On Tue, Feb 10, 2026 at 4:48 AM Leon Romanovsky leon@kernel.org wrote:

On Tue, Feb 10, 2026 at 10:05:14AM +0100, Jiri Pirko wrote:

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Mon, Feb 09, 2026 at 09:08:03PM +0100, jstultz@google.com wrote:

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On Mon, Feb 9, 2026 at 7:38 AM Jiri Pirko jiri@resnulli.us wrote:

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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.

Right, my original idea was to add a separate heap. Then I spotted the flags and seemed like a great fit. Was not aware or the history or original intention. Would be probably good to document it for future generations.

So instead of flag, I will add heap named something like "system_cc_decrypted" to implement this.

It is problematic to expose a user‑visible API that depends on a name. Such a design limits our ability to extend the functionality in the future, should new use cases arise.

Yes, how userland chooses a heap name is an open problem.

The difficulty is that userland is the only thing that knows what devices the buffer will be shared (and this knowledge may be incomplete if userland passes a buffer between processes) with, so it has to pick. But the kernel doesn't give it a way to solve the constraints of what memory types work with what devices. There have been some proposals for device sysfs directories to have links to heap types they support, but that also requires every driver to understand every heap type. And then you get to the fact that performance is what folks really want, not compatibility and that may require some system specific knowledge to decide.

The working solution right now is to have the system provide a [use] -> [heap] mapping for a specific system.

I think of this as similar to the vfs and /etc/fstab. So /home/ might be /dev/sdb1 on one device or dev/sda1 on another. You need some system specific configuration.

In Android, this mapping is done by Gralloc, so buffers are requested for a use and then Gralloc decides which heap to allocated from.

Unfortunately there doesn't seem to be a similar standard convention elsewhere. And I'll admit even then the enumeration of uses/pipelines in some general form is also difficult problem (and is somewhat more bounded for Android).

thanks -john