On 11/3/2021 14:38, Jordan Justen wrote:
John Harrison <john.c.harrison@xxxxxxxxx> writes:
On 11/1/2021 08:39, Jordan Justen wrote:
<John.C.Harrison@xxxxxxxxx> writes:
From: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
GuC contains a consolidated table with a bunch of information about the
current device.
Previously, this information was spread and hardcoded to all the components
including GuC, i915 and various UMDs. The goal here is to consolidate
the data into GuC in a way that all interested components can grab the
very latest and synchronized information using a simple query.
As per most of the other queries, this one can be called twice.
Once with item.length=0 to determine the exact buffer size, then
allocate the user memory and call it again for to retrieve the
table data. For example:
struct drm_i915_query_item item = {
.query_id = DRM_I915_QUERY_HWCONCFIG_TABLE;
};
query.items_ptr = (int64_t) &item;
query.num_items = 1;
ioctl(fd, DRM_IOCTL_I915_QUERY, query, sizeof(query));
if (item.length <= 0)
return -ENOENT;
data = malloc(item.length);
item.data_ptr = (int64_t) &data;
ioctl(fd, DRM_IOCTL_I915_QUERY, query, sizeof(query));
// Parse the data as appropriate...
The returned array is a simple and flexible KLV (Key/Length/Value)
formatted table. For example, it could be just:
enum device_attr {
ATTR_SOME_VALUE = 0,
ATTR_SOME_MASK = 1,
};
static const u32 hwconfig[] = {
ATTR_SOME_VALUE,
1, // Value Length in DWords
8, // Value
ATTR_SOME_MASK,
3,
0x00FFFFFFFF, 0xFFFFFFFF, 0xFF000000,
};
Seems simple enough, so why doesn't i915 define the format of the
returned hwconfig blob in i915_drm.h?
Because the definition is nothing to do with i915. This table comes from
the hardware spec. It is not defined by the KMD and it is not currently
used by the KMD. So there is no reason for the KMD to be creating
structures for it in the same way that the KMD does not document,
define, struct, etc. every other feature of the hardware that the UMDs
might use.
So, i915 wants to wash it's hands completely of the format? There is
obviously a difference between hardware features and a blob coming from
closed source software. (Which i915 just happens to be passing along.)
The hardware is a lot more difficult to change...
Actually, no. The table is not "coming from closed source software". The
table is defined by hardware specs. It is a table of hardware specific
values. It is not being invented by the GuC just for fun or as a way to
subvert the universe into the realms of closed source software. As per
KMD, GuC is merely passing the table through. The table is only
supported on newer hardware platforms and all GuC does is provide a
mechanism for the KMD to retrieve it because the KMD cannot access it
directly. The table contents are defined by hardware architects same as
all the other aspects of the hardware.
It seems like these details should be dropped from the i915 patch commit
message since i915 wants nothing to do with it.
Sure. Can remove comments.
I would think it'd be preferable for i915 to stand behind the basic blob
format as is (even if the keys/values can't be defined), and make a new
query item if the closed source software changes the format.
Close source software is not allowed to change the format because closed
source software has no say in defining the format. The format is
officially defined as being fixed in the spec. New key values can be
added to the key enumeration but existing values cannot be deprecated
and re-purposed. The table must be stable across all OSs and all
platforms. No software can arbitrarily decide to change it.
Of course, it'd be even better if i915 could define some keys/values as
well. (Or if a spec could be released to help document / tie down the
format.)
See the corresponding IGT test that details all the currently defined keys.
struct drm_i915_hwconfig {
uint32_t key;
uint32_t length;
uint32_t values[];
};
It sounds like the kernel depends on the closed source guc being loaded
to return this information. Is that right? Will i915 also become
dependent on some of this data such that it won't be able to initialize
without the firmware being loaded?
At the moment, the KMD does not use the table at all. We merely provide
a mechanism for the UMDs to retrieve it from the hardware.
In terms of future direction, that is something you need to take up with
the hardware architects.
Why do you keep saying hardware, when only software is involved?
See above - because the table is defined by hardware. No software,
closed or open, has any say in the specification of the table.
John.
The attribute ids are defined in a hardware spec.
Which spec?
Unfortunately, it is not one that is currently public. We are pushing
the relevant people to get it included in the public bspec / HRM. It is
a slow process :(.
Right. I take it no progress has been made in the 1.5 months since you
posted this, so it'll probably finally be documented 6~12 months after
hardware is available? :)
-Jordan
