set_ptes() spec implies that it can only be used to set a present pte
because it interprets the PFN field to increment it. However,
set_pte_at() has been implemented on top of set_ptes() since set_ptes()
was introduced, and set_pte_at() allows setting a pte to a not-present
state. So clarify the spec to state that when nr==1, new state of pte
may be present or not present. When nr>1, new state of all ptes must be
present.

While we are at it, tighten the spec to set requirements around the
initial state of ptes; when nr==1 it may be either present or
not-present. But when nr>1 all ptes must initially be not-present. All
set_ptes() callsites already conform to this requirement. Stating it
explicitly is useful because it allows for a simplification to the
upcoming arm64 contpte implementation.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
  include/linux/pgtable.h | 4 ++++
  1 file changed, 4 insertions(+)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index f0feae7f89fb..5e7eaf8f2b97 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h

@@ -229,6 +229,10 @@ static inline pte_t pte_next_pfn(pte_t pte)
   * @pte: Page table entry for the first page.
   * @nr: Number of pages to map.
   *
+ * When nr==1, initial state of pte may be present or not present, and new state
+ * may be present or not present. When nr>1, initial state of all ptes must be
+ * not present, and new state must be present.
+ *
   * May be overridden by the architecture, or the architecture can define
   * set_pte() and PFN_PTE_SHIFT.
   *