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

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Date:   Fri, 30 Jul 2021 09:48:59 +0800
From:   Xu Yilun <yilun.xu@...el.com>
To:     Tom Rix <trix@...hat.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 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.

Thanks
Yilun

> 
> 
> Tom
> 
> 
> > 
> > - Moritz
> >