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Message-ID: <20251214103928.2dc0690b@fedora>
Date: Sun, 14 Dec 2025 10:39:28 +0100
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Adrián Larumbe <adrian.larumbe@...labora.com>
Cc: linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org, Steven
Price <steven.price@....com>, kernel@...labora.com, Liviu Dudau
<liviu.dudau@....com>, Maarten Lankhorst
<maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
Simona Vetter <simona@...ll.ch>
Subject: Re: [PATCH v3 1/1] drm/panthor: Support partial unmaps of huge
pages
On Sat, 13 Dec 2025 19:08:33 +0000
Adrián Larumbe <adrian.larumbe@...labora.com> wrote:
> Commit 33729a5fc0ca ("iommu/io-pgtable-arm: Remove split on unmap
> behavior") did away with the treatment of partial unmaps of huge IOPTEs.
>
> In the case of Panthor, that means an attempt to run a VM_BIND unmap
> operation on a memory region whose start address and size aren't 2MiB
> aligned, in the event it intersects with a huge page, would lead to ARM
> IOMMU management code to fail and a warning being raised.
>
> Presently, and for lack of a better alternative, it's best to have
> Panthor handle partial unmaps at the driver level, by unmapping entire
> huge pages and remapping the difference between them and the requested
> unmap region.
>
> This could change in the future when the VM_BIND uAPI is expanded to
> enforce huge page alignment and map/unmap operational constraints that
> render this code unnecessary.
>
> Signed-off-by: Adrián Larumbe <adrian.larumbe@...labora.com>
> ---
> drivers/gpu/drm/panthor/panthor_mmu.c | 66 +++++++++++++++++++++++++++
> 1 file changed, 66 insertions(+)
>
> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
> index 183da30fa500..f11340a7f59e 100644
> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
> @@ -2110,6 +2110,44 @@ static int panthor_gpuva_sm_step_map(struct drm_gpuva_op *op, void *priv)
> return 0;
> }
>
> +static bool
> +iova_mapped_as_huge_page(const struct panthor_vma *vma, u64 addr)
> +{
> + const struct page *pg;
> + pgoff_t bo_offset;
> +
> + bo_offset = addr - vma->base.va.addr + vma->base.gem.offset;
> + pg = to_panthor_bo(vma->base.gem.obj)->base.pages[bo_offset >> PAGE_SHIFT];
> +
> + return (folio_size(page_folio(pg)) >= SZ_2M);
nit: you can drop the extra ()
return folio_size(page_folio(pg)) >= SZ_2M;
> +}
> +
> +static void
> +get_map_unmap_intervals(const struct drm_gpuva_op_remap *op,
> + const struct panthor_vma *unmap_vma,
> + u64 *unmap_start, u64 *unmap_range)
> +{
> + u64 aligned_unmap_start, aligned_unmap_end, unmap_end;
> +
> + drm_gpuva_op_remap_to_unmap_range(op, unmap_start, unmap_range);
> + unmap_end = *unmap_start + *unmap_range;
> +
> + aligned_unmap_start = ALIGN_DOWN(*unmap_start, SZ_2M);
> + if (op->prev && aligned_unmap_start < *unmap_start &&
> + op->prev->va.addr <= aligned_unmap_start &&
> + iova_mapped_as_huge_page(unmap_vma, *unmap_start)) {
> + *unmap_range += *unmap_start - aligned_unmap_start;
> + *unmap_start = aligned_unmap_start;
> + }
> +
> + aligned_unmap_end = ALIGN(unmap_end, SZ_2M);
> + if (op->next && aligned_unmap_end > unmap_end &&
> + op->next->va.addr + op->next->va.range >= aligned_unmap_end &&
> + iova_mapped_as_huge_page(unmap_vma, unmap_end - 1)) {
> + *unmap_range += aligned_unmap_end - unmap_end;
> + }
> +}
> +
> static int panthor_gpuva_sm_step_remap(struct drm_gpuva_op *op,
> void *priv)
> {
> @@ -2121,16 +2159,44 @@ static int panthor_gpuva_sm_step_remap(struct drm_gpuva_op *op,
> int ret;
>
> drm_gpuva_op_remap_to_unmap_range(&op->remap, &unmap_start, &unmap_range);
> +
> + /*
> + * ARM IOMMU page table management code disallows partial unmaps of huge pages,
> + * so when a partial unmap is requested, we must first unmap the entire huge
> + * page and then remap the difference between the huge page minus the requested
> + * unmap region. Calculating the right offsets and ranges for the different unmap
> + * and map operations is the responsibility of the following function.
> + */
> + get_map_unmap_intervals(&op->remap, unmap_vma, &unmap_start, &unmap_range);
Unfortunately, after 5b8fcf4777e7 ("drm/panthor: Add support for atomic
page table updates"), that's not enough, you also need to extend the
locked region (see [1]).
> +
> ret = panthor_vm_unmap_pages(vm, unmap_start, unmap_range);
> if (ret)
> return ret;
>
> if (op->remap.prev) {
> + ret = panthor_vm_map_pages(vm, unmap_start,
> + flags_to_prot(unmap_vma->flags),
> + to_drm_gem_shmem_obj(op->remap.prev->gem.obj)->sgt,
> + op->remap.prev->gem.offset +
> + (unmap_start - op->remap.prev->va.addr),
> + op->remap.prev->va.addr + op->remap.prev->va.range -
> + unmap_start);
> + if (ret)
> + return ret;
> +
> prev_vma = panthor_vm_op_ctx_get_vma(op_ctx);
> panthor_vma_init(prev_vma, unmap_vma->flags);
> }
>
> if (op->remap.next) {
> + ret = panthor_vm_map_pages(vm, op->remap.next->va.addr,
> + flags_to_prot(unmap_vma->flags),
> + to_drm_gem_shmem_obj(op->remap.next->gem.obj)->sgt,
> + op->remap.next->gem.offset,
> + unmap_start + unmap_range - op->remap.next->va.addr);
> + if (ret)
> + return ret;
> +
> next_vma = panthor_vm_op_ctx_get_vma(op_ctx);
> panthor_vma_init(next_vma, unmap_vma->flags);
> }
[1]https://gitlab.freedesktop.org/bbrezillon/linux/-/commit/b4b677796c8c33b5be60184bca099ef8fd8c5548
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