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

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Message-ID: <d5623e70-7971-d729-f395-7259ab0efdb0@ti.com>
Date:   Fri, 14 Dec 2018 16:43:24 +0530
From:   Sekhar Nori <nsekhar@...com>
To:     Felipe Balbi <balbi@...nel.org>,
        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 Felipe,

On 14/12/18 4:17 PM, Felipe Balbi wrote:
> Hi,
> 
> Sekhar Nori <nsekhar@...com> writes:
> 
> <snip>
> 
>>>>> 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.

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.

> Personally, I wouldn't bother. Just like dwc3 never bothered and nothing
> bad came about because of it. Well, there are quirks which are
> undetectable and for those we have quirk flags. I much prefer that
> arrangement.

Yes, quirk flags will work too for undetectable quirks.

Thanks,
Sekhar