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Date:   Mon, 7 Nov 2022 16:21:33 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Jeffrey Hugo <quic_jhugo@quicinc.com>
Cc:     Oded Gabbay <ogabbay@kernel.org>, David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        Jason Gunthorpe <jgg@nvidia.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Yuji Ishikawa <yuji2.ishikawa@toshiba.co.jp>,
        Jiho Chu <jiho.chu@samsung.com>,
        Daniel Stone <daniel@fooishbar.org>,
        Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
        Christoph Hellwig <hch@infradead.org>,
        Kevin Hilman <khilman@baylibre.com>,
        Jagan Teki <jagan@amarulasolutions.com>,
        Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>,
        Maciej Kwapulinski <maciej.kwapulinski@linux.intel.com>,
        Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>,
        Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [RFC PATCH v3 0/3] new subsystem for compute accelerator devices
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On Mon, Nov 07, 2022 at 09:07:28AM -0700, Jeffrey Hugo wrote:
> > Another important change is that I have reverted back to use IDR for minor
> > handling instead of xarray. This is because I have found that xarray doesn't
> > handle well the scenario where you allocate a NULL entry and then exchange it
> > with a real pointer. It appears xarray still considers that entry a "zero"
> > entry. This is unfortunate because DRM works that way (first allocates a NULL
> > entry and then replaces the entry with a real pointer).
> > 
> > I decided to revert to IDR because I don't want to hold up these patches,
> > as many people are blocked until the support for accel is merged. The xarray
> > issue should be fixed as a separate patch by either fixing the xarray code or
> > changing how DRM + ACCEL do minor id handling.
> 
> This sounds sane to me.  However, this appears to be something that Matthew
> Wilcox should be aware of (added for visibility).  Perhaps he has a very
> quick solution.  If not, at-least he might have ideas on how to best address
> in the future.

Thanks for cc'ing me.  I wasn't aware of this problem because I hadn't
seen Oded's email yet.  The "problem" is simply a mis-use of the API.