Re: [PATCH v2 4/4] scsi: ufs: probe and init of variant driver from the platform device

[James Bottomley]

On Wed, 2015-06-17 at 14:21 +0000, Dov Levenglick wrote:
> Hi James,
> Rob raises a point that we don't agree with. On the other hand, we are not
> capable of convincing him in the validity of our approach - we are at an
> impasse.
> I would like to point out that our approach was reviewed by Paul and Mita
> (external reviewers) and neither of them had the objection that Rob has.
> I urge you to go over this thread, where both sides raised points and
> argued their cases. We are going to need your call on this so that we can
> move forward.

The dispute is about device tree bindings.  While I could override a NAK
in the subsystem I maintain, I'm not going to when it comes from the
maintainer of the device tree subsystem on a subject that's his province
of expertise, not mine.

Please agree on what the bindings should look like and then resubmit the
driver.

James


> Thanks,
> Dov
> 
> 
> > On Wed, Jun 17, 2015 at 8:17 AM, Dov Levenglick <dovl@xxxxxxxxxxxxxx>
> > wrote:
> >>> On Wed, Jun 17, 2015 at 2:42 AM, Dov Levenglick <dovl@xxxxxxxxxxxxxx>
> >>> wrote:
> >>>>> On Tue, Jun 9, 2015 at 12:53 AM, Dov Levenglick <dovl@xxxxxxxxxxxxxx>
> >>>>> wrote:
> >>>>>>> On Sun, Jun 7, 2015 at 10:32 AM,  <ygardi@xxxxxxxxxxxxxx> wrote:
> >>>>>>>>> 2015-06-05 5:53 GMT+09:00  <ygardi@xxxxxxxxxxxxxx>:
> >>>>>
> >>>>> [...]
> >>>>>
> >>>>>>>> If ufshcd-pltfrm driver is loaded before ufs-qcom, (what actually
> >>>>>>>> happens
> >>>>>>>> always), then the calling to of_platform_populate() which is
> >>>>>>>> added,
> >>>>>>>> guarantees that ufs-qcom probe will be called and finish, before
> >>>>>>>> ufshcd_pltfrm probe continues.
> >>>>>>>> so ufs_variant device is always there, and ready.
> >>>>>>>> I think it means we are safe - since either way, we make sure
> >>>>>>>> ufs-qcom
> >>>>>>>> probe will be called and finish before dealing with ufs_variant
> >>>>>>>> device
> >>>>>>>> in
> >>>>>>>> ufshcd_pltfrm probe.
> >>>>>>>
> >>>>>>> This is due to the fact that you have 2 platform drivers. You
> >>>>>>> should
> >>>>>>> only have 1 (and 1 node). If you really think you need 2, then you
> >>>>>>> should do like many other common *HCIs do and make the base UFS
> >>>>>>> driver
> >>>>>>> a set of library functions that drivers can use or call. Look at
> >>>>>>> EHCI,
> >>>>>>> AHCI, SDHCI, etc. for inspiration.
> >>>>>>
> >>>>>> Hi Rob,
> >>>>>> We did look at SDHCI and decided to go with this design due to its
> >>>>>> simplicity and lack of library functions. Yaniv described the proper
> >>>>>> flow
> >>>>>> of probing and, as we understand things, it is guaranteed to work as
> >>>>>> designed.
> >>>>>>
> >>>>>> Furthermore, the design of having a subcore in the dts is used in
> >>>>>> the
> >>>>>> Linux kernel. Please have a look at drivers/usb/dwc3 where - as an
> >>>>>> example
> >>>>>> - both dwc3-msm and dwc3-exynox invoke the probing function in
> >>>>>> core.c
> >>>>>> (i.e. the shared underlying Synopsys USB dwc3 core) by calling
> >>>>>> of_platform_populate().
> >>>>>
> >>>>> That binding has the same problem. Please don't propagate that. There
> >>>>> is no point in a sub-node in this case.
> >>>>>
> >>>>>> Do you see a benefit in the SDHCi implementation?
> >>>>>
> >>>>> Yes, it does not let the kernel driver design dictate the hardware
> >>>>> description.
> >>>>>
> >>>>> Rob
> >>>>>
> >>>>
> >>>> Hi Rob,
> >>>> We appear to be having a philosophical disagreement on the
> >>>> practicality
> >>>> of
> >>>> designing the ufshcd variant's implementation - in other words, we
> >>>> disagree on the proper design pattern to follow here.
> >>>> If I understand correctly, you are concerned with a design pattern
> >>>> wherein
> >>>> a generic implementation is wrapped - at the device-tree level - in a
> >>>> variant implementation. The main reason for your concern is that you
> >>>> don't
> >>>> want the "kernel driver design dictate the hardware description".
> >>>>
> >>>> We considered this point when we suggested our implementation (both
> >>>> before
> >>>> and after you raised it) and reached the conclusion that - while an
> >>>> important consideration - it should not be the prevailing one. I
> >>>> believe
> >>>> that you will agree once you read the reasoning. What guided us was
> >>>> the
> >>>> following:
> >>>> 1. Keep our change minimal.
> >>>> 2. Keep our patch in line with known design patterns in the kernel.
> >>>> 3. Have our patch extend the existing solution rather than reinvent
> >>>> it.
> >>>>
> >>>> It is the 3rd point that is most important to this discussion, since
> >>>> UFS
> >>>> has already been deployed by various vendors and is used by OEM.
> >>>> Changing
> >>>> ufshcd to a set of library functions that would be called by variants
> >>>> would necessarily introduce a significant change to the code flow in
> >>>> many
> >>>> places and would pose a backward compatibility issue. By using the
> >>>> tried
> >>>> and tested pattern of subnodes in the dts we were able to keep the
> >>>> change
> >>>> simple, succinct, understandable, maintainable and backward
> >>>> compatible.
> >>>> In
> >>>> fact, the entire logic tying of the generic implementation to the
> >>>> variant
> >>>> takes ~20 lines of code - both short and elegant.
> >>>
> >>> The DWC3 binding does this and nothing else that I'm aware of. This
> >>> hardly makes for a common pattern. If you really want to split this to
> >>> 2 devices, you can create platform devices without having a DT node.
> >>>
> >>> If you want to convince me this is the right approach for the binding
> >>> then you need to convince me the h/w is actually split this way and
> >>> there is functionality separate from the licensed IP.
> >>>
> >>> Rob
> >>>
> >>
> >> I don't understand the challenge that you just posed. It is clear from
> >> our
> >> implementation that there is the standard and variants thereof. I know
> >> this to be a fact on the processors that we are working on.
> >>
> >> Furthermore, although I didn't check each and every result in the
> >> search,
> >> of_platform_populate is used in more locations than dwc3 and at least a
> >> few of them seem have be using the same paradigm as ours
> >> (http://lxr.free-electrons.com/ident?i=of_platform_populate).
> >
> > You can ignore everything under arch/ as those are root level calls.
> > Most of the rest of the list are devices which have multiple unrelated
> > functions such as PMICs or system controllers which are perfectly
> > valid uses of of_platform_populate. That leaves at most 10 examples
> > that may or may not have good bindings. 10 out of hundreds of drivers
> > in the kernel hardly makes for a pattern to follow.
> >
> > Let me be perfectly clear on the binding as is: NAK. I'm done replying
> > until you propose something different.
> >
> > Rob
> > --
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> >
> 
> 
> QUALCOMM ISRAEL, on behalf of Qualcomm Innovation Center, Inc.
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
> 
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