Date:   Wed, 4 Aug 2021 06:21:55 -0700
From:   Tom Rix <trix@...hat.com>
To:     Xu Yilun <yilun.xu@...el.com>
Cc:     Moritz Fischer <mdf@...nel.org>, "Wu, Hao" <hao.wu@...el.com>,
        "Weight, Russell H" <russell.h.weight@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fpga@...r.kernel.org" <linux-fpga@...r.kernel.org>
Subject: Re: [PATCH] fpga: region: handle compat_id as an uuid


On 8/3/21 6:44 PM, Xu Yilun wrote:
> On Tue, Aug 03, 2021 at 05:21:54AM -0700, Tom Rix wrote:
>> 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@...hat.com wrote:
>>>>>>>>>>> From: Tom Rix <trix@...hat.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 ?
> As I mentioned before, I assume there should be a unified output format
> for all vendors. So we need to choose one from raw hex, uuid_t, string
> and all other formats. Raw hex is the fundamental format, which could be
> further interpreted by user as a uuid_t, a string or other objects.

Having one format is not a requirement, the format is not listed in the 
docs.

How would converting an OF string to hex work and why would a vendor 
want to display it as hex ?

For the user, instead of reading an OF string directly they would be 
required to decrypt it.

So it they had a compat_id they would not use this one, they would stick 
it somewhere else.

Defeating the purpose of a having it a common area.

Tom

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

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