[PATCH 0/2] Document how userspace should use plane format list and IN_FORMATS

[PATCH 0/2] Document how userspace should use plane format list and IN_FORMATS

[Leandro Ribeiro]

This patch is to emphasize how userspace should use the plane format list and
the IN_FORMATS blob. The plane format list contains the formats that do not
require modifiers, and the blob property has the formats that support
modifiers.

Note that these are not disjoint sets. If a format supports modifiers but the
driver can also handle it without a modifier, it should be present in both the
IN_FORMATS blob property and the plane format list.

This is important for userspace, as there are situations in which we need to
find out if the KMS driver can handle a certain format without any modifiers.

Leandro Ribeiro (2):
  drm/doc: document drm_mode_get_plane
  drm/doc: emphasize difference between plane formats and IN_FORMATS
    blob

 drivers/gpu/drm/drm_plane.c |  4 ++++
 include/uapi/drm/drm_mode.h | 22 ++++++++++++++++++++++
 2 files changed, 26 insertions(+)

-- 
2.31.1

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[PATCH 1/2] drm/doc: document drm_mode_get_plane

[Leandro Ribeiro]

Add a small description and document struct fields of
drm_mode_get_plane.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
---
 include/uapi/drm/drm_mode.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index d1a93d2a85f9..96fc9a6da608 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -312,16 +312,35 @@ struct drm_mode_set_plane {
 	__u32 src_w;
 };
 
+/**
+ * struct drm_mode_get_plane - Get plane metadata.
+ *
+ * Userspace can perform a GETPLANE ioctl to retrieve information about a
+ * plane.
+ */
 struct drm_mode_get_plane {
+	/** @plane_id: Object ID of the plane. */
 	__u32 plane_id;
 
+	/** @crtc_id: Object ID of the current CRTC. */
 	__u32 crtc_id;
+	/** @fb_id: Object ID of the current fb. */
 	__u32 fb_id;
 
+	/**
+	 * @possible_crtcs: Pointer to ``__u32`` array of CRTC's that are
+	 * compatible with the plane.
+	 */
 	__u32 possible_crtcs;
+	/** @gamma_size: Size of the legacy gamma table. */
 	__u32 gamma_size;
 
+	/** @count_format_types: Number of formats. */
 	__u32 count_format_types;
+	/**
+	 * @format_type_ptr: Pointer to ``__u32`` array of formats that are
+	 * supported by the plane. These formats do not require modifiers.
+	 */
 	__u64 format_type_ptr;
 };
 
-- 
2.31.1

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Re: [PATCH 1/2] drm/doc: document drm_mode_get_plane

[Ville Syrjälä]

On Tue, Apr 06, 2021 at 04:21:17PM -0300, Leandro Ribeiro wrote:
> Add a small description and document struct fields of
> drm_mode_get_plane.
> 
> Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
> ---
>  include/uapi/drm/drm_mode.h | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index d1a93d2a85f9..96fc9a6da608 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -312,16 +312,35 @@ struct drm_mode_set_plane {
>  	__u32 src_w;
>  };
>  
> +/**
> + * struct drm_mode_get_plane - Get plane metadata.
> + *
> + * Userspace can perform a GETPLANE ioctl to retrieve information about a
> + * plane.
> + */
>  struct drm_mode_get_plane {
> +	/** @plane_id: Object ID of the plane. */
>  	__u32 plane_id;
>  
> +	/** @crtc_id: Object ID of the current CRTC. */
>  	__u32 crtc_id;
> +	/** @fb_id: Object ID of the current fb. */
>  	__u32 fb_id;
>  
> +	/**
> +	 * @possible_crtcs: Pointer to ``__u32`` array of CRTC's that are
> +	 * compatible with the plane.
> +	 */

It's a bitmask.

>  	__u32 possible_crtcs;
> +	/** @gamma_size: Size of the legacy gamma table. */
>  	__u32 gamma_size;
>  
> +	/** @count_format_types: Number of formats. */
>  	__u32 count_format_types;
> +	/**
> +	 * @format_type_ptr: Pointer to ``__u32`` array of formats that are
> +	 * supported by the plane. These formats do not require modifiers.
> +	 */
>  	__u64 format_type_ptr;
>  };
>  
> -- 
> 2.31.1
> 
> _______________________________________________
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> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
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Re: [PATCH 1/2] drm/doc: document drm_mode_get_plane

[Leandro Ribeiro]

On 4/7/21 10:45 AM, Ville Syrjälä wrote:
> On Tue, Apr 06, 2021 at 04:21:17PM -0300, Leandro Ribeiro wrote:
>> Add a small description and document struct fields of
>> drm_mode_get_plane.
>>
>> Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
>> ---
>>  include/uapi/drm/drm_mode.h | 19 +++++++++++++++++++
>>  1 file changed, 19 insertions(+)
>>
>> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
>> index d1a93d2a85f9..96fc9a6da608 100644
>> --- a/include/uapi/drm/drm_mode.h
>> +++ b/include/uapi/drm/drm_mode.h
>> @@ -312,16 +312,35 @@ struct drm_mode_set_plane {
>>  	__u32 src_w;
>>  };
>>  
>> +/**
>> + * struct drm_mode_get_plane - Get plane metadata.
>> + *
>> + * Userspace can perform a GETPLANE ioctl to retrieve information about a
>> + * plane.
>> + */
>>  struct drm_mode_get_plane {
>> +	/** @plane_id: Object ID of the plane. */
>>  	__u32 plane_id;
>>  
>> +	/** @crtc_id: Object ID of the current CRTC. */
>>  	__u32 crtc_id;
>> +	/** @fb_id: Object ID of the current fb. */
>>  	__u32 fb_id;
>>  
>> +	/**
>> +	 * @possible_crtcs: Pointer to ``__u32`` array of CRTC's that are
>> +	 * compatible with the plane.
>> +	 */
> 
> It's a bitmask.

Thank you, I'll fix this in the next version.

> 
>>  	__u32 possible_crtcs;
>> +	/** @gamma_size: Size of the legacy gamma table. */
>>  	__u32 gamma_size;
>>  
>> +	/** @count_format_types: Number of formats. */
>>  	__u32 count_format_types;
>> +	/**
>> +	 * @format_type_ptr: Pointer to ``__u32`` array of formats that are
>> +	 * supported by the plane. These formats do not require modifiers.
>> +	 */
>>  	__u64 format_type_ptr;
>>  };
>>  
>> -- 
>> 2.31.1
>>
>> _______________________________________________
>> dri-devel mailing list
>> dri-devel@lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 
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Re: [PATCH 1/2] drm/doc: document drm_mode_get_plane

[Simon Ser]

Hi Leandro,

Any chance you could re-spin at least the first patch?

Thanks,

Simon
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[PATCH 2/2] drm/doc: emphasize difference between plane formats and IN_FORMATS blob

[Leandro Ribeiro]

Emphasize how userspace should use the plane format list
(format_type_ptr) and the IN_FORMATS blob property.

Formats exposed in the plane format list (format_type_ptr) do not
require modifiers, and formats that are present in the IN_FORMATS blob
property support modifiers.

Note that these are not disjoint sets. If a format supports modifiers
but the driver can also handle it without a modifier, it should be
present in both the IN_FORMATS blob property and the plane format list.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
---
 drivers/gpu/drm/drm_plane.c | 4 ++++
 include/uapi/drm/drm_mode.h | 3 +++
 2 files changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
index 0dd43882fe7c..b48d9bd81a59 100644
--- a/drivers/gpu/drm/drm_plane.c
+++ b/drivers/gpu/drm/drm_plane.c
@@ -128,6 +128,10 @@
  *     pairs supported by this plane. The blob is a struct
  *     drm_format_modifier_blob. Without this property the plane doesn't
  *     support buffers with modifiers. Userspace cannot change this property.
+ *
+ *     To find out the list of buffer formats which are supported without a
+ *     modifier, userspace should not look at this blob property, but at the
+ *     formats list of the plane: &drm_mode_get_plane.format_type_ptr.
  */
 
 static unsigned int drm_num_planes(struct drm_device *dev)
diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index 96fc9a6da608..4293800ec095 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -340,6 +340,9 @@ struct drm_mode_get_plane {
 	/**
 	 * @format_type_ptr: Pointer to ``__u32`` array of formats that are
 	 * supported by the plane. These formats do not require modifiers.
+	 *
+	 * To find out the list of formats that support modifiers, userspace
+	 * must use the plane IN_FORMATS blob property.
 	 */
 	__u64 format_type_ptr;
 };
-- 
2.31.1

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Re: [PATCH 2/2] drm/doc: emphasize difference between plane formats and IN_FORMATS blob

[Ville Syrjälä]

On Tue, Apr 06, 2021 at 04:21:18PM -0300, Leandro Ribeiro wrote:
> Emphasize how userspace should use the plane format list
> (format_type_ptr) and the IN_FORMATS blob property.
> 
> Formats exposed in the plane format list (format_type_ptr) do not
> require modifiers, and formats that are present in the IN_FORMATS blob
> property support modifiers.
> 
> Note that these are not disjoint sets. If a format supports modifiers
> but the driver can also handle it without a modifier, it should be
> present in both the IN_FORMATS blob property and the plane format list.
> 
> Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
> ---
>  drivers/gpu/drm/drm_plane.c | 4 ++++
>  include/uapi/drm/drm_mode.h | 3 +++
>  2 files changed, 7 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> index 0dd43882fe7c..b48d9bd81a59 100644
> --- a/drivers/gpu/drm/drm_plane.c
> +++ b/drivers/gpu/drm/drm_plane.c
> @@ -128,6 +128,10 @@
>   *     pairs supported by this plane. The blob is a struct
>   *     drm_format_modifier_blob. Without this property the plane doesn't
>   *     support buffers with modifiers. Userspace cannot change this property.
> + *
> + *     To find out the list of buffer formats which are supported without a
> + *     modifier, userspace should not look at this blob property, but at the
> + *     formats list of the plane: &drm_mode_get_plane.format_type_ptr.
>   */
>  
>  static unsigned int drm_num_planes(struct drm_device *dev)
> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index 96fc9a6da608..4293800ec095 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -340,6 +340,9 @@ struct drm_mode_get_plane {
>  	/**
>  	 * @format_type_ptr: Pointer to ``__u32`` array of formats that are
>  	 * supported by the plane. These formats do not require modifiers.
> +	 *
> +	 * To find out the list of formats that support modifiers, userspace
> +	 * must use the plane IN_FORMATS blob property.
>  	 */

Addfb2+modifiers predates the IN_FORMATS blob, so this doesn't
match reality.

>  	__u64 format_type_ptr;
>  };
> -- 
> 2.31.1
> 
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Re: [PATCH 2/2] drm/doc: emphasize difference between plane formats and IN_FORMATS blob

[Simon Ser]

On Wednesday, April 7th, 2021 at 3:51 PM, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:

> > +	 * To find out the list of formats that support modifiers, userspace
> > +	 * must use the plane IN_FORMATS blob property.
> >  	 */
>
> Addfb2+modifiers predates the IN_FORMATS blob, so this doesn't
> match reality.

TBH, I'm inclined not to care about this edge-case. It's already
complicated enough for user-space to figure out what's the right thing
to do when supporting both implicit modifiers and explicit modifiers.
Using modifiers without IN_FORMATS is risky, since a whole part of the
modifier negotiation mechanism is missing.

Maybe we can just stick a "since kernel x.y.z" in here to address your
concern.
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Re: [PATCH 2/2] drm/doc: emphasize difference between plane formats and IN_FORMATS blob

[Pekka Paalanen]

[-- Attachment #1.1: Type: text/plain, Size: 2723 bytes --]

On Thu, 08 Apr 2021 08:39:10 +0000
Simon Ser <contact@emersion.fr> wrote:

> On Wednesday, April 7th, 2021 at 3:51 PM, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> 
> > > +	 * To find out the list of formats that support modifiers, userspace
> > > +	 * must use the plane IN_FORMATS blob property.
> > >  	 */  
> >
> > Addfb2+modifiers predates the IN_FORMATS blob, so this doesn't
> > match reality.  
> 
> TBH, I'm inclined not to care about this edge-case. It's already
> complicated enough for user-space to figure out what's the right thing
> to do when supporting both implicit modifiers and explicit modifiers.
> Using modifiers without IN_FORMATS is risky, since a whole part of the
> modifier negotiation mechanism is missing.
> 
> Maybe we can just stick a "since kernel x.y.z" in here to address your
> concern.

Yeah, or we could word it less "must", e.g. "The list of supported
formats with their explicit modifiers is advertised via IN_FORMATS blob."

We don't have to require userspace to not use the explicit modifier
uAPI if IN_FORMATS does not exist. There might be other ways how
userspace determines the explicit modifiers, like a Wayland compositor
advertising those format-modifier pairs that EGL can import. Then
clients use those, and the compositor can try to import those to KMS as
well. Maybe it works, maybe it doesn't (the same even if IN_FORMATS
exists).

IN_FORMATS just gives better chances of picking something that ends up
working.

The thing userspace *must not* do is to try to use the no-modifiers uAPI
when it has an explicit modifier. But do we then have exceptions for
MOD_LINEAR?

If a buffer has been allocated with explicit modifier MOD_LINEAR, does
it mean it is ok to import to KMS using the no-modifiers uAPI or not?

The other things userspace *must not* do is use the explicit modifier
uAPI when it does not have an explicit modifier, in essence pulling a
modifier out of a hat. It might be tempting to use MOD_LINEAR in that
case, sometimes it might work, but it's wrong. Userspace must use the
no-modifiers uAPI instead.

Right?

The point of these documentation patches is to establish the convention
that:

- drm_mode_get_plane::format_type_ptr is the list of pixel formats that
  can work via the no-modifiers uAPI, but says nothing about the
  explicit modifiers uAPI.

- IN_FORMATS is a list of format-modifier pairs that can work via the
  explicit modifiers API, but says nothing about the no-modifiers uAPI.

Is that a reasonable expectation?

Is it also so that passing MOD_INVALID to the explicit modifier uAPI
(ADDFB2) is invalid argument? Do we have that documented?


Thanks,
pq

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Re: [PATCH 2/2] drm/doc: emphasize difference between plane formats and IN_FORMATS blob

[Daniel Vetter]

On Thu, Apr 08, 2021 at 12:59:19PM +0300, Pekka Paalanen wrote:
> On Thu, 08 Apr 2021 08:39:10 +0000
> Simon Ser <contact@emersion.fr> wrote:
> 
> > On Wednesday, April 7th, 2021 at 3:51 PM, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> > 
> > > > +	 * To find out the list of formats that support modifiers, userspace
> > > > +	 * must use the plane IN_FORMATS blob property.
> > > >  	 */  
> > >
> > > Addfb2+modifiers predates the IN_FORMATS blob, so this doesn't
> > > match reality.  
> > 
> > TBH, I'm inclined not to care about this edge-case. It's already
> > complicated enough for user-space to figure out what's the right thing
> > to do when supporting both implicit modifiers and explicit modifiers.
> > Using modifiers without IN_FORMATS is risky, since a whole part of the
> > modifier negotiation mechanism is missing.
> > 
> > Maybe we can just stick a "since kernel x.y.z" in here to address your
> > concern.
> 
> Yeah, or we could word it less "must", e.g. "The list of supported
> formats with their explicit modifiers is advertised via IN_FORMATS blob."
> 
> We don't have to require userspace to not use the explicit modifier
> uAPI if IN_FORMATS does not exist. There might be other ways how
> userspace determines the explicit modifiers, like a Wayland compositor
> advertising those format-modifier pairs that EGL can import. Then
> clients use those, and the compositor can try to import those to KMS as
> well. Maybe it works, maybe it doesn't (the same even if IN_FORMATS
> exists).
> 
> IN_FORMATS just gives better chances of picking something that ends up
> working.

Yup.

> The thing userspace *must not* do is to try to use the no-modifiers uAPI
> when it has an explicit modifier. But do we then have exceptions for
> MOD_LINEAR?
> 
> If a buffer has been allocated with explicit modifier MOD_LINEAR, does
> it mean it is ok to import to KMS using the no-modifiers uAPI or not?

Maybe it work with current userspace, but not guaranteed. I think we
should strongly discourage this at least.

The case this can go boom is if userspace allocates a big bo, with some
implicit metadata. And then suballocates some linear buffer out of that
with explicit modifiers. This could happen if userspace does realize
modifiers work, and then does some funky optimization to linear e.g. as
part of a resolve pass. Not such hw/driver currently exists, but not
something I'd guarantee never happens.

If you then create a drm_fb with that with no modifier specified, you get
the implicit one from the metadata.
> 
> The other things userspace *must not* do is use the explicit modifier
> uAPI when it does not have an explicit modifier, in essence pulling a
> modifier out of a hat. It might be tempting to use MOD_LINEAR in that
> case, sometimes it might work, but it's wrong. Userspace must use the
> no-modifiers uAPI instead.

Yes. Userspace can't guess the modifier if it doesn't have it.


> 
> Right?
> 
> The point of these documentation patches is to establish the convention
> that:
> 
> - drm_mode_get_plane::format_type_ptr is the list of pixel formats that
>   can work via the no-modifiers uAPI, but says nothing about the
>   explicit modifiers uAPI.
> 
> - IN_FORMATS is a list of format-modifier pairs that can work via the
>   explicit modifiers API, but says nothing about the no-modifiers uAPI.
> 
> Is that a reasonable expectation?

I'm not sure. I thought they're the same list underneath in the kernel, at
least for drivers that do support modifiers. The current wording I think
suggests more meaning than is actually there.

> Is it also so that passing MOD_INVALID to the explicit modifier uAPI
> (ADDFB2) is invalid argument? Do we have that documented?

We'd need to check that, currently it's an out-of-band flag in the struct.
Atm DRM_FORMAT_MOD_INVALID is entirely an internal sentinel value to
denote end-of-array entries.

In practice it wont pass because we validate the modifiers against the
advertised list.
-Daniel

> 
> 
> Thanks,
> pq



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Re: [PATCH 2/2] drm/doc: emphasize difference between plane formats and IN_FORMATS blob

[Pekka Paalanen]

[-- Attachment #1.1: Type: text/plain, Size: 3432 bytes --]

On Thu, 8 Apr 2021 13:30:16 +0200
Daniel Vetter <daniel@ffwll.ch> wrote:

> On Thu, Apr 08, 2021 at 12:59:19PM +0300, Pekka Paalanen wrote:

> > The point of these documentation patches is to establish the convention
> > that:
> > 
> > - drm_mode_get_plane::format_type_ptr is the list of pixel formats that
> >   can work via the no-modifiers uAPI, but says nothing about the
> >   explicit modifiers uAPI.
> > 
> > - IN_FORMATS is a list of format-modifier pairs that can work via the
> >   explicit modifiers API, but says nothing about the no-modifiers uAPI.
> > 
> > Is that a reasonable expectation?  
> 
> I'm not sure. I thought they're the same list underneath in the kernel, at
> least for drivers that do support modifiers. The current wording I think
> suggests more meaning than is actually there.

They may be the same list in the kernel today, but do you want to force
all future drivers and future formats-modifiers to have that too?
Or did the boat sail already?

The existing uAPI considers these two to be independent lists, no
documentation saying otherwise, is there?

Should a kernel driver not have a way to say "this format won't work
via the no-modifiers uAPI"?

The practical consequence in userspace is how should userspace collect
the lists of supported format-modifier pairs, when the kernel has two
independent format lists, one carries modifiers explicitly and the
other does not. The one that carries explicit modifiers cannot denote
"no modifier" AFAIU.

So the "obvious" interpretation in userspace is that:
- the format list that does not carry any modifier information should
  be used with the no-modifiers uAPI, and
- the format list that does carry explicit modifiers should be used
  with the explicit modifiers uAPI.

If you were to say, that if IN_FORMATS exists, use it and ignore the
old no-modifiers format list, then the conclusion in userspace when
IN_FORMATS exists is that you cannot use the no-modifiers uAPI, because
all formats that are listed as supported carry an explicit modifier.

I understand that the format or format-modifier lists are not
authoritative. Formats outside of the lists *could* work. But why would
anyone bother trying something that is not suggested to work?

Or, is the intention such, that all formats in IN_FORMATS list imply
some support through the no-modifiers uAPI too, iff buffer
allocation does not give you an explicit modifier?

Or, should there be an i-g-t test to ensure that both the old and
IN_FORMATS lists have the exact same pixel formats always, carving that
fact into stone and resolving all this ambiguity?

> > Is it also so that passing MOD_INVALID to the explicit modifier uAPI
> > (ADDFB2) is invalid argument? Do we have that documented?  
> 
> We'd need to check that, currently it's an out-of-band flag in the struct.
> Atm DRM_FORMAT_MOD_INVALID is entirely an internal sentinel value to
> denote end-of-array entries.
> 
> In practice it wont pass because we validate the modifiers against the
> advertised list.

Right, so while at it, would be good to document that one cannot
substitute no-modifiers uAPI with explicit modifier uAPI using
MOD_INVALID. This should be documented, because other userspace APIs
have tendency to gravitate towards having just one explicit modifiers
function allowing MOD_INVALID, meaning "no modifier".


Thanks,
pq

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Re: [PATCH 2/2] drm/doc: emphasize difference between plane formats and IN_FORMATS blob

[Ville Syrjälä]

On Thu, Apr 08, 2021 at 04:57:51PM +0300, Pekka Paalanen wrote:
> On Thu, 8 Apr 2021 13:30:16 +0200
> Daniel Vetter <daniel@ffwll.ch> wrote:
> 
> > On Thu, Apr 08, 2021 at 12:59:19PM +0300, Pekka Paalanen wrote:
> 
> > > The point of these documentation patches is to establish the convention
> > > that:
> > > 
> > > - drm_mode_get_plane::format_type_ptr is the list of pixel formats that
> > >   can work via the no-modifiers uAPI, but says nothing about the
> > >   explicit modifiers uAPI.
> > > 
> > > - IN_FORMATS is a list of format-modifier pairs that can work via the
> > >   explicit modifiers API, but says nothing about the no-modifiers uAPI.
> > > 
> > > Is that a reasonable expectation?  
> > 
> > I'm not sure. I thought they're the same list underneath in the kernel, at
> > least for drivers that do support modifiers. The current wording I think
> > suggests more meaning than is actually there.
> 
> They may be the same list in the kernel today, but do you want to force
> all future drivers and future formats-modifiers to have that too?
> Or did the boat sail already?
> 
> The existing uAPI considers these two to be independent lists, no
> documentation saying otherwise, is there?
> 
> Should a kernel driver not have a way to say "this format won't work
> via the no-modifiers uAPI"?
> 
> The practical consequence in userspace is how should userspace collect
> the lists of supported format-modifier pairs, when the kernel has two
> independent format lists, one carries modifiers explicitly and the
> other does not. The one that carries explicit modifiers cannot denote
> "no modifier" AFAIU.
> 
> So the "obvious" interpretation in userspace is that:
> - the format list that does not carry any modifier information should
>   be used with the no-modifiers uAPI, and
> - the format list that does carry explicit modifiers should be used
>   with the explicit modifiers uAPI.
> 
> If you were to say, that if IN_FORMATS exists, use it and ignore the
> old no-modifiers format list, then the conclusion in userspace when
> IN_FORMATS exists is that you cannot use the no-modifiers uAPI, because
> all formats that are listed as supported carry an explicit modifier.
> 
> I understand that the format or format-modifier lists are not
> authoritative. Formats outside of the lists *could* work. But why would
> anyone bother trying something that is not suggested to work?

IMO formats not listed by any plane should just be rejected by addfb2.
I tried to put that check in the drm core actually but there was some
weird pushback, so for the moment it's handled by each driver. Some
drivers (like i915) will reject anything not supported by any plane,
other drivers might not (and probably no one knows how badly they
might blow up if you pass in some exotic format...).

I also had some igt patches to test that addfb2 behaviour but
they didn't go in either.

> 
> Or, is the intention such, that all formats in IN_FORMATS list imply
> some support through the no-modifiers uAPI too, iff buffer
> allocation does not give you an explicit modifier?
> 
> Or, should there be an i-g-t test to ensure that both the old and
> IN_FORMATS lists have the exact same pixel formats always, carving that
> fact into stone and resolving all this ambiguity?
> 
> > > Is it also so that passing MOD_INVALID to the explicit modifier uAPI
> > > (ADDFB2) is invalid argument? Do we have that documented?  
> > 
> > We'd need to check that, currently it's an out-of-band flag in the struct.
> > Atm DRM_FORMAT_MOD_INVALID is entirely an internal sentinel value to
> > denote end-of-array entries.
> > 
> > In practice it wont pass because we validate the modifiers against the
> > advertised list.

We don't actually. If the driver provides the .format_mod_supported()
hook then it's up to the driver to validate the modifier in said hook.
This was done so that people can embed metadata inside the modifier
while only having the base modifier on the modifier list. How userspace
is supposed to figure out which values for this extra metadata are valid
I have no idea.

-- 
Ville Syrjälä
Intel
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Re: [PATCH 2/2] drm/doc: emphasize difference between plane formats and IN_FORMATS blob

[Pekka Paalanen]

[-- Attachment #1.1: Type: text/plain, Size: 1556 bytes --]

On Thu, 8 Apr 2021 17:39:22 +0300
Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:

> On Thu, Apr 08, 2021 at 04:57:51PM +0300, Pekka Paalanen wrote:
> > On Thu, 8 Apr 2021 13:30:16 +0200
> > Daniel Vetter <daniel@ffwll.ch> wrote:
> >   

> > > > Is it also so that passing MOD_INVALID to the explicit modifier uAPI
> > > > (ADDFB2) is invalid argument? Do we have that documented?    
> > > 
> > > We'd need to check that, currently it's an out-of-band flag in the struct.
> > > Atm DRM_FORMAT_MOD_INVALID is entirely an internal sentinel value to
> > > denote end-of-array entries.
> > > 
> > > In practice it wont pass because we validate the modifiers against the
> > > advertised list.  
> 
> We don't actually. If the driver provides the .format_mod_supported()
> hook then it's up to the driver to validate the modifier in said hook.
> This was done so that people can embed metadata inside the modifier
> while only having the base modifier on the modifier list. How userspace
> is supposed to figure out which values for this extra metadata are valid
> I have no idea.

Maybe it's the difference between generic userspace and userspace
drivers? I've been having the feeling that these two have different
"rules". Maybe that distinction should be formalised in documentation
somewhere?

Generic userspace never looks into modifiers, it just relays them and
compares them as opaque 64-bit words.

Userspace drivers are allowed to look into what a modifier actually
means and fiddle with it.


Thanks,
pq

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Re: [PATCH 2/2] drm/doc: emphasize difference between plane formats and IN_FORMATS blob

[Daniel Vetter]

On Thu, Apr 08, 2021 at 05:39:22PM +0300, Ville Syrjälä wrote:
> On Thu, Apr 08, 2021 at 04:57:51PM +0300, Pekka Paalanen wrote:
> > On Thu, 8 Apr 2021 13:30:16 +0200
> > Daniel Vetter <daniel@ffwll.ch> wrote:
> > 
> > > On Thu, Apr 08, 2021 at 12:59:19PM +0300, Pekka Paalanen wrote:
> > 
> > > > The point of these documentation patches is to establish the convention
> > > > that:
> > > > 
> > > > - drm_mode_get_plane::format_type_ptr is the list of pixel formats that
> > > >   can work via the no-modifiers uAPI, but says nothing about the
> > > >   explicit modifiers uAPI.
> > > > 
> > > > - IN_FORMATS is a list of format-modifier pairs that can work via the
> > > >   explicit modifiers API, but says nothing about the no-modifiers uAPI.
> > > > 
> > > > Is that a reasonable expectation?  
> > > 
> > > I'm not sure. I thought they're the same list underneath in the kernel, at
> > > least for drivers that do support modifiers. The current wording I think
> > > suggests more meaning than is actually there.
> > 
> > They may be the same list in the kernel today, but do you want to force
> > all future drivers and future formats-modifiers to have that too?
> > Or did the boat sail already?
> > 
> > The existing uAPI considers these two to be independent lists, no
> > documentation saying otherwise, is there?
> > 
> > Should a kernel driver not have a way to say "this format won't work
> > via the no-modifiers uAPI"?
> > 
> > The practical consequence in userspace is how should userspace collect
> > the lists of supported format-modifier pairs, when the kernel has two
> > independent format lists, one carries modifiers explicitly and the
> > other does not. The one that carries explicit modifiers cannot denote
> > "no modifier" AFAIU.
> > 
> > So the "obvious" interpretation in userspace is that:
> > - the format list that does not carry any modifier information should
> >   be used with the no-modifiers uAPI, and
> > - the format list that does carry explicit modifiers should be used
> >   with the explicit modifiers uAPI.

Imo the right interpretation is "it's the same list".

> > If you were to say, that if IN_FORMATS exists, use it and ignore the
> > old no-modifiers format list, then the conclusion in userspace when
> > IN_FORMATS exists is that you cannot use the no-modifiers uAPI, because
> > all formats that are listed as supported carry an explicit modifier.

So formats without modifiers are very funny in their semantics. It means
- implied modifier is untiled
- except on i915 and radeon/amdgpu, where there's magic hidden information
  and you might get something else. But it won't work in multi-gpu
  situations
- except bugs (like imo mesa trying to use tiling without modifiers and
  without magic in-kernel tiling information forwarding is just broken,
  and surprise, it's broken). We've had some where parts of mesa where
  assuming modifiers are ok, but that wasn't the case.

Now maybe we can expose to userspace which drivers have magic device-local
tiling information sharing, but I don't expect this list to ever grow.

Anything beyond that sounds like the kernel should maintain a bug list of
things which are broken in userspace.

> > I understand that the format or format-modifier lists are not
> > authoritative. Formats outside of the lists *could* work. But why would
> > anyone bother trying something that is not suggested to work?
> 
> IMO formats not listed by any plane should just be rejected by addfb2.
> I tried to put that check in the drm core actually but there was some
> weird pushback, so for the moment it's handled by each driver. Some
> drivers (like i915) will reject anything not supported by any plane,
> other drivers might not (and probably no one knows how badly they
> might blow up if you pass in some exotic format...).
>
> I also had some igt patches to test that addfb2 behaviour but
> they didn't go in either.
>
> > Or, is the intention such, that all formats in IN_FORMATS list imply
> > some support through the no-modifiers uAPI too, iff buffer
> > allocation does not give you an explicit modifier?
> > 
> > Or, should there be an i-g-t test to ensure that both the old and
> > IN_FORMATS lists have the exact same pixel formats always, carving that
> > fact into stone and resolving all this ambiguity?
> > 
> > > > Is it also so that passing MOD_INVALID to the explicit modifier uAPI
> > > > (ADDFB2) is invalid argument? Do we have that documented?  
> > > 
> > > We'd need to check that, currently it's an out-of-band flag in the struct.
> > > Atm DRM_FORMAT_MOD_INVALID is entirely an internal sentinel value to
> > > denote end-of-array entries.
> > > 
> > > In practice it wont pass because we validate the modifiers against the
> > > advertised list.
> 
> We don't actually. If the driver provides the .format_mod_supported()
> hook then it's up to the driver to validate the modifier in said hook.
> This was done so that people can embed metadata inside the modifier
> while only having the base modifier on the modifier list. How userspace
> is supposed to figure out which values for this extra metadata are valid
> I have no idea.

It took me a while to figure this out again, but now I remember. It's
because of combinatorial explosion of modifiers with stuff like AFBC. So
the modifier list is not necessarily an exhaustive list of all
combinations, some of the modifiers count more like flags. Which is kinda
hilarious since it means userspace can't do anything with modifiers it
gets :-/
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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Re: [PATCH 0/2] Document how userspace should use plane format list and IN_FORMATS

[Daniel Vetter]

On Tue, Apr 06, 2021 at 04:21:16PM -0300, Leandro Ribeiro wrote:
> This patch is to emphasize how userspace should use the plane format list and
> the IN_FORMATS blob. The plane format list contains the formats that do not
> require modifiers, and the blob property has the formats that support
> modifiers.

Uh this is a very strong statement that I don't think is supported by
kernel driver code. Where is this from.

> Note that these are not disjoint sets. If a format supports modifiers but the
> driver can also handle it without a modifier, it should be present in both the
> IN_FORMATS blob property and the plane format list.

Same here ...

I thought these two lists are 100% consistent. If not sounds like driver
bugs that we need to maybe validate in drm_plane_init.

> This is important for userspace, as there are situations in which we need to
> find out if the KMS driver can handle a certain format without any modifiers.

I don't think you can rely on this. No modifiers means implicit modifier,
and the only thing that can give you such buffers is defacto mesa
userspace drivers (since that all depends upon driver private magic, with
maybe some kernel metadata passed around in private ioctls on the render
node).

Maybe for more context, what's the problem you've hit and trying to
clarify here?
-Daniel

> 
> Leandro Ribeiro (2):
>   drm/doc: document drm_mode_get_plane
>   drm/doc: emphasize difference between plane formats and IN_FORMATS
>     blob
> 
>  drivers/gpu/drm/drm_plane.c |  4 ++++
>  include/uapi/drm/drm_mode.h | 22 ++++++++++++++++++++++
>  2 files changed, 26 insertions(+)
> 
> -- 
> 2.31.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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Re: [PATCH 0/2] Document how userspace should use plane format list and IN_FORMATS

[Leandro Ribeiro]

On 4/8/21 8:35 AM, Daniel Vetter wrote:
> On Tue, Apr 06, 2021 at 04:21:16PM -0300, Leandro Ribeiro wrote:
>> This patch is to emphasize how userspace should use the plane format list and
>> the IN_FORMATS blob. The plane format list contains the formats that do not
>> require modifiers, and the blob property has the formats that support
>> modifiers.
>
> Uh this is a very strong statement that I don't think is supported by
> kernel driver code. Where is this from.
>
>> Note that these are not disjoint sets. If a format supports modifiers but the
>> driver can also handle it without a modifier, it should be present in both the
>> IN_FORMATS blob property and the plane format list.
> 
> Same here ...
> 

Yes, sorry. The wording was not good. To clarify:

I'm trying to document a good approach that userspace *can* (not must)
take to be able to tell if a certain format can be used in the
pre-modifier kernel uAPI or if it only works with modifiers.

The background is that we are reworking the way that Weston stores the
formats and modifiers supported by the planes, and there were some wrong
assumptions in the code related to what we can assume that the KMS
driver supports.

We've discussed and decided to send a patch to raise a discussion and
check if the conclusions that we've made were reasonable. And if not,
what would be a better approach.

This is part of a MR in which we add support for the dmabuf-hints
protocol extension in Weston. In sort, in Weston we store the formats
and modifiers supported by the planes. Then we send them to the client
and it may pick one of these format/modifier pairs to allocate its
buffers, increasing the chances of its surface ending up in a plane.

Here are two commits of the MR that are related to this discussion:

https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/544/diffs?commit_id=de6fc18bc35c2e43dff936dd85f310d1f778a7b8

https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/544/diffs?commit_id=75363bdb121bda2f326109afca5f4c3259423b7d

Thanks!

> I thought these two lists are 100% consistent. If not sounds like driver
> bugs that we need to maybe validate in drm_plane_init.
> 
>> This is important for userspace, as there are situations in which we need to
>> find out if the KMS driver can handle a certain format without any modifiers.
> 
> I don't think you can rely on this. No modifiers means implicit modifier,
> and the only thing that can give you such buffers is defacto mesa
> userspace drivers (since that all depends upon driver private magic, with
> maybe some kernel metadata passed around in private ioctls on the render
> node).
> 
> Maybe for more context, what's the problem you've hit and trying to
> clarify here?
> -Daniel
> 
>>
>> Leandro Ribeiro (2):
>>   drm/doc: document drm_mode_get_plane
>>   drm/doc: emphasize difference between plane formats and IN_FORMATS
>>     blob
>>
>>  drivers/gpu/drm/drm_plane.c |  4 ++++
>>  include/uapi/drm/drm_mode.h | 22 ++++++++++++++++++++++
>>  2 files changed, 26 insertions(+)
>>
>> -- 
>> 2.31.1
>>
> 
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Re: [PATCH 0/2] Document how userspace should use plane format list and IN_FORMATS

[Daniel Vetter]

On Thu, Apr 08, 2021 at 07:24:30PM -0300, Leandro Ribeiro wrote:
> 
> 
> On 4/8/21 8:35 AM, Daniel Vetter wrote:
> > On Tue, Apr 06, 2021 at 04:21:16PM -0300, Leandro Ribeiro wrote:
> >> This patch is to emphasize how userspace should use the plane format list and
> >> the IN_FORMATS blob. The plane format list contains the formats that do not
> >> require modifiers, and the blob property has the formats that support
> >> modifiers.
> >
> > Uh this is a very strong statement that I don't think is supported by
> > kernel driver code. Where is this from.
> >
> >> Note that these are not disjoint sets. If a format supports modifiers but the
> >> driver can also handle it without a modifier, it should be present in both the
> >> IN_FORMATS blob property and the plane format list.
> > 
> > Same here ...
> > 
> 
> Yes, sorry. The wording was not good. To clarify:

Ok I think this context helps.

> I'm trying to document a good approach that userspace *can* (not must)
> take to be able to tell if a certain format can be used in the
> pre-modifier kernel uAPI or if it only works with modifiers.

I think the short summary is "use modifiers everywhere you can".

> The background is that we are reworking the way that Weston stores the
> formats and modifiers supported by the planes, and there were some wrong
> assumptions in the code related to what we can assume that the KMS
> driver supports.
> 
> We've discussed and decided to send a patch to raise a discussion and
> check if the conclusions that we've made were reasonable. And if not,
> what would be a better approach.
> 
> This is part of a MR in which we add support for the dmabuf-hints
> protocol extension in Weston. In sort, in Weston we store the formats
> and modifiers supported by the planes. Then we send them to the client
> and it may pick one of these format/modifier pairs to allocate its
> buffers, increasing the chances of its surface ending up in a plane.
> 
> Here are two commits of the MR that are related to this discussion:
> 
> https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/544/diffs?commit_id=de6fc18bc35c2e43dff936dd85f310d1f778a7b8

  - drmModePlane's format list (the older, which does not advertise
    modifiers). Formats exposed through this support implicit modifiers.

The above isn't an accurate statement imo. Implied modifiers is a pretty
good mess:
- On most SoC platforms addfb1 actually implies linear. Except mesa got
  that wrong in a bunch of cases, and now everyone is unhappy.
- On i915/amdgpu/radeon there's implicit modifiers. Maybe also on nouveau
  I guess, not sure about any of the others. These don't generally work
  across device instances.
- On the kernel side, for drivers supporting modifiers, figuring out which
  implied modifier to pick is driver specific. There's bugs where
  essentially depending upon use case things wont work out.
- There are currently at least formats which never work with untiled
  modifier, so essentiall useless on addfb1. This applies to some afbc
  compressed formats.

In short: implied modifier is best effort trying to make stuff work,
somewhat, no guarantees.

The real recommendation is to not use implied modifiers if you can, so
also not use addfb1.

> https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/544/diffs?commit_id=75363bdb121bda2f326109afca5f4c3259423b7d

Again this isn't reflecting current reality. Right now the kernel puts the
same list into both. There is no meaning attached to these two lists being
different, they are not. Userspace starting to attach meaning pretty much
means we cannot, ever, make them different, and any additional hints would
need to be conveyed through new uapi somewhere else, maybe entire new list
of formats.

Some more fun things around modifiers:
- Some formats don't work at all with untile, so useless with addfb1.
  These got mostly added for afbc support I think.
- What's even more fun and I don't think documented anywhere: The modifier
  list is treated as a bitmask for some drivers, e.g. afbc drivers
  generally don't list all combinations, but just the flags they support.
  So you might get a format+modifier combo that's not even in the list you
  have, and it will actually work (with addfb2)

I think before we add new meaning to these two lists and somehow imply
they can be different (right now they are never different, in any kernel
that shipped ever since modifier support landed) is to document the
current modifier rules.

Cheers, Daniel

> 
> Thanks!
> 
> > I thought these two lists are 100% consistent. If not sounds like driver
> > bugs that we need to maybe validate in drm_plane_init.
> > 
> >> This is important for userspace, as there are situations in which we need to
> >> find out if the KMS driver can handle a certain format without any modifiers.
> > 
> > I don't think you can rely on this. No modifiers means implicit modifier,
> > and the only thing that can give you such buffers is defacto mesa
> > userspace drivers (since that all depends upon driver private magic, with
> > maybe some kernel metadata passed around in private ioctls on the render
> > node).
> > 
> > Maybe for more context, what's the problem you've hit and trying to
> > clarify here?
> > -Daniel
> > 
> >>
> >> Leandro Ribeiro (2):
> >>   drm/doc: document drm_mode_get_plane
> >>   drm/doc: emphasize difference between plane formats and IN_FORMATS
> >>     blob
> >>
> >>  drivers/gpu/drm/drm_plane.c |  4 ++++
> >>  include/uapi/drm/drm_mode.h | 22 ++++++++++++++++++++++
> >>  2 files changed, 26 insertions(+)
> >>
> >> -- 
> >> 2.31.1
> >>
> > 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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