Re: [RFC/PATCH v2 1/8] Add arch_phys_wc_{add,del} to manipulate WCMTRRs if needed

[Daniel Vetter]

On Fri, May 10, 2013 at 11:00:56AM -0700, Andy Lutomirski wrote:
> On Fri, May 10, 2013 at 2:19 AM, Daniel Vetter <daniel@xxxxxxxx> wrote:
> > On Thu, May 09, 2013 at 12:46:20PM -0700, Andy Lutomirski wrote:
> >> Several drivers currently use mtrr_add through various #ifdef guards
> >> and/or drm wrappers.  The vast majority of them want to add WC MTRRs
> >> on x86 systems and don't actually need the MTRR if PAT (i.e.
> >> ioremap_wc, etc) are working.
> >>
> >> arch_phys_wc_add and arch_phys_wc_del are new functions, available
> >> on all architectures and configurations, that add WC MTRRs on x86 if
> >> needed (and handle errors) and do nothing at all otherwise.  They're
> >> also easier to use than mtrr_add and mtrr_del, so the call sites can
> >> be simplified.
> >>
> >> As an added benefit, this will avoid wasting MTRRs and possibly
> >> warning pointlessly on PAT-supporting systems.
> >>
> >> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> >> ---
> >>  arch/x86/include/asm/io.h       |  7 ++++++
> >>  arch/x86/include/asm/mtrr.h     |  5 ++++-
> >>  arch/x86/kernel/cpu/mtrr/main.c | 48 +++++++++++++++++++++++++++++++++++++++++
> >>  include/linux/io.h              | 25 +++++++++++++++++++++
> >>  4 files changed, 84 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
> >> index d8e8eef..34f69cb 100644
> >> --- a/arch/x86/include/asm/io.h
> >> +++ b/arch/x86/include/asm/io.h
> >> @@ -345,4 +345,11 @@ extern bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
> >>
> >>  #define IO_SPACE_LIMIT 0xffff
> >>
> >> +#ifdef CONFIG_MTRR
> >> +extern int __must_check arch_phys_wc_add(unsigned long base,
> >> +                                      unsigned long size);
> >> +extern void arch_phys_wc_del(int handle);
> >> +#define arch_phys_wc_add arch_phys_wc_add
> >> +#endif
> >> +
> >>  #endif /* _ASM_X86_IO_H */
> >> diff --git a/arch/x86/include/asm/mtrr.h b/arch/x86/include/asm/mtrr.h
> >> index e235582..10d0fba 100644
> >> --- a/arch/x86/include/asm/mtrr.h
> >> +++ b/arch/x86/include/asm/mtrr.h
> >> @@ -26,7 +26,10 @@
> >>  #include <uapi/asm/mtrr.h>
> >>
> >>
> >> -/*  The following functions are for use by other drivers  */
> >> +/*
> >> + * The following functions are for use by other drivers that cannot use
> >> + * arch_phys_wc_add and arch_phys_wc_del.
> >> + */
> >>  # ifdef CONFIG_MTRR
> >>  extern u8 mtrr_type_lookup(u64 addr, u64 end);
> >>  extern void mtrr_save_fixed_ranges(void *);
> >> diff --git a/arch/x86/kernel/cpu/mtrr/main.c b/arch/x86/kernel/cpu/mtrr/main.c
> >> index 726bf96..23bd49a 100644
> >> --- a/arch/x86/kernel/cpu/mtrr/main.c
> >> +++ b/arch/x86/kernel/cpu/mtrr/main.c
> >> @@ -51,6 +51,7 @@
> >>  #include <asm/e820.h>
> >>  #include <asm/mtrr.h>
> >>  #include <asm/msr.h>
> >> +#include <asm/pat.h>
> >>
> >>  #include "mtrr.h"
> >>
> >> @@ -524,6 +525,53 @@ int mtrr_del(int reg, unsigned long base, unsigned long size)
> >>  }
> >>  EXPORT_SYMBOL(mtrr_del);
> >>
> >> +/**
> >> + * arch_phys_wc_add - add a WC MTRR and handle errors if PAT is unavailable
> >> + * @base: Physical base address
> >> + * @size: Size of region
> >> + *
> >> + * If PAT is available, this does nothing.  If PAT is unavailable, it
> >> + * attempts to add a WC MTRR covering size bytes starting at base and
> >> + * logs an error if this fails.
> >> + *
> >> + * Drivers must store the return value to pass to mtrr_del_wc_if_needed,
> >> + * but drivers should not try to interpret that return value.
> >> + */
> >> +int arch_phys_wc_add(unsigned long base, unsigned long size)
> >> +{
> >> +     int ret;
> >> +
> >> +     if (pat_enabled)
> >> +             return 0;  /* Success!  (We don't need to do anything.) */
> >
> > Shouldn't we #define a big number for this case since mtrr_add returns
> > 0-based mtrr indices? Rather unlikely that the very first mtrr is unused I
> > know, but still feels like a cleaner interface. And we don't need to leak
> > that #define out at all to users of this interface.
> 
> I did something more like that in v1, but I like having the property
> that arch_phys_wc_del(0) does nothing as opposed to possibly breaking
> the system.  Given that almost all of these things are stored in
> kzalloced space, this seems safer to me.  The return value here could
> just as easily be -ENOSYS, but nothing should care.
> 
> There is an issue, though: the drm maps interface is leaking the
> return values to userspace via a couple of ioctls, so I should fix it
> to at least return the correct value.  (Why that interface includes an
> mtrr number is a mystery to me.)

Old drm interfaces are horrible. Best approach is to simply stay far away
from them ...

> Have I convinced you that my bike shed color is okay?  I'll send out a
> v3 later today with a fix for the leaking-to-userspace issue and I'll
> fix the i915 thing.

A comment in your arch_phys_wc_add/del functions that we do have an mtrr
with index 0, but won't ever get that would be fine with me. Still feels a
bit irky though ;-)

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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