[U-Boot] [linux-sunxi] Re: [PATCH 0/3] sunxi: sun8i-emac: Update DT bindings

Wed Jan 31 08:36:43 UTC 2018

Hi Julian,

On Wed, Jan 31, 2018 at 07:29:13PM +1100, Julian Calaby wrote:
> Hi Maxime,
> 
> On Wed, Jan 31, 2018 at 7:21 PM, Maxime Ripard
> <maxime.ripard at free-electrons.com> wrote:
> > Hi,
> >
> > On Mon, Jan 29, 2018 at 10:38:25AM +0000, Andre Przywara wrote:
> >> On 29/01/18 09:58, Maxime Ripard wrote:
> >> > On Mon, Jan 29, 2018 at 09:44:44AM +0000, Andre Przywara wrote:
> >> >> On 29/01/18 08:51, Maxime Ripard wrote:
> >> >>> On Mon, Jan 29, 2018 at 01:15:19AM +0000, Andre Przywara wrote:
> >> >>>> The existing sun8i-emac driver in U-Boot uses some preliminary bindings,
> >> >>>> which matched our own DTs. Now that the Linux kernel got a driver, lets
> >> >>>> update our probe code to handle those Linux DTs as well.
> >> >>>> The first patch adds the missing compatible strings for the pinctrl drivers,
> >> >>>> which is needed for using the sunxi_name_to_gpio() lookup function.
> >> >>>> Patch 2/3 updates the pinctrl parser used in the sun8i-emac driver, to cope
> >> >>>> with the new, generic Allwinner pinctrl bindings.
> >> >>>> The final patch extends the probe routine in the Ethernet driver to deal
> >> >>>> with both the old and the new bindings.
> >> >>>
> >> >>> Thanks for posting this
> >> >>>
> >> >>>> This series allows to copy in the DTs from the latest kernel. Unfortunately
> >> >>>> right now updating the DTs for the H5 and A64 breaks the build, as the
> >> >>>> resulting binary (which embeds the DT) gets to large and triggers our new
> >> >>>> image size check.
> >> >>>
> >> >>> Sigh...
> >> >>>
> >> >>>> As the H5 and H3 share most of the DT, we can't just update the H3
> >> >>>> DTs either. Hopefully we find some neat trick to work around that.
> >> >>>
> >> >>> Is it just because of the DT size, or because there's more code?
> >> >>
> >> >> My impression the code itself is always growing a tiny bit over the
> >> >> weeks, but this time around it's really the DT update.
> >> >> The current A64 .dtbs in U-Boot are around 8KB, mainline is at 13KB.
> >> >> Similar for the H5: going from 9.5KB to 14.5KB.
> >> >>
> >> >> Since you did a pretty good job already in identifying the code hogs, I
> >> >> couldn't find *easy* mitigations over the weekend.
> >> >> One possible fix is to remove the second .dtb in the Pine64 case, for
> >> >> which I sent a patch Friday night.
> >> >
> >> > Since the DT is fed to the C preprocessor, we could also put some
> >> > #ifdef 0 around the nodes that are never used by U-Boot (like the
> >> > clocks, timer, psci, dma, GIC, RTC, RSB, etc.)
> >>
> >> Well yes, U-Boot itself actually only requires a *tiny* .dtb (I think
> >> /aliases, /chosen, the reg of USB and Ethernet). But to be honest I
> >> don't want to go there. First it would be a constant churn to keep this
> >> up-to-date,
> >
> > I'm not too worried about the churn, it would be there only for the
> > time until we fully migrate to the FAT environment, so one-two release
> > now. And we're not syncing the DT very often these days (now that we
> > have support for the EMAC and USB that is all U-Boot cares about).
> >
> >> but more importantly for proper UEFI boot we just reuse U-Boot's
> >> .dtb to pass it on to the kernel. That is actually the purpose of
> >> this whole exercise. That already works today (at least for A64),
> >> but would benefit from some updates.
> >>
> >> So I would refrain from tinkering with U-Boot's .dtbs.
> >
> > That sucks :/
> >
> >> > This should give us some room.
> >> >
> >> >> Another thing that stuck out is the sha256 checksum. It's "default y" if
> >> >> you have FIT. We need FIT for the SPL loader - but we don't do or need
> >> >> the checksum there.
> >> >> Some people do FIT loading for the kernel and initrd in U-Boot proper, I
> >> >> suppose, but I am not sure how many depend on SHA256 checksums in their
> >> >> images.
> >> >
> >> > I think there was someone (Tom?) that said that it was useful in some
> >> > circumstances?
> >>
> >> Yes, I clearly see that it is *useful*, but I wonder how many people
> >> would actually miss it today? We would bring it back once we dumped
> >> ENV_IS_IN_MMC, so it's only temporarily.
> >
> > His words were stronger actually, and he said that we want to keep it.
> >
> >> I think we can just disable it in some defconfigs, to avoid collateral
> >> damage to other boards.
> >> If people have a special need, they can always disable the MMC env and
> >> enable stuff at their likings, it's just the standard "make
> >> .._defconfig; make" process that needs to be fixed with some band-aids
> >> for now.
> >
> > I really don't want to go down the "let's fix each defconfig when we
> > find out it broke", this is very likely to be broken with no-one
> > noticing.
> >
> > Is this issue happening when you sync the whole DT, and would it break
> > if you just convert the EMAC binding?
> >
> > Otherwise, we might try to revive the DTC garbage collection of unused
> > nodes patches. This would prevent us from using the overlays on such a
> > DT, but that doesn't like like an unfair tradeoff.
> 
> Stupid question:

It's not really stupid :)

> As I understand it, the boot process is SPL => Full U-Boot => Linux.
> 
> Would it therefore be possible to use a cut-down DT for the SPL (just
> the bits it cares about) then use a full one afterwards?

The thing is, we're not using the DT for the SPL, and the DT size
we're discussing about is the one in the main U-Boot binary.

> I'm guessing that the SPL wants to patch the DT we pass to Linux,
> would we be able to handle that using overlays?

In a "standard" setup (or at least the one you described), U-Boot will
patch, or apply the overlay to, the DT provided by Linux, not its own,
so even if we prevent the overlay usage on U-Boot's own, the only
downside would be that the UEFI case Andre was describing would not
work with overlays anymore.

Actually, we're not building U-boot's DT with overlay support anyway
at the moment, so it's not a regression.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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