Re: [RFC PATCH v3 0/3] new subsystem for compute accelerator devices

From: Matthew Wilcox <willy@infradead.org>
To: Jeffrey Hugo <quic_jhugo@quicinc.com>
Cc: dri-devel@lists.freedesktop.org,
	Maciej Kwapulinski <maciej.kwapulinski@linux.intel.com>,
	Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>,
	Kevin Hilman <khilman@baylibre.com>,
	Christoph Hellwig <hch@infradead.org>,
	Jagan Teki <jagan@amarulasolutions.com>,
	Jason Gunthorpe <jgg@nvidia.com>,
	John Hubbard <jhubbard@nvidia.com>, Arnd Bergmann <arnd@arndb.de>,
	Jiho Chu <jiho.chu@samsung.com>,
	Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>,
	Yuji Ishikawa <yuji2.ishikawa@toshiba.co.jp>,
	Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Oded Gabbay <ogabbay@kernel.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	linux-kernel@vger.kernel.org,
	Thomas Zimmermann <tzimmermann@suse.de>,
	Alex Deucher <alexander.deucher@amd.com>
Subject: Re: [RFC PATCH v3 0/3] new subsystem for compute accelerator devices
Date: Mon, 7 Nov 2022 16:21:33 +0000	[thread overview]
Message-ID: <Y2kwjawUuFGDvr20@casper.infradead.org> (raw)
In-Reply-To: <6f756dec-b386-a822-5f52-c121c00525b4@quicinc.com>

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.