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From:   Felipe Balbi <balbi@kernel.org>
To:     Sekhar Nori <nsekhar@ti.com>, Peter Chen <hzpeterchen@gmail.com>,
        pawell@cadence.com
Cc:     rogerq@ti.com, devicetree@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-usb@vger.kernel.org, lkml <linux-kernel@vger.kernel.org>,
        adouglas@cadence.com, jbergsagel@ti.com, nm@ti.com,
        sureshp@cadence.com, peter.chen@nxp.com, pjez@cadence.com,
        kurahul@cadence.com
Subject: Re: [RFC PATCH v2 08/15] usb:cdns3: Implements device operations part of the API
In-Reply-To: <d5623e70-7971-d729-f395-7259ab0efdb0@ti.com>
References: <1542535751-16079-1-git-send-email-pawell@cadence.com> <1542535751-16079-9-git-send-email-pawell@cadence.com> <5BFE8883.7090802@ti.com> <CAL411-oKvu3EADStit=JtbcxVzM_fFGF2uPn67-wmXxPzUK6GQ@mail.gmail.com> <6b19b55c-66d7-439e-df8f-7b311b45af5e@ti.com> <BYAPR07MB470972A525D85B03C3037D3BDDA60@BYAPR07MB4709.namprd07.prod.outlook.com> <CAL411-oc-_0H88egpHsc06ptL12UsaE63rWwjN7novuFFx_qbA@mail.gmail.com> <5a41de27-cd1f-0cfd-ccdc-dccbf0854fcb@ti.com> <87bm5ol6zt.fsf@linux.intel.com> <d5623e70-7971-d729-f395-7259ab0efdb0@ti.com>
Date:   Fri, 14 Dec 2018 13:26:02 +0200
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Hi,

Sekhar Nori <nsekhar@ti.com> writes:
>>>>>> All this should be part of comments in code along with information a=
bout
>>>>>> 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=E2=80=99t ha=
ve 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 compati=
ble.
>>>
>>> Agree with detecting IP version at runtime.
>>>
>>> But please have some indication of version in compatible string too,
>>=20
>> why? Runtime detection by revision register should be the way to go if
>> the HW provides it. Why duplicate the information in compatible string?
>>=20
>>> 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.
>>=20
>> 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?=20

> 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.

=2D-=20
balbi

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