[U-Boot] [PATCH 1/3] config: introduce a generic $bootcmd

[Simon Glass]

Hi Stephen,

On 31 July 2014 17:00, Stephen Warren <swarren at wwwdotorg.org> wrote:
> On 07/31/2014 04:03 PM, Simon Glass wrote:
>>
>> Hi Stephen,
>>
>> On 30 July 2014 23:37, Stephen Warren <swarren at wwwdotorg.org> wrote:
>>>
>>> From: Dennis Gilmore <dennis at ausil.us>
>>>
>>> This generic $bootcmd, and associated support macros, automatically
>>> searches a defined set of storage devices (or network protocols) for an
>>> extlinux configuration file or U-Boot boot script in various standardized
>>> locations. Distros that install such a boot config file/script in those
>>> standard locations will get easy-to-set-up booting on HW that enables
>>> this generic $bootcmd.
>>>
>>> Boards can define the set of devices from which boot is attempted, and
>>> the order in which they are attempted. Users may later customize this
>>> set/order by edting $boot_targets.
>>>
>>> Users may interrupt the boot process and boot from a specific device
>>> simply by executing e.g.:
>>>
>>> $ run bootcmd_mmc1
>>> or:
>>> $ run bootcmd_pxe
>>>
>>> This patch was originally written by Dennis Gilmore based on Tegra and
>>> rpi_b boot scripts. I have made the following modifications since then:
>>>
>>> * Boards must define the BOOT_TARGET_DEVICES macro in order to specify
>>>    the set of devices (and order) from which to attempt boot. If needed,
>>>    we can define a default directly in config_distro_bootcmd.h.
>>>
>>> * Removed $env_import and related variables; nothing used them, and I
>>>    think it's better for boards to pre-load an environment customization
>>>    file using CONFIG_PREBOOT if they need.
>>>
>>> * Renamed a bunch of variables to suit my whims:-)
>>>
>>> Signed-off-by: Dennis Gilmore <dennis at ausil.us>
>>> Signed-off-by: Stephen Warren <swarren at nvidia.com>
>>
>>
>> I do understand the desirability of getting something sorted in this
>> area. But I am not thrilled with all the macro magic. How does this
>> fit with the new Kconfig setup? It encourages a single setting for
>> each variable, and I feel that the #ifdefs are not compatible with
>> that.
>
>
> I think Kconfig would be completely unsuitable for this kind of detailed
> configuration. Kconfig is great for enabling/disabling features, but
> applying it here feels too much to me. Equally, Kconfig is rather new in
> U-Boot. It'd be better to enable the feature so that distros can rely on it,
> and then refactor it later if required.

Are you saying that we can reimplement this in a nicer way and distros
will still see the same behaviour?

>
> I do feel that actually implementing the boot script as U-Boot environment
> variables is much preferred over hard-coding any algorithm into U-Boot. That
> way, the user can easily modify the scripts as they desire. If we just put
> e.g. $boot_targets into the environment or DT, but none of the other scripts
> in this patch, the user would be much more severely restricted in how they
> could reconfigure the system to act how they want.

But that worries me. It means that it is easy for one board to deviate
from what is essentially an undocumented boot standard, and then we
will end up having to support that use case in the future.

Or if it is documented, where is that?

>
>
>> Would it be possible to put the settings in the device tree somehow
>> instead of CONFIGs?
>
>
> At least part of the information isn't a HW description, but rather
> user-/vendor configuration. So, I don't think it's appropriate to put this
> into DT. Equally, very few U-Boot platforms currently use DT, and I
> certainly hope it doesn't become mandatory in any way. So, using DT for this
> purpose would severely limit the platforms where this feature was available.

The only platforms I see support for in your series are Tegra (which
has DT) and Raspberry PI. Adding basic DT support is not really
onerous so doesn't impose any real limits on adoption. In any case I'm
mostly just responding to what I see might become a large and
mandatory script setup in U-Boot. Would it not be better now to
document this clearly and specify that changes are not supported? Then
we might be able to refactor it later and still retain compatibility.
If we have a clear API separate from the implementation then it is
easier to live with the scripts knowing we can do things a different
way later.

Looking at this from a driver model perspective we would probably want
to have a list of devices to try, in a certain order. Certainly driver
model would support the approach in this series, but it would be a
very ugly way of achieving the goal IMO.

BTW in your series bootpart is always 1 - is that always the case for
all boards?

As to the question of HW description, I'm not sure I follow. The
devices that are attached to the hardware presumably appear in the DT,
the partition is fixed anyway, all you need to know is whether it is a
bootable device or not. What part of the description is not related to
the hardware?

In trying to figure out what was going on, I was hampered by the fact
that autoconf.mk does not get the full environment (e.g. since BOOTDEV
doesn't have a CONFIG_ prefix). I can see what it doesn't, I think.

>
>
>> I did send a series some time ago that aimed to improve the default
>> environment specification in config files - it was parked while
>> Kconfig was going on, but we could revisit it.
>
>
> I think we'd still need to use a C pre-processor (or some other code
> generation/templating tool) even with that scheme in place. So, I think the
> two are orthogonal.

Yes, but the more of this sort of stuff we add to U-Boot the harder it
becomes to revisit. As you may recall the tegra config files were
already pretty hard to move over.

Regards,
Simon