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    SubjectRe: [PATCH] fpga: region: handle compat_id as an uuid
    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.

    Thanks,
    Yilun

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

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