linux-kernel - [PATCH V3 6/6] dt-bindings: xen: Clarify "reg" purpose

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Date:   Wed, 24 Nov 2021 22:53:43 +0200
From:   Oleksandr Tyshchenko <olekstysh@...il.com>
To:     xen-devel@...ts.xenproject.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>
Cc:     Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Julien Grall <julien@....org>
Subject: [PATCH V3 6/6] dt-bindings: xen: Clarify "reg" purpose

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>

Xen on Arm has gained new support recently to calculate and report
extended regions (unused address space) safe to use for external
mappings. These regions are reported via "reg" property under
"hypervisor" node in the guest device-tree. As region 0 is reserved
for grant table space (always present), the indexes for extended
regions are 1...N.

No device-tree bindings update is needed (except clarifying the text)
as guest infers the presence of extended regions from the number
of regions in "reg" property.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
---
According to the recent update to Xen's guest.txt:
https://xenbits.xen.org/gitweb/?p=xen.git;a=blob_plain;f=docs/misc/arm/device-tree/guest.txt;hb=refs/heads/master

Changes V2 -> V3:
   - new patch
---
 Documentation/devicetree/bindings/arm/xen.txt | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/Documentation/devicetree/bindings/arm/xen.txt b/Documentation/devicetree/bindings/arm/xen.txt
index db5c56d..156fe10b 100644
--- a/Documentation/devicetree/bindings/arm/xen.txt
+++ b/Documentation/devicetree/bindings/arm/xen.txt
@@ -7,10 +7,14 @@ the following properties:
 	compatible = "xen,xen-<version>", "xen,xen";
   where <version> is the version of the Xen ABI of the platform.
 
-- reg: specifies the base physical address and size of a region in
-  memory where the grant table should be mapped to, using an
-  HYPERVISOR_memory_op hypercall. The memory region is large enough to map
-  the whole grant table (it is larger or equal to gnttab_max_grant_frames()).
+- reg: specifies the base physical address and size of the regions in memory
+  where the special resources should be mapped to, using an HYPERVISOR_memory_op
+  hypercall.
+  Region 0 is reserved for mapping grant table, it must be always present.
+  The memory region is large enough to map the whole grant table (it is larger
+  or equal to gnttab_max_grant_frames()).
+  Regions 1...N are extended regions (unused address space) for mapping foreign
+  GFNs and grants, they might be absent if there is nothing to expose.
   This property is unnecessary when booting Dom0 using ACPI.
 
 - interrupts: the interrupt used by Xen to inject event notifications.
-- 
2.7.4