Re: [FFmpeg-devel] [PATCH] hwcontext_vulkan: fix exporting multi-plane DRM modifiers

[Russell Greene <russellgreene8@gmail.com>]

> vkGetPhysicalDeviceToolPropertiesEXT(dev, nb, array)
> If pToolProperties is NULL, then the number of tools currently active on
> physicalDevice is returned in pToolCount. Otherwise, pToolCount must
> point to a variable set by the application to the number of elements in
> the pToolProperties array, and on return the variable is overwritten
> with the number of structures actually written to pToolProperties. If
> pToolCount is less than the number of currently active tools, at most
> pToolCount structures will be written.
>
> It's in the spec. Different function, but all functions which write to
> an array follow the same signature.

Sorry, I guess I wasn't clear about my question.

I understand that you can fetch the size by setting the data pointer
to NULL, etc. The question is "is there way to fetch the number of
memory planes a modifier uses without allocating memory?"

My understanding is:
1. There is no fixed number of modifiers a driver can expose (this may
be wrong, but seems to me it would be a strange limitation, and I
don't see anything in the vk spec or drivers that indicat that this
could be the case)
2. We don't know where in the modifier list the one we want info about is

So therefore, we have to fetch the info for *all* modifiers, which as
I understand it probably requires at least a slow path with an
allocation for drivers that potentially have a lot of modifiers.

If we're OK with a practical "we don't support drivers with more than
X modifiers because we doubt it'll ever exist" then that's a valid
answer as well.
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