Re: [PATCH 4/5] dma-buf: heaps: allow heap to specify valid heap flags

[John Stultz <jstultz@google.com>]

On Mon, Feb 9, 2026 at 7:38 AM Jiri Pirko <jiri@resnulli.us> wrote:
>
> From: Jiri Pirko <jiri@nvidia.com>
>
> Currently the flags, which are unused, are validated for all heaps.
> Since the follow-up patch introduces a flag valid for only one of the
> heaps, allow to specify the valid flags per-heap.

I'm not really in this space anymore, so take my feedback with a grain of salt.

While the heap allocate flags argument is unused, it was intended to
be used for generic allocation flags that would apply to all or at
least a wide majority of heaps.

It was definitely not added to allow for per-heap or heap specific
flags (as this patch tries to utilize it). That was the mess we had
with ION driver that we were trying to avoid.

The intent of dma-buf heaps is to try to abstract all the different
device memory constraints so there only needs to be a [usage] ->
[heap] mapping, and otherwise userland can be generalized so that it
doesn't need to be re-written to work with different devices/memory
types.  Adding heap-specific allocation flags prevents that
generalization.

So instead of adding heap specific flags, the general advice has been
to add a separate heap name for the flag property.

Now, there has been many discussions around "protected buffers" (which
doesn't seem to map exactly to this confidental computing primitive,
but sounds like it might be related) , which have bounced between
being a allocation flag or a device specific heap without much
resolution. I appreciate in this patch seires you've pushed your
concept down into a DMA_ATTR_, as I do feel the kernel should have a
deeper sense of protected buffers (or any general propery like this)
as a concept if it is going to be a generic allocation flag, instead
of it being a somewhat thin creation of the outer heap-driver layer.

But, it seems like the use case here is still far too narrow for a top
level allocation flag.

So I'd advocate against introducing heap-specific flags like this.

thanks
-john