Message-ID: <875zvwl585.fsf@linux.intel.com>
Date:   Fri, 14 Dec 2018 13:26:02 +0200
From:   Felipe Balbi <balbi@...nel.org>
To:     Sekhar Nori <nsekhar@...com>, Peter Chen <hzpeterchen@...il.com>,
        pawell@...ence.com
Cc:     rogerq@...com, devicetree@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
        adouglas@...ence.com, jbergsagel@...com, nm@...com,
        sureshp@...ence.com, peter.chen@....com, pjez@...ence.com,
        kurahul@...ence.com
Subject: Re: [RFC PATCH v2 08/15] usb:cdns3: Implements device operations part of the API


Hi,

Sekhar Nori <nsekhar@...com> writes:
>>>>>> All this should be part of comments in code along with information about
>>>>>> controller versions which suffer from the errata.
>>>>>>
>>>>>> Is there a version of controller available which does not have the
>>>>>> defect? Is there a future plan to fix this?
>>>>>>
>>>>>> If any of that is yes, you probably want to handle this with runtime
>>>>>> detection of version (like done with DWC3_REVISION_XXX macros).
>>>>>> Sometimes the hardware-read versions themselves are incorrect, so its
>>>>>> better to introduce a version specific compatible too like
>>>>>> "cdns,usb-1.0.0" (as hinted to by Rob Herring as well).
>>>>>>
>>>>>
>>>>> custom match_ep is used and works with all versions of the gen1
>>>>> controller. Future (gen2) releases of the controller won’t have such
>>>>> limitation but there is no plan to change current (gen1) functionality
>>>>> of the controller.
>>>>>
>>>>> I will add comment before cdns3_gadget_match_ep function.
>>>>> Also I will change cdns,usb3 to cdns,usb3-1.0.0 and add additional
>>>>> cdns,usb3-1.0.1 compatible.
>>>>>
>>>>> cdns,usb3-1.0.1 will be for current version of controller which I use.
>>>>> cdns,usb3-1.0.0 will be for older version - Peter Chan platform.
>>>>> I now that I have some changes in controller, and one of them require
>>>>> some changes in DRD driver. It will be safer to add two separate
>>>>> version in compatibles.
>>>>>
>>>>
>>>> Pawel, could we have correct register to show controller version? It is
>>>> better we could version judgement at runtime instead of static compatible.
>>>
>>> Agree with detecting IP version at runtime.
>>>
>>> But please have some indication of version in compatible string too,
>> 
>> why? Runtime detection by revision register should be the way to go if
>> the HW provides it. Why duplicate the information in compatible string?
>> 
>>> especially since you already know there is going to be another revision
>>> of hardware. It has the advantage that one can easily grep to see which
>>> hardware is running current version of controller without having access
>>> to the hardware itself. Becomes useful later on when its time to
>>> clean-up unused code when boards become obsolete or for requesting
>>> testing help.
>> 
>> This doesn't sound like a very strong argument, actually. Specially when
>> you consider that, since driver will do revision checking based on
>> revision register, you already have strings to grep. Moreover, we don't
>> usually drop support just like that.
>
> AFAICS, it is impossible to know just by grep'ing if there is any
> hardware still supported in kernel and using DWC3_REVISION_194A, for
> example.

but why do you even care? 

> If we are never going to drop support for any revision, this does not
> matter much.
>
> Also, once you have the controller supported behind PCI, then I guess
> you are pretty much tied to having to read hardware revision at runtime.

that's another argument *for* using runtime detection, not against it.

-- 
balbi

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