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CC:     nd <nd@arm.com>
Subject: [RFC PATCH] drm:- Add a modifier to denote 'protected' framebuffer
Thread-Topic: [RFC PATCH] drm:- Add a modifier to denote 'protected'
 framebuffer
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Date:   Mon, 9 Sep 2019 13:42:53 +0000
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Add a modifier 'DRM_FORMAT_MOD_ARM_PROTECTED' which denotes that the frameb=
uffer
is allocated in a protected system memory.
Essentially, we want to support EGL_EXT_protected_content in our komeda dri=
ver.

Signed-off-by: Ayan Kumar Halder <ayan.halder@arm.com>

/-- Note to reviewer
Komeda driver is capable of rendering DRM (Digital Rights Management) prote=
cted
content. The DRM content is stored in a framebuffer allocated in system mem=
ory
(which needs some special hardware signals for access).

Let us ignore how the protected system memory is allocated and for the scop=
e of
this discussion, we want to figure out the best way possible for the usersp=
ace
to communicate to the drm driver to turn the protected mode on (for accessi=
ng the
framebuffer with the DRM content) or off.

The possible ways by which the userspace could achieve this is via:-

1. Modifiers :- This looks to me the best way by which the userspace can
communicate to the kernel to turn the protected mode on for the komeda driv=
er
as it is going to access one of the protected framebuffers. The only proble=
m is
that the current modifiers describe the tiling/compression format. However,=
 it
does not hurt to extend the meaning of modifiers to denote other attributes=
 of
the framebuffer as well.

The other reason is that on Android, we get an info from Gralloc
(GRALLOC_USAGE_PROTECTED) which tells us that the buffer is protected. This=
 can
be used to set up the modifier/s (AddFB2) during framebuffer creation.

2. Framebuffer flags :- As of today, this can be one of the two values
ie (DRM_MODE_FB_INTERLACED/DRM_MODE_FB_MODIFIERS). Unlike modifiers, the dr=
m
framebuffer flags are generic to the drm subsystem and ideally we should no=
t
introduce any driver specific constraint/feature.

3. Connector property:- I could see the following properties used for DRM
protected content:-
DRM_MODE_CONTENT_PROTECTION_DESIRED / ENABLED :- "This property is used by
userspace to request the kernel protect future content communicated over
the link". Clearly, we are not concerned with the protection attributes of =
the
transmitter. So, we cannot use this property for our case.

4. DRM plane property:- Again, we want to communicate that the framebuffer(=
which
can be attached to any plane) is protected. So introducing a new plane prop=
erty
does not help.

5. DRM crtc property:- For the same reason as above, introducing a new crtc
property does not help.

--/

---
 include/uapi/drm/drm_fourcc.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
index 3feeaa3f987a..38e5e81d11fe 100644
--- a/include/uapi/drm/drm_fourcc.h
+++ b/include/uapi/drm/drm_fourcc.h
@@ -742,6 +742,15 @@ extern "C" {
  */
 #define AFBC_FORMAT_MOD_BCH     (1ULL << 11)
=20
+/*
+ * Protected framebuffer
+ *
+ * The framebuffer is allocated in a protected system memory which can be =
accessed
+ * via some special hardware signals from the dpu. This is used to support
+ * 'GRALLOC_USAGE_PROTECTED' in our framebuffer for EGL_EXT_protected_cont=
ent.
+ */
+#define DRM_FORMAT_MOD_ARM_PROTECTED	fourcc_mod_code(ARM, (1ULL << 55))
+
 /*
  * Allwinner tiled modifier
  *
--=20
2.23.0