linux-kernel - RE: [RFC PATCH v2 08/15] usb:cdns3: Implements device operations part of the API

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Message-ID: <BYAPR07MB4709122D890CEA548523752FDDA30@BYAPR07MB4709.namprd07.prod.outlook.com>
Date:   Sun, 16 Dec 2018 13:31:47 +0000
From:   Pawel Laszczak <pawell@...ence.com>
To:     Sekhar Nori <nsekhar@...com>, Felipe Balbi <balbi@...nel.org>,
        Peter Chen <hzpeterchen@...il.com>
CC:     "rogerq@...com" <rogerq@...com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Alan Douglas <adouglas@...ence.com>,
        "jbergsagel@...com" <jbergsagel@...com>, "nm@...com" <nm@...com>,
        Suresh Punnoose <sureshp@...ence.com>,
        "peter.chen@....com" <peter.chen@....com>,
        Pawel Jez <pjez@...ence.com>, Rahul Kumar <kurahul@...ence.com>
Subject: RE: [RFC PATCH v2 08/15] usb:cdns3: Implements device operations part
 of the API

Hi

>
>On 14/12/18 4:56 PM, Felipe Balbi wrote:
>> 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?
>
>When, for example, its coming in the way of some clean-up I am
>attempting to do.
>
>>
>>> 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.
>
>I know :). I should have stated that in last e-mail itself, I am okay
>with just runtime detection.

I agree with you. Controller has usb_cap6 register that keep 
device controller version. It's not a problem doing such detection
at runtime. I will do it in this way.

But also I will add extra compatible  to dt-binding. Even if this will not be used 
in driver, it informs that there are several versions of controller. 

Thanks,
Pawel