On Wed, Feb 28, 2024 at 7:24 AM Maxime Ripard mripard@redhat.com wrote:
I'm currently working on a platform that seems to have togglable RAM ECC support. Enabling ECC reduces the memory capacity and memory bandwidth, so while it's a good idea to protect most of the system, it's not worth it for things like framebuffers that won't really be affected by a bitflip.
It's currently setup by enabling ECC on the entire memory, and then having a region of memory where ECC is disabled and where we're supposed to allocate from for allocations that don't need it.
My first thought to support this was to create a reserved memory region for the !ECC memory, and to create a heap to allocate buffers from that region. That would leave the system protected by ECC, while enabling userspace to be nicer to the system by allocating buffers from the !ECC region if it doesn't need it.
However, this creates basically a new combination compared to the one we already have (ie, physically contiguous vs virtually contiguous), and we probably would want to throw in cacheable vs non-cacheable too.
If we had to provide new heaps for each variation, we would have 8 heaps (and 6 new ones), which could be fine I guess but would still increase quite a lot the number of heaps we have so far.
Is it something that would be a problem? If it is, do you see another way to support those kind of allocations (like providing hints through the ioctl maybe?)?
So, the dma-buf heaps interface uses chardevs so that we can have a lot of flexibility in the types of heaps (and don't have the risk of bitmask exhaustion like ION had). So I don't see adding many differently named heaps as particularly problematic.
That said, if there are truly generic properties (cacheable vs non-cachable is maybe one of those) which apply to most heaps, I'm open to making use of the flags. But I want to avoid having per-heap flags, it really needs to be a generic attribute.
And I personally don't mind the idea of having things added as heaps initially, and potentially upgrading them to flags if needed (allowing heap drivers to optionally enumerate the old chardevs behind a config option for backwards compatibility).
How common is the hardware that is going to provide this configurable ECC option, and will you really want the option on all of the heap types? Will there be any hardware constraint limitations caused by the ECC/!ECC flags? (ie: Devices that can't use !ECC allocated buffers?)
If not, I wonder if it would make sense to have something more along the lines using a fcntl() like how F_SEAL_* is used with memfds? With some of the discussion around "restricted"/"secure" heaps that can change state, I've liked this idea of just allocating dmabufs from normal heaps and then using fcntl or something similar to modify properties of the buffer that are separate from the type of memory that is needed to be allocated to satisfy device constraints.
thanks -john