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

From: Rob Herring
Date: Wed Jun 17 2015 - 08:46:47 EST

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.

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