On Tue, Nov 24, 2020 at 04:41:04PM +1100, David Gibson wrote:
> Hi Tom,
>
> Sorry I've taken so long to reply to this. I was pretty busy, and
> then on holiday away from my email.
Thanks for explaining.
> Overall I think this looks good, but there are some nits and some
> inaccuracies in comments.
>
> On Thu, Nov 05, 2020 at 11:57:07AM -0500, Tom Rini wrote:
> > Commits 6dcb8ba4 "libfdt: Add helpers for accessing unaligned words"
> > introduced changes to support unaligned reads for ARM platforms and
> > 11738cf01f15 "libfdt: Don't use memcpy to handle unaligned reads on ARM"
> > improved the performance of these helpers.
> >
> > On further discussion, while there are potential cases where we could be
> > used on platforms that do not fixup unaligned reads for us, making this
> > choice the default is very expensive in terms of binary size and access
> > time. To address this, introduce and use new _fdt{32,64}_ld functions
> > that call fdt{32,64}_to_cpu() as was done prior to the above mentioned
> > commits. Leave the existing load functions as unaligned-safe and
> > include comments in both cases.
>
> So, leading underscore identifiers are off limits for libfdt - they're
> reserved for the system libraries (the kernel can get away with it
> because it doesn't use the system libraries, but we sometimes do).
>
> The usual workaround is to use a trailing underscore instead
> (e.g. fdt_add_property_(), fdt_splice_struct_()). Although.. the
> convention with those (similar to the kernel's) is that foo() is
> usually a safer wrapper implemented in terms of foo_(). In this case
> fdtXX_ld() is not, and cannot, be implemented in terms of fdtXX_ld_().
Alright, so not _foo(). foo__() to indicate it's special?
> [snip]
> > const char *fdt_get_alias_namelen(const void *fdt,
> > diff --git a/libfdt/libfdt.h b/libfdt/libfdt.h
> > index b600c8d6dd41..3f36ed6d690f 100644
> > --- a/libfdt/libfdt.h
> > +++ b/libfdt/libfdt.h
> > @@ -126,12 +126,10 @@ static inline void *fdt_offset_ptr_w(void *fdt, int offset, int checklen)
> > uint32_t fdt_next_tag(const void *fdt, int offset, int *nextoffset);
> >
> > /*
> > - * Alignment helpers:
> > - * These helpers access words from a device tree blob. They're
> > - * built to work even with unaligned pointers on platforms (ike
> > - * ARM) that don't like unaligned loads and stores
> > + * External helpers to access words from a device tree blob. These functions
> > + * work in a manner that is safe on platforms that do not handle unaligned
> > + * memory accesses and need special care in those cases.
>
> I actually prefer the old wording for the most part. Could you just
> tweak it for the changes, rather than rewriting the whole thing?
I rewrote it because I didn't like the level of accuracy in the existing
comment. As Rob detailed in the other thread, yes, ARM could be a
problem, but only if you don't enable the feature that's virtually
always enabled on cores that have it. But, I don't feel so strongly
about it that it's worth delaying this either. Do you prefer:
External helpers to access words from a device tree blob. They're built
to work even with unaligned pointers on platforms (like ARM) that don't
like unaligned loads and stores.
Or:
External helpers to access words from a device tree blob. They're built
to work even with unaligned pointers on platforms that don't like
unaligned loads and stores.
> > */
> > -
> > static inline uint32_t fdt32_ld(const fdt32_t *p)
> > {
> > const uint8_t *bp = (const uint8_t *)p;
> > diff --git a/libfdt/libfdt_internal.h b/libfdt/libfdt_internal.h
> > index d4e0bd49c037..785b8b45ad1c 100644
> > --- a/libfdt/libfdt_internal.h
> > +++ b/libfdt/libfdt_internal.h
> > @@ -46,6 +46,23 @@ static inline struct fdt_reserve_entry *fdt_mem_rsv_w_(void *fdt, int n)
> > return (void *)(uintptr_t)fdt_mem_rsv_(fdt, n);
> > }
> >
> > +/*
> > + * Internal helpers to access words from a device tree blob. Assume that we
> > + * are on a platform where unaligned memory reads will be handled in a graceful
> > + * manner and that we do not need to ensure our reads are aligned. If this is
> > + * not the case there are _unaligned versions of these functions that follow
> > + * and can be used.
>
> Can you adjust this to emphasise a couple of points:
> * These helpers are intended for structural elements of the DT,
> rather than for reading integers from within property values
> * These are safe if EITHER the platform handles unaligned loads OR
> they're given naturally aligned addresses. Currently you only
> mention the first, but the second condition is really the one we
> rely on, since it should be true in practice for the users of these
> assuming a compliant dtb loaded at an 8-byte aligned address.
OK.
> Finally, given the history, I'm just a little paranoid that somebody
> in future is going to hit some weird platform with some new weird
> alignment issue. So, I'm rather tempted to tie this to a new ASSUME
> flag (though I'd be willing to have it default to on). I'd envision:
> 1. You use new internal load wrappers
> 2. If can_assume(ALIGNED) or whatever, those wrappers expand to the
> load and byteswap only
> 3. if !can_assume(ALIGNED), they call the external, unaligned-safe
> helpers instead
But the entirety of the history of the problem is caught with the other
patch, fail directly if we're not 8 byte aligned.
That said, if we renumber the ASSUME values so that
ASSUME_SAFE_UNALIGNED_LOAD, I would hope that the compiler would do the
right thing and optimize things the way we need them over in U-Boot.
But I have to experiment first there to be sure.
--
Tom
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