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

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Message-ID: <YQL4qyAmqj322HTz@epycbox.lan>
Date:   Thu, 29 Jul 2021 11:51:23 -0700
From:   Moritz Fischer <mdf@...nel.org>
To:     "Wu, Hao" <hao.wu@...el.com>
Cc:     Tom Rix <trix@...hat.com>,
        "Weight, Russell H" <russell.h.weight@...el.com>,
        "mdf@...nel.org" <mdf@...nel.org>,
        "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 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?

- Moritz