Re: [FFmpeg-devel] [PATCH 55/97] Vulkan patchset part 2 - hwcontext rewrite and filtering

[Lynne <dev@lynne.ee>]

May 18, 2023, 10:30 by anton@khirnov.net:

> Quoting Lynne (2023-05-16 16:46:45)
>
>> May 16, 2023, 15:41 by anton@khirnov.net:
>>
>> > Quoting Lynne (2023-05-11 20:13:29)
>> >
>> >> >> diff --git a/libavutil/vulkan.h b/libavutil/vulkan.h
>> >> >> index 4bd1c9fc00..4c38dbc2e6 100644
>> >> >> --- a/libavutil/vulkan.h
>> >> >> +++ b/libavutil/vulkan.h
>> >> >> @@ -216,6 +216,9 @@ typedef struct FFVulkanContext {
>> >> >>  VkPhysicalDeviceProperties2 props;
>> >> >>  VkPhysicalDeviceDriverProperties driver_props;
>> >> >>  VkPhysicalDeviceMemoryProperties mprops;
>> >> >> +    VkQueueFamilyQueryResultStatusPropertiesKHR *query_props;
>> >> >> +    VkQueueFamilyVideoPropertiesKHR *video_props;
>> >> >> +    VkQueueFamilyProperties2 *qf_props;
>> >> >>
>> >> >
>> >> > How does the user of these know how many elements are in each array?
>> >> >
>> >>
>> >> They don't have to, we read the total number of queues available
>> >> for the device, so all indices are always available.
>> >>
>> >
>> > "all indices"?
>> >
>> > The allocated size of these arrays is purely local to
>> > ff_vk_load_props(), so there is no safe way for any outside caller to
>> > know it.
>> >
>>
>> The init function queries the driver for the total number of queue family indices,
>> allocates an array of that amount for each structure, and reads the properties
>> into the array.
>> API users then index into the array based on the queue family index.
>> API users cannot index outside of the array ever, as the queue family index
>> they receive is always AVVulkanDeviceContext.queue_family_index (or the
>> transfer, compute, encode, or decode queue family index member of that structure).
>> The queue family index members of that structure are checked upon initialization
>> to not be larger than what the driver returns.
>>
>> Hence, there's no need for them to know how large the array is, as
>> it is allocated for all possible indices.
>>
>
> That's way too much indirection and way too much code that has to make
> the exact same unstated assumption on what the actual size is. In my
> experience, it is almost always a good idea to be explicit: store the
> exact array size right next to the array itself.
>
> If nothing else, it will be very helpful for the person debugging the
> inevitable invalid accesses.
>

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