Re: [PATCH v6 3/5] arm64: dts: sdm845: add Inline Crypto Engine registers and clock

[Bjorn Andersson <bjorn.andersson@linaro.org>]

On Tue 14 Jul 10:31 PDT 2020, Bjorn Andersson wrote:

> On Tue 14 Jul 10:12 PDT 2020, Eric Biggers wrote:
> 
> > On Tue, Jul 14, 2020 at 10:59:44AM -0600, Rob Herring wrote:
> > > On Tue, Jul 14, 2020 at 10:43 AM Eric Biggers <ebiggers@kernel.org> wrote:
> > > >
> > > > On Tue, Jul 14, 2020 at 10:35:12AM -0600, Rob Herring wrote:
> > > > > On Tue, Jul 14, 2020 at 10:15 AM Eric Biggers <ebiggers@kernel.org> wrote:
> > > > > >
> > > > > > On Tue, Jul 14, 2020 at 10:16:04AM -0400, Martin K. Petersen wrote:
> > > > > > >
> > > > > > > Eric,
> > > > > > >
> > > > > > > > Add the vendor-specific registers and clock for Qualcomm ICE (Inline
> > > > > > > > Crypto Engine) to the device tree node for the UFS host controller on
> > > > > > > > sdm845, so that the ufs-qcom driver will be able to use inline crypto.
> > > > > > >
> > > > > > > I would like to see an Acked-by for this patch before I merge it.
> > > > > > >
> > > > > >
> > > > > > Andy, Bjorn, or Rob: can you give Acked-by?
> > > > >
> > > > > DTS changes should go in via the QCom tree.
> > > > >
> > > >
> > > > So, the DTS patch can't be applied without the driver patches since then the
> > > > driver would misinterpret the ICE registers as the dev_ref_clk_ctrl registers.
> > > 
> > > That sounds broken, but there's no context here for me to comment
> > > further. DTS changes should work with old/stable kernels. I'd suggest
> > > you get a review from Bjorn on the driver first.
> > > 
> > 
> > The "breaking" change is that the dev_ref_clk_ctrl registers are now identified
> > by name instead of assumed to be index 1.
> > 
> > A reviewer had complained about the device-mapper bindings of this driver before
> > (https://lkml.kernel.org/r/158334171487.7173.5606223900174949177@swboyd.mtv.corp.google.com).
> > Changing to identifying the registers by name seemed like an improvement.
> > 
> > If needed I can add a hole at index 1 to make the DTS changes work with
> > old/stable kernels too, but I didn't know that is a requirement.  (Normally for
> > Linux kernel development, kernel-internal refactoring is always allowed
> > upstream.)  If I do this, would this hack have to be carried forever, or would
> > we be able to fix it up eventually?  Is there any deprecation period for DTS, or
> > do the latest DTS have to work with a 20 year old kernel?
> > 
> 
> The problem here is that DT binding refactoring is not kernel-internal.
> It's two different projects living in the same git.
> 
> There's a wish from various people that we make sure that new DTS
> continues to work with existing kernels. This is a nice in theory
> there's a lot of examples where we simply couldn't anticipate how future
> bindings would look. A particular example is that this prohibits most
> advancement in power management.
> 
> 
> But afaict what you describe above would make a new kernel failing to
> operate with the old DTS and that we have agreed to avoid.
> So I think the appropriate way to deal with this is to request the reg
> byname to detect the new binding and if that fails then assume that
> index 1 is dev_ref_clk_ctrl.
> 

I took another look at the git history and I can't find a single dts -
either upstream or in any downstream tree - that specifies that second
reg.

So per my argument below, if you could include a patch that just removes
the "dev_ref_clk_ctrl_mem" reference from the binding and driver I would
be happy to r-b that and ack this patch.

Regards,
Bjorn

> 
> There are cases where we just decide not to be backwards compatible, but
> it's pretty rare. As for deprecation, I think 1-2 LTS releases is
> sufficient, at that time scale it doesn't make sense to sit with an old
> DTB anyways (given the current pace of advancements in the kernel).
> 
> Regards,
> Bjorn