Hi Tom,
On Mon, 27 Jan 2020 at 08:05, Tom Rini <trini@xxxxxxxxxxxx> wrote:
>
> On Mon, Jan 27, 2020 at 02:23:51PM +1100, David Gibson wrote:
> > On Thu, Jan 23, 2020 at 07:16:50AM -0500, Tom Rini wrote:
> > > On Thu, Jan 23, 2020 at 08:14:40PM +1100, David Gibson wrote:
> > > > On Fri, Jan 17, 2020 at 10:31:06AM -0500, Tom Rini wrote:
> > > > > 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.
> > > > >
> > > > > Ultimately however, these helpers should not exist. Unaligned access
> > > > > only occurs when images are forced out of alignment by the user. This
> > > > > unalignment is not supported and introduces problems later on as other
> > > > > parts of the system image are unaligned and they too require alignment.
> > > > >
> > > > > Revert both of these changes.
> > > > >
> > > > > Signed-off-by: Tom Rini <trini@xxxxxxxxxxxx>
> > > > > ---
> > > > > By way of a little more explanation, looking at the archives it seems
> > > > > that the initial bug reporter said that they had a platform that was
> > > > > using U-Boot and had the "fdt_high=0xffffffff" set in the environment.
> > > > > What that does is to tell U-Boot to not do any of the sanity checks and
> > > > > relocation to ensure alignment that it would normally do because the
> > > > > user knows best. This later came up on the U-Boot list as once the DTB
> > > > > was loaded, Linux is unhappy because it demands correct alignment.
> > > > >
> > > > > I only realized libfdt had introduced changes here when it was reported
> > > > > that boot time had gotten much slower once we merged this change in. It
> > > > > would be best to just drop it.
> > > >
> > > > Hmm. I'm not sure about this. The commit message makes a case for
> > > > why the unaligned handling isn't necessary, but not a case for why
> > > > it's bad. Even if handling an unaligned tree isn't a common case,
> > > > isn't it better to be able to than not?
> > > >
> > > > I gather from the previous discussion that there's a significant
> > > > performance impact, but that rationale needs to go into the commit
> > > > message for posterity.
> > >
> > > I wanted to emphasize that the code simply isn't ever needed, not that
> > > it's a performance problem. A performance problem implies that we would
> > > keep this, if it was fast enough. That's why people noticed it (it
> > > slows things down to an unusable level). But it's functionally wrong.
> > >
> > > > [snip]
> > > > > diff --git a/libfdt/libfdt.h b/libfdt/libfdt.h
> > > > > index fc4c4962a01c..d4ebe915cf46 100644
> > > > > --- a/libfdt/libfdt.h
> > > > > +++ b/libfdt/libfdt.h
> > > > > @@ -117,23 +117,6 @@ 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
> > > > > - */
> > > > > -
> > > > > -static inline uint32_t fdt32_ld(const fdt32_t *p)
> > > > > -{
> > > > > - const uint8_t *bp = (const uint8_t *)p;
> > > > > -
> > > > > - return ((uint32_t)bp[0] << 24)
> > > > > - | ((uint32_t)bp[1] << 16)
> > > > > - | ((uint32_t)bp[2] << 8)
> > > > > - | bp[3];
> > > > > -}
> > > >
> > > > In particular, I definitely think removing the helpers entirely is a
> > > > no go. They're now part of the published interface of the library.
> > >
> > > Perhaps "mistakes were made" ? I don't think we need to worry about
> > > removing an interface here as projects are avoiding upgrading libfdt
> > > now (TF-A, formerly ATF) or reverting the change (U-Boot).
> > >
> > > > Even if they're not used for reading the internal tags, they can be
> > > > used to load integers from within particular properties. Those are
> > > > frequently unaligned, since properties generally have packed
> > > > representations.
> > >
> > > I don't see the relevance. Go back to the initial problem report. It's
> > > not "I have a new unique platform I'm using libfdt on and I have
> > > problems". It's "I keep jabbing myself with a rusty nail and now I have
> > > problems".
> >
> > The initial report isn't the only relevant thing here. Although it
> > prompted the change, it wasn't the only consideration in making it.
> >
> > There's also two separate questions here:
> > 1) Do we want byteswapping integer load helpers?
> > 2) Should those handle unaligned accesses?
> >
> > In working out how to address the (as it turns out, non existent)
> > problem, I realized an abstraction for loading big-endian integers
> > from the blob would be a useful thing in its own right. I also
> > realized that it's a useful thing not just inside the libfdt code, but
> > as an external interface. Callers have always needed to interpret the
> > contents of individual dt properties, and loading integers from them
> > is often a part of that.
> >
> > I know of people using those fdtXX_ld() helpers right now, so I'm not
> > going to take them away.
> >
> > For the case of external users we absolutely do need to handle
> > unaligned accesses. There are a number of existing bindings that mix
> > strings and integers in packed format, in a single encoded property.
> > So regardless of the alignment of the whole property, we can easily
> > get unaligned integers in there, and I don't want to expose multiple
> > different helpers for different cases.
> >
> > Now, we don't *have* to use the same helpers for internal use. We
> > could open code the internal loads, or use a special aligned-only
> > version inside. But using the existing external helpers is the
> > obvious and simple choice.
> >
> > So, we're back to: I need a case for changing this now, not just a
> > case for claiming it wasn't needed in the first place.
>
> For U-Boot, I'm just going to revert this part of the changes. I would
> suggest that you look in to some way to fix the "fast path" here to go
> back to the previous way of working so that other project can continue
> to use libfdt as well and when callers need to access these helpers and
> are not otherwise in the fast path can do so.
>
> You're adding visible boot time delay with things the way they exist
> today. That's not OK. I wouldn't have updated U-Boot with these
> changes if we had automated timing tests like the TF-A folks do.
>
> Please do let us know if you have any changes to try that might
> alleviate the overall problem. Thanks!
For my part I think it is best to revert the change in U-Boot for now.
The code difference thus created is small. Hopefully it can be
reverted in libfdt, perhaps along the lines that David suggests.
Regards,
Simon
