linux-kernel - Re: [PATCH] fpga: region: handle compat

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Message-ID: <dd2bc08b-3610-b14f-59fc-ab444845d0f7@redhat.com>
Date:   Fri, 30 Jul 2021 05:07:00 -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 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 ?

Tom

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