Messages in this thread | | | Subject | Re: [PATCH 2/3] ACPI, APEI: Add RAM mapping support to ACPI | From | Huang Ying <> | Date | Mon, 30 Jan 2012 08:35:38 +0800 |
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On Sun, 2012-01-29 at 13:41 -0700, Bjorn Helgaas wrote: > On Sat, Jan 28, 2012 at 5:58 PM, Huang Ying <ying.huang@intel.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 think that is about the semantics of ioremap().
Hi, Ingo,
Can you describe why normal RAM is not allowed to be mapped by ioremap()?
> >> 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?
Copy from Tony's mail,
This (Ying: use kmap insted of ioremap to map RAM) might be a problem on ia64 - it is s/w responsibility to make sure that we don't map the same underlying physical address using different cache attributes - e.g. we must not map memory both cacheable and uncacheable at the same time. Accessing such a mis-attributed page will result in a machine check.
So I'd worry that if the memory in question was being used as uncacheable, this code might result in a cached access, which would crash the machine.
Best Regards, Huang Ying
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