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

[Simon Glass]

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!

Regards,
Simon