[U-Boot] U-Boot proper(not SPL) relocate option

Mon Nov 27 18:53:50 UTC 2017

On Mon, Nov 27, 2017 at 10:13:09AM -0700, Simon Glass wrote:
> (Tom - any thoughts about a more expansive cc list on this?)

Not really, sorry.

> Hi Masahiro,
> 
> On 26 November 2017 at 07:16, Masahiro Yamada
> <yamada.masahiro at socionext.com> wrote:
> > 2017-11-26 20:38 GMT+09:00 Simon Glass <sjg at chromium.org>:
> >> Hi Philipp,
> >>
> >> On 25 November 2017 at 16:31, Dr. Philipp Tomsich
> >> <philipp.tomsich at theobroma-systems.com> wrote:
> >>> Hi,
> >>>
> >>>> On 25 Nov 2017, at 23:34, Simon Glass <sjg at chromium.org> wrote:
> >>>>
> >>>> +Tom, Masahiro, Philipp
> >>>>
> >>>> Hi,
> >>>>
> >>>> On 22 November 2017 at 03:27, Wolfgang Denk <wd at denx.de> wrote:
> >>>>> Dear Kever Yang,
> >>>>>
> >>>>> In message <fd0bb500-80c4-f317-cc18-f7aaf1344fd8 at rock-chips.com> you wrote:
> >>>>>>
> >>>>>> I can understand this feature, we always do dram_init_banks() first,
> >>>>>> then we relocate to 'known' area, then will be no risk to access memory.
> >>>>>> I believe there must be some historical reason for some kind of device,
> >>>>>> the relocate feature is a wonderful idea for it.
> >>>>>
> >>>>> This is actuallyu not so much a feature needed to support some
> >>>>> specific device (in this case much simpler approahces would be
> >>>>> possible), but to support a whole set of features.  Unfortunately
> >>>>> these appear to get forgotten / ignored over time.
> >>>>>
> >>>>>>     many other SoCs should be similar.
> >>>>>> - Without relocate we can save many step, some of our customer really
> >>>>>>     care much about the boot time duration.
> >>>>>>     * no need to relocate everything
> >>>>>>     * no need to copy all the code
> >>>>>>     * no need init the driver more than once
> >>>>>
> >>>>> Please have a look at the README, section "Memory Management".
> >>>>> The reloaction is not done to any _fixed_ address, but the address
> >>>>> is actually computed at runtime, depending on a number features
> >>>>> enabled (at least this is how it used to be - appearently little of
> >>>>> this is tested on a regular base, so I would not be surprised if
> >>>>> things are broken today).
> >>>>>
> >>>>> The basic idea was to reserve areas of memory at the top of RAM,
> >>>>> that would not be initialized / modified by U-Boot and Linux, not
> >>>>> even across a reset / warm boot.
> >>>>>
> >>>>> This was used for exaple for:
> >>>>>
> >>>>> - pRAM (Protected RAM) which could be used to store all kind of data
> >>>>>  (for example, using a pramfs [Protected and Persistent RAM
> >>>>>  Filesystem]) that could be kept across reboots of the OS.
> >>>>>
> >>>>> - shared frame buffer / video memory. U-Boot and Linux would be able
> >>>>>  to initialize the video memory just once (in U-Boot) and then
> >>>>>  share it, maybe even across reboots.  especially, this would allow
> >>>>>  for a very early splash screen that gets passed (flicker free) to
> >>>>>  Linux until some Linux GUI takes over (much more difficult today).
> >>>>>
> >>>>> - shared log buffer: U-Boot and Linux used to use the same syslog
> >>>>>  buffer mechanism, so you could share it between U-Boot and Linux.
> >>>>>  this allows for example to
> >>>>>  * read the Linux kernel panic messages after reset in U-Boot; this
> >>>>>    is very useful when you bring up a new system and Linux crashes
> >>>>>    before it can display the log buffer on the console
> >>>>>  * pass U-Boot POST results on to Linux, so the application code
> >>>>>    can read and process these
> >>>>>  * process the system log of the previous run (especially after a
> >>>>>    panic) in Lunux after it rebootet.
> >>>>>
> >>>>> etc.
> >>>>>
> >>>>> There are a number of such features which require to reserve room at
> >>>>> the top of RAM, the size of which is calculatedat runtime, often
> >>>>> depending on user settable environment data.
> >>>>>
> >>>>> All this cannot be done without relocation to a (dynmaically
> >>>>> computed) target address.
> >>>>>
> >>>>>
> >>>>> Yes, the code could be simpler and faster without that - but then,
> >>>>> you cut off a number of features.
> >>>>
> >>>> I would be interested in seeing benchmarks showing the cost of
> >>>> relocation in terms of boot time. Last time I did this was on Exynos 5
> >>>> and it was some years ago. The time was pretty small provided the
> >>>> cache was on for the memory copies associated with relocation itself.
> >>>> Something like 10-20ms but I don't have the numbers handy.
> >>>>
> >>>> I think it is useful to be able to allocate memory in board_init_f()
> >>>> for use by U-Boot for things like the display and the malloc() region.
> >>>>
> >>>> Options we might consider:
> >>>>
> >>>> 1. Don't relocate the code and data. Thus we could avoid the copy and
> >>>> relocation cost. This is already supported with the GD_FLG_SKIP_RELOC
> >>>> used when U-Boot runs as an EFI app
> >>>>
> >>>> 2. Rather than throwing away the old malloc() region, keep it around
> >>>> so existing allocated blocks work. Then new malloc() region would be
> >>>> used for future allocations. We could perhaps ignore free() calls in
> >>>> that region
> >>>>
> >>>> 2a. This would allow us to avoid re-init of driver model in most cases
> >>>> I think. E.g. we could init serial and timer before relocation and
> >>>> leave them inited after relocation. We could just init the
> >>>> 'additional' devices not done before relocation.
> >>>>
> >>>> 2b. I suppose we could even extend this to SPL if we wanted to. I
> >>>> suspect it would just be a pain though, since SPL might use memory
> >>>> that U-Boot wants.
> >>>>
> >>>> 3. We could turn on the cache earlier. This removes most of the
> >>>> boot-time penalty. Ideally this should be turned on in SPL and perhaps
> >>>> redone in U-Boot which has more memory available. If SPL is not used,
> >>>> we could turn on the cache before relocation.
> >>>
> >>> Both turning on the cache and initialising the clocking could be of benefit
> >>> to boot-time.
> >>>
> >>> However, the biggest possible gain will come from utilising Falcon mode
> >>> to skip the full U-Boot stage and directly boot into the OS from SPL.  This
> >>> assumes that the drivers involved are fully optimised, so loading up the
> >>> OS image does not take longer than necessary.
> >>
> >> I'd like to see numbers on that. From my experience, loading and
> >> running U-Boot does not take very long...
> >>
> >>>
> >>>> 4. Rather than the reserving memory in board_init_f() we could have it
> >>>> call malloc() from the expanded region. We could then perhaps then
> >>>> move this reserve/allocate code in to particular drivers or
> >>>> subsystems, and drop a good chunk of the init sequence. We would need
> >>>> to have a larger malloc() region than is currently the case.
> >>>>
> >>>> There are still some arch-specific bits in board_init_f() which make
> >>>> these sorts of changes a bit tricky to support generically. IMO it
> >>>> would be best to move to 'generic relocation' written in C, where all
> >>>> archs work basically the same way, before attempting any of the above.
> >>>>
> >>>> Still, I can see some benefits and even some simplifications.
> >>>>
> >>>> Regards,
> >>>> Simon
> >>>
> >
> >
> >
> > This discussion should have happened.
> > U-Boot boot sequence is crazily inefficient.
> >
> >
> >
> > When we talk about "relocation", two things are happening.
> >
> >  [1] U-Boot proper copies itself to the very end of DRAM
> >  [2] Fix-up the global symbols
> >
> > In my opinion, only [2] is useful.
> >
> >
> > SPL initializes the DRAM, so it knows the base and size of DRAM.
> > SPL should be able to load the U-Boot proper to the final destination.
> > So, [1] is unnecessary.
> >
> >
> > [2] is necessary because SPL may load the U-Boot proper
> > to a different place than CONFIG_SYS_TEXT_BASE.
> > This feature is useful for platforms
> > whose DRAM base/size is only known at run-time.
> > (Of course, it should be user-configurable by CONFIG_RELOCATE
> > or something.)
> >
> > Moreover, board_init_f() is unneeded -
> > everything in board_init_f() is already done by SPL.
> > Multiple-time DM initialization is really inefficient and ugly.
> >
> >
> > The following is how the ideal boot loader would work.
> >
> >
> > Requirement for U-Boot proper:
> > U-Boot never changes the location by itself.
> > So, SPL or a vendor loader must load U-Boot proper
> > to the final destination directly.
> > (You can load it to the very end of DRAM if you like,
> > but the actual place does not matter here.)
> >
> >
> > Boot sequence of U-Boot proper:
> > If CONFIG_RELOCATE (or something) is enabled,
> > it fixes the global symbols at the very beginning
> > of the boot.
> > (In this case, CONFIG_SYS_TEXT_BASE can be arbitrary)
> >
> > That's it.  Proceed to the rest of init code.
> > (= board_init_r)
> > board_init_f() is unnecessary.
> >
> > This should work for recent platforms.
> 
> Yes that sounds reasonable to me.
> 
> We could do the symbol fixup/relocation in SPL after loading U-Boot.,
> although that would probably push us to using ELF format for U-Boot
> which is a bit limited.
> 
> Still I think the biggest performance improvement comes from turning
> on the cache in SPL. So the above is a simplification, not really a
> speed-up.

Note that in some platforms we do enable cache in SPL, so there's no
reason not to on others, if we can afford the size.

-- 
Tom
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