[U-Boot] [PATCH 1/4] Enable CONFIG_TIMER_EARLY with bootstage

[Bin Meng]

Hi Simon,

On Wed, Sep 26, 2018 at 1:42 PM Simon Glass <sjg at chromium.org> wrote:
>
> Hi Bin,
>
> On 4 September 2018 at 03:06, Bin Meng <bmeng.cn at gmail.com> wrote:
> > Hi Simon,
> >
> > On Mon, Sep 3, 2018 at 7:02 AM Simon Glass <sjg at chromium.org> wrote:
> >>
> >> In initr_bootstage() we call bootstage_mark_name() which ends up calling
> >> timer_get_us(). This call happens before initr_dm(), which inits driver
> >> model.
> >>
> >> On x86 we set gd->timer to NULL in the transition from board_init_f()
> >
> > It's not just x86 we set gd->timer to NULL. It applied to all
> > architectures when CONFIG_TIMER is on.
> >
> >> to board_init_r(). See board_init_f_r() for this assignment. So U-Boot
> >> knows there is no timer available in the period immediately after
> >> relocation.
> >>
> >> On x86 the timer_get_us() call is implemented as calls to get_ticks() and
> >> get_tbclk(). Both of these call dm_timer_init() to set up the timer, if
> >> gd->timer is NULL and the early timer is not available.
> >>
> >> However dm_timer_init() cannot succeed before initr_dm() is called.
> >>
> >> So it seems that on x86 if we want to use CONFIG_BOOTSTAGE we must enable
> >> CONFIG_TIMER_EARLY. Update the Kconfig to handle this.
> >>
> >> Note: On most architectures we can rely on the pre-relocation memory still
> >> being available, so that gd->timer pointers to a valid timer device and
> >> everything works correctly. Admittedly this is not strictly correct since
> >> the timer device is set up by pre-relocation U-Boot, but normally this is
> >> fine. On x86 the 'CAR' (cache-as-RAM) memory used by pre-relocation U-Boot
> >> disappears in board_init_f_r() and any attempt to access it will hang.
> >> This is the reason why we must mark the timer as invalid when we get to
> >> board_init_f_r().
> >>
> >> Signed-off-by: Simon Glass <sjg at chromium.org>
> >> ---
> >>
> >>  drivers/timer/Kconfig | 3 +++
> >>  1 file changed, 3 insertions(+)
> >>
> >> diff --git a/drivers/timer/Kconfig b/drivers/timer/Kconfig
> >> index 5ab6749193c..ff434de6f7c 100644
> >> --- a/drivers/timer/Kconfig
> >> +++ b/drivers/timer/Kconfig
> >> @@ -30,6 +30,9 @@ config TPL_TIMER
> >>  config TIMER_EARLY
> >>         bool "Allow timer to be used early in U-Boot"
> >>         depends on TIMER
> >> +       # initr_bootstage() requires a timer and is called before initr_dm()
> >> +       # so only the early timer is available
> >> +       default y if X86 && BOOTSTAGE
> >
> > Since this applies not only on x86, and given without TIMER_EARLY
> > BOOTSTAGE is broken, shouldn't we do this in BOOTSTAGE config instead:
> >
> > config BOOTSTAGE
> >          select TIMER_EARLY
>
> Well we could, but I suspect that would break things since the early
> timer is not supported by many boards. Also for most boards this
> doesn't actually work fine. x86 is really quite awful in that it has
> no SRAM and the CAR becomes inaccessible as soon as you turn on the
> cache!

It's true that early timer is supported by some limited boards, but
that's a different issue. For now that patch does not fix anything. If
we add BOOTSTAGE from either defconfig or 'menuconfig' for a board,
without adding TIMER_EARLY, it will for sure break.

Regards,
Bin