[U-Boot] [PATCH 2/2] arm64: booti: allow to place kernel image anywhere in physical memory

[Tom Rini]

On Thu, Feb 23, 2017 at 06:17:38PM +0900, Masahiro Yamada wrote:
> Hi Tom,
> 
> 
> 2017-02-23 1:19 GMT+09:00 Tom Rini <trini at konsulko.com>:
> > On Wed, Feb 22, 2017 at 11:34:26AM +0900, Masahiro Yamada wrote:
> >
> >> At first, the ARM64 Linux booting requirement recommended that the
> >> kernel image be placed text_offset bytes from 2MB aligned base near
> >> the start of usable system RAM because memory below that base address
> >> was unusable at that time.
> >>
> >> This requirement was relaxed by Linux commit a7f8de168ace ("arm64:
> >> allow kernel Image to be loaded anywhere in physical memory").
> >> Since then, the bit 3 of the flags field indicates the tolerance
> >> of the kernel physical placement.  If this bit is set, the 2MB
> >> aligned base may be anywhere in physical memory.  For details, see
> >> Documentation/arm64/booting.txt of Linux.
> >>
> >> The booti command should be also relaxed to not expect the kernel
> >> image at the start of the system RAM.  Even when booting older
> >> kernel versions, it still makes sense to have some space below the
> >> kernel.  For example, some firmware may sit at the start of the
> >> system RAM.
> >>
> >> After all, the most flexible way for booting the kernel is to respect
> >> the original images->ep instead of gd->bd->bi_dram[0].start.  If
> >> image->ep (which is the address given to the booti command) already
> >> meets the address requirement, just use it.  If not, relocate the
> >> kernel to the next 2MB aligned address.
> >>
> >> Signed-off-by: Masahiro Yamada <yamada.masahiro at socionext.com>
> >> ---
> >>
> >>  cmd/booti.c | 6 +++++-
> >>  1 file changed, 5 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/cmd/booti.c b/cmd/booti.c
> >> index f65f0e7..9408c34 100644
> >> --- a/cmd/booti.c
> >> +++ b/cmd/booti.c
> >> @@ -11,6 +11,8 @@
> >>  #include <image.h>
> >>  #include <lmb.h>
> >>  #include <mapmem.h>
> >> +#include <linux/kernel.h>
> >> +#include <linux/sizes.h>
> >>
> >>  DECLARE_GLOBAL_DATA_PTR;
> >>
> >> @@ -54,7 +56,9 @@ static int booti_setup(bootm_headers_t *images)
> >>        * If we are not at the correct run-time location, set the new
> >>        * correct location and then move the image there.
> >>        */
> >> -     dst = gd->bd->bi_dram[0].start + le64_to_cpu(ih->text_offset);
> >> +     dst = images->ep - ih->text_offset;
> >> +     dst = ALIGN(dst, SZ_2M);
> >> +     dst += ih->text_offset;
> >
> > I think the code will be slightly more complex here but I would rather
> > see us check for the presence of the flag which allows for us to
> > relocate things rather than assume that we can always use the address
> > provided, or round it up.  The 'contract' wwith the kernel previously
> > said it must be from start of memory and I'd rather not change that.
> 
> 
> At first, I tried this approach.
> 
> The problem (at least for me) is
> commit a7f8de168ace is quite new; this is only included in Linux 4.5 and later.
> Linux 4.4 LTS will be used on Socionext's products for a while.
> However, I need to avoid the relocation of the kernel image.
> 
> The gd->bd->bi_dram[0].start points to the start of the DRAM.
> Here, some firmware is sitting at the start of the DRAM.
> To hide the head of the memory from Linux,
> the memory node in the device tree is carved out.
> 
> If CONFIG_ARCH_FIXUP_FDT_MEMORY is not set,
> U-Boot passes the memory node as-is to Linux.
> As a result, the Image is placed out of the available memory region
> specified by DT,
> then Linux fails to boot.
> 
> 
> Somehow I want to achieve,
> "Even when booting older kernel versions, it still makes sense to have
> some space below the
> kernel.  For example, some firmware may sit at the start of the system RAM."
> 
> 
> Perhaps, can we introduce a CONFIG
> to enable/disable the relocation?
> Or, any other good idea?

I guess the answer is that I need to find some time to re-read the
history on Documentation/arm64/booting.txt to see when various
restrictions changed / were introduced.  I'm also willing to say that
perhaps my initial implementation (or the follow up when text_offset was
introduced) was incorrectly too strict.

-- 
Tom
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