Re: [Qemu-devel] [PATCH v2] hw/misc/zynq_slcr: Change CPU clock rate for Linux boots

[Peter Maydell]

On 13 September 2015 at 23:42, Peter Crosthwaite
<address@hidden> wrote:
> On Sun, Sep 13, 2015 at 1:47 PM, Peter Maydell <address@hidden> wrote:
>> On 13 September 2015 at 21:22, Peter Crosthwaite
>> <address@hidden> wrote:
>>> There may be more changes worth making on is_linux. I don't have the
>>> patch with the full list of FSBL-related SLCR changes handy and can't
>>> seem to find it in any modern Yocto trees. Wondering if Yocto still
>>> supports booting Zynq without FSBL (Nathan/Alistair may know more)?
>>
>> I'd prefer us not to propagate lots of "only if Linux boot"
>> changes into devices. The GIC *must* have these because the
>> kernel can't configure it otherwise from non-secure mode.
>> I'm not sure that applies here.
>>
>
> At least this change is a must. I have had this discussion with kernel
> people before and they insist that initing the PLLs and clocks to
> desired values is the job of the bootloader and the kernel reads back
> the values from this core. It is same philosophy at the GIC init,
> which is at the end of the day, done by some pre-boot software. The
> same bootloader (FSBL) makes other changes that kernels past present
> and future may rely on and it would be good to have those.

The thing is that if we go down this path we end up incorporating
most of a boot firmware into QEMU, scattered across different
devices (and what do we do if we find that two boards want a
single device set up differently?). The current in-QEMU ARM
bootloader basically assumes a traditional 32-bit ARM setup,
where the kernel didn't really trust the firmware or bootloader
and did a lot of hardware setup itself. This model is starting
to break down as modern kernels assume more that the firmware
has done certain setup, but it's what QEMU's design here is based
on.

The other approach would be to actually run some firmware
blob at startup, and let that do the setup.

thanks
-- PMM