linux-kernel - Re: [PATCH 2/3] ACPI, APEI: Add RAM mapping support to ACPI

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Date:	Sun, 29 Jan 2012 13:41:53 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Huang Ying <ying.huang@...el.com>
Cc:	Myron Stowe <myron.stowe@...hat.com>, lenb@...nel.org,
	linux-acpi@...r.kernel.org, rjw@...k.pl, trenn@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] ACPI, APEI: Add RAM mapping support to ACPI

On Sat, Jan 28, 2012 at 5:58 PM, Huang Ying <ying.huang@...el.com> wrote:
> Hi, Bjorn,
>
> Sorry for late.  Just return from Chinese new year holiday.
>
> On Sat, 2012-01-21 at 08:04 -0700, Bjorn Helgaas wrote:
> [snip]
>> > +
>> > +static void __iomem *acpi_map(acpi_physical_address pg_off, unsigned long pg_sz)
>> > +{
>> > +       unsigned long pfn;
>> > +
>> > +       pfn = pg_off >> PAGE_SHIFT;
>> > +       if (should_use_kmap(pfn)) {
>> > +               if (pg_sz > PAGE_SIZE)
>> > +                       return NULL;
>> > +               return (void __iomem __force *)kmap(pfn_to_page(pfn));
>> > +       } else
>> > +               return acpi_os_ioremap(pg_off, pg_sz);
>>
>> This implies that ioremap() works differently on ia64 than on x86.
>> Apparently one can ioremap() RAM on x86, but not on ia64.  Why is this
>> different?  Shouldn't we instead fix ioremap() on ia64 so it works the
>> same as on x86?
>
> If my understanding were correct, ioremap can not work for RAM on x86.
> So we need to use kmap for RAM.  And on IA64, ioremap works for RAM and
> will take care of cache attributes while kmap will not.  So ioremap is
> used on IA64, while kmap is used on x86.

My point is that the *user* of ioremap() shouldn't need to care what
architecture we're on.  For example, maybe the ioremap()
implementation could be changed so that it uses kmap() internally when
necessary.

>> I looked at the ia64 ioremap(), and I can't see the reason it fails
>> for RAM.  Huang, do you remember the details from 76da3fb3575?

This question is still open.  Do you remember anything about it?

Bjorn
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