From: Boris Brezillon <boris.brezillon@collabora.com>
To: Steven Price <steven.price@arm.com>
Cc: "Liviu Dudau" <liviu.dudau@arm.com>,
"Adrián Larumbe" <adrian.larumbe@collabora.com>,
dri-devel@lists.freedesktop.org, kernel@collabora.com,
"Eric Smith" <eric.smith@collabora.com>
Subject: Re: [PATCH v2 4/4] drm/panthor: Fix an off-by-one in the heap context retrieval logic
Date: Thu, 2 May 2024 16:47:45 +0200 [thread overview]
Message-ID: <20240502164745.0f11ed7c@collabora.com> (raw)
In-Reply-To: <20240502163602.70f554b5@collabora.com>
On Thu, 2 May 2024 16:36:02 +0200
Boris Brezillon <boris.brezillon@collabora.com> wrote:
> On Thu, 2 May 2024 15:26:55 +0100
> Steven Price <steven.price@arm.com> wrote:
>
> > On 02/05/2024 15:15, Boris Brezillon wrote:
> > > On Thu, 2 May 2024 15:03:51 +0100
> > > Steven Price <steven.price@arm.com> wrote:
> > >
> > >> On 30/04/2024 12:28, Boris Brezillon wrote:
> > >>> ID 0 is reserved to encode 'no-tiler-heap', the heap ID range is
> > >>> [1:MAX_HEAPS_PER_POOL], which we occasionally need to turn into an index
> > >>> in the [0:MAX_HEAPS_PER_POOL-1] when we want to access the context object.
> > >>
> > >> This might be a silly question, but do we need ID 0 to be
> > >> "no-tiler-heap"? Would it be easier to e.g. use a negative number for
> > >> that situation and avoid all the off-by-one problems?
> > >>
> > >> I'm struggling to find the code which needs the 0 value to be special -
> > >> where is it exactly that we encode this "no-tiler-heap" value?
> > >
> > > Hm, I thought we were passing the heap handle to the group creation
> > > ioctl, but heap queue/heap association is actually done through a CS
> > > instruction, so I guess you have a point. The only thing that makes a
> > > bit hesitant is that handle=0 is reserved for all other kind of handles
> > > we return, and I think I'd prefer to keep it the same for heap handles.
> > >
> > > This being said, we could do the `+- 1` in
> > > panthor_ioctl_tiler_heap_{create,destroy}() to keep things simple in
> > > panthor_heap.c.
> >
> > The heap handles returned to user space have the upper 16 bits encoding
> > the VM ID - so hopefully no one is doing anything crazy and splitting it
> > up to treat the lower part specially. And (unless I'm mistaken) the VM
> > IDs start from 1 so we'd still not have IDs of 0. So I don't think we
> > need the +- 1 part anywhere for tiler heaps.
>
> Ah, I forgot about that too. Guess we're all good with a
> [0,MAX_HEAPS_PER_POOL-1] range then.
>
> >
> > I'd certainly consider it a user space bug to treat the handles as
> > anything other than opaque. Really user space shouldn't be treating 0 as
> > special either: the uAPI doesn't say it's not valid. But I'd be open to
> > updating the uAPI to say 0 is invalid if there's some desire for that.
>
> Will do that in v3 then.
Taking that back. I don't think it needs to be enforced in the uAPI. As
you said, it's supposed to be opaque, so I'm tempted to update the
drm_panthor_tiler_heap_destroy::handle kerneldoc saying it must be
a valid handle returned by DRM_IOCTL_PANTHOR_TILER_HEAP_CREATE instead.
It's just that making the handle non-zero is kinda nice for debugging
purposes, and as I said, this way it's consistent with other kind of
handles (GEMs, VMs, syncobjs, ...).
prev parent reply other threads:[~2024-05-02 14:47 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-30 11:28 [PATCH v2 0/4] drm/panthor: Collection of tiler heap related fixes Boris Brezillon
2024-04-30 11:28 ` [PATCH v2 1/4] drm/panthor: Fix tiler OOM handling to allow incremental rendering Boris Brezillon
2024-04-30 15:27 ` Liviu Dudau
2024-05-02 14:03 ` Steven Price
2024-04-30 11:28 ` [PATCH v2 2/4] drm/panthor: Make sure the tiler initial/max chunks are consistent Boris Brezillon
2024-04-30 15:31 ` Liviu Dudau
2024-05-02 14:03 ` Steven Price
2024-04-30 11:28 ` [PATCH v2 3/4] drm/panthor: Relax the constraints on the tiler chunk size Boris Brezillon
2024-04-30 13:08 ` Adrián Larumbe
2024-05-02 14:03 ` Steven Price
2024-04-30 16:10 ` Liviu Dudau
2024-04-30 11:28 ` [PATCH v2 4/4] drm/panthor: Fix an off-by-one in the heap context retrieval logic Boris Brezillon
2024-04-30 16:40 ` Liviu Dudau
2024-04-30 17:07 ` Boris Brezillon
2024-05-02 14:03 ` Steven Price
2024-05-02 14:15 ` Boris Brezillon
2024-05-02 14:26 ` Steven Price
2024-05-02 14:36 ` Boris Brezillon
2024-05-02 14:47 ` Boris Brezillon [this message]
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