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Subject: Re: [PATCH] fpga: region: handle compat_id as an uuid
To:     Xu Yilun <yilun.xu@intel.com>
Cc:     Moritz Fischer <mdf@kernel.org>, "Wu, Hao" <hao.wu@intel.com>,
        "Weight, Russell H" <russell.h.weight@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-fpga@vger.kernel.org" <linux-fpga@vger.kernel.org>
References: <20210726202650.4074614-1-trix@redhat.com>
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 <ba28bac6-9c6d-de73-523f-b8ba4bef84de@redhat.com>
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 <YQL4qyAmqj322HTz@epycbox.lan>
 <a5b4b303-7d9b-27d7-4c1e-cd29fea8cdb9@redhat.com>
 <20210730014859.GA436611@yilunxu-OptiPlex-7050>
 <dd2bc08b-3610-b14f-59fc-ab444845d0f7@redhat.com>
 <20210803033224.GA461389@yilunxu-OptiPlex-7050>
From:   Tom Rix <trix@redhat.com>
Message-ID: <fa35d07e-f455-a9c9-1d01-b115d114de9b@redhat.com>
Date:   Tue, 3 Aug 2021 05:21:54 -0700
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On 8/2/21 8:32 PM, Xu Yilun wrote:
> On Fri, Jul 30, 2021 at 05:07:00AM -0700, Tom Rix wrote:
>> On 7/29/21 6:48 PM, Xu Yilun wrote:
>>> On Thu, Jul 29, 2021 at 12:16:47PM -0700, Tom Rix wrote:
>>>> On 7/29/21 11:51 AM, Moritz Fischer wrote:
>>>>> On Wed, Jul 28, 2021 at 01:36:56AM +0000, Wu, Hao wrote:
>>>>>>> On 7/26/21 3:12 PM, Russ Weight wrote:
>>>>>>>> On 7/26/21 1:26 PM, trix@redhat.com wrote:
>>>>>>>>> From: Tom Rix <trix@redhat.com>
>>>>>>>>>
>>>>>>>>> An fpga region's compat_id is exported by the sysfs
>>>>>>>>> as a 128 bit hex string formed by concatenating two
>>>>>>>>> 64 bit values together.
>>>>>>>>>
>>>>>>>>> The only user of compat_id is dfl.  Its user library
>>>>>>>>> opae converts this value into a uuid.
>>>>>>>>>
>>>>>>>>> ex/
>>>>>>>>> $ cat /sys/class/fpga_region/region1/compat_id
>>>>>>>>> f3c9941350814aadbced07eb84a6d0bb
>>>>>>>>>
>>>>>>>>> Is reported as
>>>>>>>>> $ fpgainfo bmc
>>>>>>>>> ...
>>>>>>>>> Pr Interface Id                  : f3c99413-5081-4aad-bced-07eb84a6d0bb
>>>>>>>>>
>>>>>>>>> Storing a uuid as 2 64 bit values is vendor specific.
>>>>>>>>> And concatenating them together is vendor specific.
>>>>>>>>>
>>>>>>>>> It is better to store and print out as a vendor neutral uuid.
>>>>>>>>>
>>>>>>>>> Change fpga_compat_id from a struct to a union.
>>>>>>>>> Keep the old 64 bit values for dfl.
>>>>>>>>> Sysfs output is now
>>>>>>>>> f3c99413-5081-4aad-bced-07eb84a6d0bb
>>>>>>>> I'm fowarding feedback from Tim Whisonant, one of the OPAE userspace
>>>>>>>> developers:
>>>>>>>>
>>>>>>>> I think that this change to the sysfs for the compat_id node will
>>>>>>>> end up breaking the SDK, which does not expect the '-' characters to
>>>>>>>> be included when parsing the sysfs value. Currently, it is parsed as
>>>>>>>> a raw hex string without regard to any '-' characters. This goes for
>>>>>>>> any "guid" currently exported by sysfs and for what we read in the
>>>>>>>> device MMIO space.
>>>>>>> Yes, it will.
>>>>>>>
>>>>>>> And there are other places, like dfl-afu-main.c:afu_id_show()
>>>>>>>
>>>>>>> outputs raw hex that sdk turns into a uuid.
>>>>>>>
>>>>>>>
>>>>>>> Some options.
>>>>>>>
>>>>>>> If no one but dfl will ever use it, then v1 of patchset.
>>>>>>>
>>>>>>> If others can use it but don't want to change dfl, then v2 of patchset,
>>>>>>> my favorite.
>>>>>>>
>>>>>>> Or this one for uuid for everyone, what have been v3 but changed too much.
>>>>>>>
>>>>>>>
>>>>>>> could dfl change generally to output uuid's to the sysfs ?
>>>>>>>
>>>>>>> this would be generally helpful and a one time disruption to the sdk.
>>>>>> This change limited the output format to uuid_t, but if any hardware doesn't
>>>>>> use uuid_t on hardware may have to convert it back from the sysfs output in
>>>>>> userspace. Leave it to print hardware values (e.g. from register), and convert
>>>>>> it in userspace should be fine too I think.
>>>>> I'm not entirely sure. I seem to recall there being examples of sysfs
>>>>> files returning different things for different drivers.
>>>>>
>>>>> That being said it seems largely cosmetic to add the '-' in between.
>>>>>
>>>>> If it breaks userspace, I'm against it. If you *need* it make a
>>>>> compat_uuid entry or something in that case?
>>>> My gripe is
>>>>
>>>> For a nominally common interface, compat_id has a vendor specific output.
>>>>
>>>> If for example another vendor wanted to use this field but their natural
>>>> format was an OF string.
>>>>
>>>> 16 bytes of raw hex would not work for them, so they would roll their own.
>>>>
>>>> which defeats the purpose of a common interface.
>>>>
>>>>
>>>> The language in the docs as-is is vague on the output format.
>>>>
>>>> DFL is the only user of the interface.
>>>>
>>>> So ver 2
>>>>
>>>> https://lore.kernel.org/linux-fpga/4ab7dd2d-c215-6333-6860-6f7d0ac64c3d@redhat.com/
>>>>
>>>> Keeps the output as-is for dfl, so nothing breaks in userspace
>>>>
>>>> And adds flexibility for vendors to output their appropriate natural form.
>>>>
>>>> So compat_id becomes generally useful.
>>> Mixing types seems be strongly against in Documentation/filesystems/sysfs.rst.
>>> So in my opinion there should be a determined format for the output. The
>>> concern for this patch is which one is a better format, uuid style or
>>> 128 bit raw hex?
>>>
>>> And I vote for 128 bit raw hex, as other vendors may not use uuid_t as
>>> the identifier, may be an OF string. So we don't have to force them
>>> decorate it as the uuid style.
>> So you would be ok with v2 of this patchset ?
> No. I prefer we keep the current implementation. I mean 128 bit raw hex
> could be a more compatible output format for all vendors, uuid style,
> string style or others.

How would 128 bit raw hex be compatible if a vendor's natural format was 
an OF string ?

Why would they even use compat_id ?

Tom

>
> Thanks,
> Yilun
>
>> Tom
>>
>>> Thanks
>>> Yilun
>>>
>>>> Tom
>>>>
>>>>
>>>>> - Moritz
>>>>>