Re: [PATCH v3 07/14] drm/panthor: Add the MMU/VM logical block

[Boris Brezillon <boris.brezillon@collabora.com>]

On Mon,  4 Dec 2023 18:33:00 +0100
Boris Brezillon <boris.brezillon@collabora.com> wrote:

> +/**
> + * panthor_vm_idle() - Flag a VM idle
> + * @VM: VM to flag as idle.
> + *
> + * When we know the GPU is done with the VM (no more jobs to process),
> + * we can relinquish the AS slot attached to this VM, if any.
> + *
> + * We don't release the slot immediately, but instead place the VM in
> + * the LRU list, so it can be evicted if another VM needs an AS slot.
> + * This way, VMs keep attached to the AS they were given until we run
> + * out of free slot, limiting the number of MMU operations (TLB flush
> + * and other AS updates).
> + */
> +void panthor_vm_idle(struct panthor_vm *vm)
> +{
> +	struct panthor_device *ptdev = vm->ptdev;
> +
> +	mutex_lock(&ptdev->mmu->as.slots_lock);
> +	if (vm->as.id >= 0 && list_empty(&vm->as.lru_node))
> +		list_add_tail(&vm->as.lru_node, &ptdev->mmu->as.lru_list);

This doesn't work as soon as we have more than one scheduling group
per VM. If one of them is being scheduled out while the other stays on
a FW slot, the VM becomes idle and its AS might be re-assigned to a
different group. We need to refcnt the active users here, and only
return the AS when this active_cnt reaches 0.

> +	mutex_unlock(&ptdev->mmu->as.slots_lock);
> +}