Re: [Ksummit-discuss] [Stable kernel] feature backporting collaboration

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Sep 06, 2016 at 06:06:48PM +0200, Bartlomiej Zolnierkiewicz wrote:
> On Tuesday, September 06, 2016 02:34:30 PM Catalin Marinas wrote:
> > On Mon, Sep 05, 2016 at 05:22:59PM +0300, Laurent Pinchart wrote:
> > > On Monday 05 Sep 2016 10:03:27 Theodore Ts'o wrote:
> > > > Maybe there will be some hope if some of the features from ARM64
> > > > server can infect the SOC community --- Jon Masters really had the
> > > > right idea when he insisted on one kernel to boot all ARM64 kernels,
> > > > with all changes pushed upstream, and not hacky, out-of-tree patches
> > > > which only work for one SOC vendor.
[...]
> > > I don't think that's a fair comparison. For server platforms end-users of the 
> > > hardware will pick a distribution and roll it out on the machines, so hardware 
> > > vendors have a strong incentive to play by our rules. Phones are completely 
> > > different in that the device vendor doesn't care about end-users being able to 
> > > pick what software in general and kernel in particular they want to install on 
> > > the device.
> > 
> > Things could be different if fewer entities control the software that
> > gets installed/updated on such hardware. E.g. Google controlling the OTA
> > updates of the Chromebook kernels, they will at some point take a
> > similar hard stance to Red Hat on upstream first, single kernel Image.
> > For phones, however, that's unlikely to happen given the multitude and
> > short life-time of new products.
[...]
> For phones it feels that upstream itself is moving too slow for
> vendors to get benefits from upstream first policy.  Each year there
> is a new flagship device and new SoC.  With the current upstream model
> (new kernel release every 2-3 months and discussions on upstreaming
> some core subsystems enhancements easily taking weeks/months) upstream
> first policy just doesn't give enough business benefits.  Before you
> have everything upstream and changes propagate itself to LTS and Android
> kernels (which should also move faster BTW) it can be easily 2 years
> since start of the effort and your SoC is now obsolete.

If there is a one-off, completely independent SoC that no-one (not even
the vendor) cares about a year after the device goes on sales, I don't
think we want it upstream either. However, SoC vendors tend to work on
SoC families with some variations within a family (like CPU upgrades,
maybe a new interconnect etc.) but in general a lot of code that can be
reused. That's where upstreaming is highly beneficial to the vendor on
the long run since such SoC family has a life span bigger than the
individual device derived from a specific SoC.

I'm also aware that vendors don't always want to disclose their SoC
details until the device goes public, so that's another business
argument against upstreaming first, especially in the mobile world.

One impediment to upstreaming in my experience is that vendors tend to
develop the initial SoC port against an old kernel version (e.g. based
on the Android version they target). Forward-porting to latest mainline
all of a sudden becomes a much larger task that companies are not always
willing to (sufficiently) invest in. So if an "upstream first" policy is
not always feasible from a (mobile) business perspective, "develop
against upstream" is a better second option. An initial SoC port doesn't
need all the additional features that Android kernels provide, so it's
usually doable with what is available upstream. There is more effort
initially since targeting certain Android versions require back-porting,
however it pays off in the long run for the SoC *family*.

Trying to get on-topic: where organisations providing kernels like LSK
(Linaro) can help is offering to integrate/maintain the SoC back-port
while encouraging the SoC vendors to focus on developing against the
latest upstream. It looks to me that on (too) many occasions SoC vendors
take LSK as their development base for new SoC ports, making the
forward-porting effort significantly larger (and potentially ignored).

-- 
Catalin