[RFC PATCH V2 2/2] kbuild: dtbs_install: new make target

[Jason Cooper]

On Tue, Nov 19, 2013 at 12:53:07PM -0700, Stephen Warren wrote:
> On 11/19/2013 11:57 AM, Jason Cooper wrote:
> > On Tue, Nov 19, 2013 at 11:42:17AM -0700, Stephen Warren wrote:
> >> On 11/19/2013 05:28 AM, Russell King - ARM Linux wrote:
> >>> On Mon, Nov 18, 2013 at 03:46:32PM -0700, Stephen Warren wrote:
> >>>> Again, the point is not that it's hard to write the script. As you
> >>>> demonstrated, it's easy. The problem is that then, everybody has to do
> >>>> something different for Tegra, forever. No matter how small the actual
> >>>> cost of doing it is, it's still non-scalable to require that everyone
> >>>> know about this special case. I'm not convinced the issue would be
> >>>> isolated to Tegra either.
> >>>
> >>> That's why there's the facility to allow an override to the script,
> >>> just like there's the facility to override the default script when
> >>> running "make install".
> >>
> >> Again, you are completely missing the point about that not scaling at all.
> >>
> >> But I will go and investigate what it takes to support renaming the
> >> DTBs. Everyone (using Tegra) will have to update their bootloader, but
> >> perhaps that can be dealt with.
> > 
> > Setting aside the idea of hard-coding filenames into any bootloader
> > binary, did you catch that the proposed solution would allow you to
> > continue using the dts filenames as they currently are?
> 
> I don't consider requiring people to install a custom /sbin/installdtbs
> in order to override renaming done by the default in-kernel
> scripts/installdtbs a solution.

Russell's point about not creating policy for userspace is on the mark.
*I* think the most-specific compatible string should be unique per
board, and thus would be a sane naming scheme.  But that is exactly as
Russell said making policy for userspace.  If you said as much, I missed
it.  Sorry about that.

Now, off on a tangent...

> > I must admit I'm a bit mystified as to what Tegra is doing that this
> > breaks it so horribly...
> 
> The idea is that a disto boot process runs as follow:
> 
> * U-Boot runs and initializes.
> 
> * U-Boot searches the boot disk (or any disk that could be a boot disk)
> for e.g. /boot/boot.scr, and executes it.
> 
> * boot.scr loads the kernel, initrd, DTB, sets up the command-line, and
> boots the kernel.
> 
> A few points to make here:
> 
> a) There must be a boot.scr or similar on disk, to provide a way for
> distros to parameterize the boot process. At the very least they need to
> configure the kernel command-line, specify whether an initrd is used at
> all, etc.
> 
> b) boot.scr is responsible for loading the zImage etc. from disk, rather
> than the default U-Boot environment doing this. This allows distros
> complete flexibility re: where to put the zImage etc. on disk. On distro
> might store a single copy in /boot/zImage, another might use
> /boot/zImage-${kernelversion}, another might use
> /boot/${kernelversion}/zImage, etc. By putting the onus on boot.scr to
> do the loading, the distro has complete flexibility in file naming here.
> 
> c) There are a number of system-specific parameters required to
> implement zImage/DTB loading. For example, the SDRAM address that they
> should be loaded to. Distros shouldn't have to detect which board
> they're running on and calculate the values of all those parameters.
> Instead, U-Boot should provide the values to boot.scr, by setting
> certain standard variables in the environment.
> 
> For example, the zImage gets loaded to $kernel_addr_r, the DTB to
> $fdt_addr_r, etc.
> 
> d) Finally, boot.scr needs to load the DTB. This requires knowing which
> DTB to load. Again, the distro shouldn't have to detect which board
> they're running on, and either install the correct DTB to a static
> filename, or make a decision on the DTB filename to load. Instead,
> distros should simply install all DTBs generated by the kernel build,
> and use some run-time information to calculate the DTB filename using a
> completely HW-agnostic and generic algorithm.
> 
> To support this, U-Boot can be configured to add certain standard
> environment variables to the default environment. These define which SoC
> and board the code is running on. These are ${soc} and ${board}. If you
> then calculate the DTB filename as ${soc}-${board}.dtb, that should work
> anywhere. This is why keeping the current in-kernel DTB filenames is
> important, so they match the assumption that this algorithm works.
> 
> Note: if the firmware contains an embedded DTB, boot.scr could detect
> this e.g. by the standard variable $fdt_addr being set, and hence skiip
> loading a DTB. IIRC, this is how Calxeda boards are set up.

Ok, so the ${soc}-${board} is a work-around until all bootloaders are
providing a dtb, or until all bootloaders are providing a stable dtb?

I ask because in tinkering with pxa-impedance-matcher [1], I've setup
selecting from many dtbs at boot (linked in, no fs support) by
specifying the desired board compatible string on the kernel
commandline (patches still a wip).  The impedance-matcher then scans
through the provided dtbs looking for a match.

It's a hack, and only useful until bootloaders are providing dtbs.  If
the dtb is upgradable, then it goes away earlier.  If the dtb isn't,
then it'll be around longer.  It's goal is to keep silly code out of the
kernel (non-standard atag-to-fdt translation, other bootloader
workarounds).  Perhaps it'll be used to translate ACPI into DT for a
bit... :)

> So finally, if the kernel "make dtbs_install" starts renaming DTB files,
> then I need to adjust the boot.scr we're using to use some other
> algorithm to calculate the DTB filename. Given the filename isn't then
> purely derived from ${soc} and ${board}, I probably need to set up some
> additional (or replacement) variables in the standard U-Boot environment
> that contain the extra fields required to calculate the DTB filename.
> The existing ${vendor} might work. However, it'd be simplest if I just
> had U-Boot export something like ${dtb_filename}, which contained the
> (or a list of) filenames to try.
> 
> You can take a look at my boot.scr generation tool at:
> https://github.com/NVIDIA/tegra-uboot-scripts
> 
> That probably works on the Raspberry Pi too (since I set up the Pi's
> U-Boot port to work the same way) and likely would require minimal
> changes to work on at least some TI and Calxeda boards.
> 
> The idea is that eventually, and board that wants to support a generic
> distro booting on it would export the standard variables I mentioned
> above, have a default U-Boot environment that simply searched for and
> executed boot.scr, and distros would use a generic boot.scr as generated
> by the tool I linked to above.

Thanks for the detailed explanation.  I'll post a non-RFC version in the
next few days.

thx,

Jason.

[1] https://github.com/zonque/pxa-impedance-matcher.git