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

Tue Nov 28 09:53:32 UTC 2017

On Mon, 27 Nov 2017 10:13:09 -0700
Simon Glass <sjg at chromium.org> wrote:

> (Tom - any thoughts about a more expansive cc list on this?)
> 
> 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.
> 
> >
> >
> >
> > We should think about old platforms that boot from a NOR flash or
> > something. There are two solutions:
> >  - execute-in-place: run the code in the flash directly
> >  - use SPL (common/spl/spl-nor.c) if you want to run
> >    it from RAM  
> 
> This seems like a big regression in functionality. For example for x86
> 32-bit we currently don't have an SPL (we do for 64-bit). So I think
> this means that everything would be forced to have an SPL?
> 
> I am wondering who else we should cc on this discussion?

Not all boards use SPL. There are some targets, which use FBL (SPL
counterpart) from vendor and only U-boot proper. Good example is Odroid
XU3.

And I also do agree - for the original post in this discussion we
should have the measurements of boot time improvement.

> 
> Regards,
> Simon
> _______________________________________________
> U-Boot mailing list
> U-Boot at lists.denx.de
> https://lists.denx.de/listinfo/u-boot

Best regards,

Lukasz Majewski

--

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