[U-Boot] [PATCH v3 2/2] TI: DaVinci DA850 EVM: support passing maximum allowed cpu clock rate information to kernel

[Nori, Sekhar]

Hi Stefano,

On Thu, Aug 19, 2010 at 13:51:46, Stefano Babic wrote:
> Sekhar Nori wrote:
> > The TI DA850/OMAP-L138/AM18x EVM can be populated with devices
> > having different maximum allowed CPU clock rating.
> >
> > The maximum clock the chip can support can only be determined from
> > the label on the package (not software readable).
> >
> > Introduce a method to pass the maximum allowed clock rate information
> > to kernel using ATAG_REVISION. The kernel uses this information to
> > determine the maximum cpu clock rate reachable using cpufreq.
> >
> > Note that U-Boot itself does not set the CPU clock rate. The CPU
> > clock is setup by a primary bootloader ("UBL"). The rate setup by
> > UBL could be different from the maximum clock rate supported by the
> > device.
> >
> > Signed-off-by: Sekhar Nori <nsekhar at ti.com>
> > ---
> > Changes in v3:
> > a) renamed maxspeed to maxcpuclk
> > b) add information regarding the new environment variable in README.davinci
> > c) use if-else instead of switch to check for value range rather than specific
> >    values of maxcpuclk
> > d) change comment to document values returned in bit[0-3] by get_board_rev() in
> >    binary
> >
> >  board/davinci/da8xxevm/da850evm.c |   33 +++++++++++++++++++++++++++++++++
> >  doc/README.davinci                |   13 +++++++++++++
> >  include/configs/da850evm.h        |    1 +
> >  3 files changed, 47 insertions(+), 0 deletions(-)
>
> Hi,
>
> sorry if I come only late in the discussion, I have a general question.
> As I see, the UBL does not take care at all of the possibility to have
> different frequencies, and sets the PLL with a fix value (300 Mhz, as I
> read in sources). From my point of view, it makes sense to use the
> maximum allowed cpu clock if we then set the CPU to increase the
> performances.

Yes, cpufreq in kernel can do this based on cpu load.

> If you set the value in a u-boot environment, the value could be easy
> overwritten and a wrong value could be displayed. So I have doubt on
> using an environment to get a so hardware-related value..

Overwritten by whom and displayed where?

> Do you plan to use the maximum cpu clock to set the PLL ? Or is it only
> for info purposes ?

The kernel will use the value to understand the maximum frequency it can
reach using cpufreq. So yes, kernel uses this to set the PLL.

Thanks,
Sekhar