linux however only uses ACPI for servers, so that case does not happen.
I guess it would be reasonable to add a run-time warning here if you
try to do DMA on a device that does not have CCA set, and you should
probably set the DMA mask to 0 in that case as well.
Note that there are lots of ways in which you could have noncoherent DMA:
the default on ARM32 is that it requires uncached access or explicit
cache flushes, but it's also possible to have an SMP system where a device
is only coherent with some of the CPUs and requires explicit synchronization
(not flushes) otherwise. In a multi-level cache hierarchy, there could be
all sorts of combinations of flushes and syncs you would need to do.
With DT, we handle this using SoC-specific overrides for platforms that
are noncoherent in funny ways, see
http://lxr.free-electrons.com/source/arch/arm/mach-mvebu/coherency.c?v=3.18#L263
for instance. If we just disallow DMA to devices that are marked with _CCA=0
in ACPI, we can avoid this case, or discuss it by the time someone has hardware
that wants it, and then make a more informed decision about it.
Arnd
next prev parent reply other threads:[~2015-04-29 15:54 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-29 13:44 [PATCH 0/2] ACPI : Introduce support for _CCA object Suravee Suthikulpanit
2015-04-29 13:44 ` [PATCH 1/2] arm/arm64: ACPI: Introduce CONFIG_ACPI_MUST_HAVE_CCA Suravee Suthikulpanit
2015-04-29 14:04 ` Catalin Marinas
2015-04-29 14:31 ` Suravee Suthikulpanit
2015-04-29 14:42 ` Catalin Marinas
2015-04-29 14:44 ` Suravee Suthikulpanit
2015-04-30 13:47 ` Hanjun Guo
2015-04-30 13:50 ` Will Deacon
2015-04-30 14:14 ` Hanjun Guo
2015-04-30 15:01 ` Lorenzo Pieralisi
2015-04-29 13:44 ` [PATCH 2/2] ACPI / scan: Parse _CCA and setup device coherency Suravee Suthikulpanit
2015-04-29 14:03 ` Arnd Bergmann
2015-04-29 14:45 ` Suravee Suthikulpanit
2015-04-29 14:47 ` [Linaro-acpi] " Arnd Bergmann
2015-04-29 14:57 ` Suthikulpanit, Suravee
2015-04-29 15:39 ` Al Stone
2015-04-29 16:15 ` Arnd Bergmann
2015-04-29 15:54 ` Arnd Bergmann [this message]
2015-05-01 11:06 ` Catalin Marinas
2015-05-08 14:08 ` Arnd Bergmann
2015-05-11 17:10 ` Catalin Marinas
2015-05-11 17:24 ` Robin Murphy
2015-04-29 16:25 ` Arnd Bergmann
2015-04-29 21:53 ` Suravee Suthikulpanit
2015-04-30 8:23 ` [Linaro-acpi] " Arnd Bergmann
2015-04-30 10:41 ` Will Deacon
2015-04-30 10:47 ` Arnd Bergmann
2015-04-30 11:07 ` Will Deacon
2015-04-30 11:24 ` Arnd Bergmann
2015-04-30 11:46 ` Will Deacon
2015-04-30 13:03 ` Arnd Bergmann
2015-04-30 13:13 ` Will Deacon
2015-04-30 13:52 ` Arnd Bergmann
2015-04-30 15:55 ` Catalin Marinas
2015-05-08 14:01 ` Arnd Bergmann
2015-04-30 23:39 ` Suravee Suthikulanit
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