[Linaro-acpi] [PATCH v8 02/21] acpi: fix acpi_os_ioremap for arm64

[Ard Biesheuvel]

On 5 February 2015 at 17:48, Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Thu, Feb 05, 2015 at 04:42:19PM +0000, Al Stone wrote:
>> On 02/05/2015 06:54 AM, Mark Salter wrote:
>> > On Thu, 2015-02-05 at 10:41 +0000, Catalin Marinas wrote:
>> >> On Wed, Feb 04, 2015 at 06:58:14PM +0000, Mark Salter wrote:
>> >>> On Wed, 2015-02-04 at 17:57 +0000, Catalin Marinas wrote:
>> >>>> On Wed, Feb 04, 2015 at 04:08:27PM +0000, Mark Salter wrote:
>> >>>>> acpi_os_remap() is used to map ACPI tables. These tables may be in ram
>> >>>>> which are already included in the kernel's linear RAM mapping. So we
>> >>>>> need ioremap_cache to avoid two mappings to the same physical page
>> >>>>> having different caching attributes.
>> >>>>
>> >>>> What's the call path to acpi_os_ioremap() on such tables already in the
>> >>>> linear mapping? I can see an acpi_map() function which already takes
>> >>>> care of the RAM mapping case but there are other cases where
>> >>>> acpi_os_ioremap() is called directly. For example,
>> >>>> acpi_os_read_memory(), can it be called on both RAM and I/O?
>> >>>
>> >>> acpi_map() is the one I've seen.
>> >>
>> >> By default, if should_use_kmap() is not patched for arm64, it translates
>> >> to page_is_ram(); acpi_map() would simply use a kmap() which returns the
>> >> current kernel linear mapping on arm64.
>> >
>> > The problem with kmap() is that it only maps a single page. I've seen
>> > tables over 4k which is why I patched acpi_map() not to use kmap() on
>> > arm64.
>>
>> Right.  Mark replied to this before I could; using kmap() enforced a 4k
>> (one page) limit that we kept breaking with some ACPI tables being larger
>> than that (DSDTs and SSDTs, fwiw).  This would lead to some very odd behaviors
>> when most but not all of a device definition was within the page; using the
>> table checksums was one way of detecting the issues.
>
> OK. So I think Mark's original patch was ok, assuming that the System
> Memory cases mentioned by Graeme are detected with page_is_ram().
>

page_is_ram() returns whether a pfn is covered by the linear mapping,
so memory before the kernel or after a mem= limit will be
misidentified.