Re: [RFC 0/9] Unrestricted media entity ID range support

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hi Mauro,

Mauro Carvalho Chehab wrote:
> Em Sun, 20 Sep 2015 10:26:09 +0300
> Sakari Ailus <sakari.ailus@linux.intel.com> escreveu:
>
>> Hi Mauro,
>>
>> Mauro Carvalho Chehab wrote:
>>> Hi Sakari,
>>>
>>> On Fri, 11 Sep 2015 13:09:03 +0300
>>> Sakari Ailus <sakari.ailus@linux.intel.com> wrote:
>>>
>>>> Hi all,
>>>>
>>>> This patchset adds an API for managing entity enumerations, i.e.
>>>> storing a bit of information per entity. The entity ID is no longer
>>>> limited to small integers (e.g. 31 or 63), but
>>>> MEDIA_ENTITY_MAX_LOW_ID. The drivers are also converted to use that
>>>> instead of a fixed number.
>>>>
>>>> If the number of entities in a real use case grows beyond the
>>>> capabilities of the approach, the algorithm may be changed. But most
>>>> importantly, the API is used to perform the same operation everywhere
>>>> it's done.
>>>>
>>>> I'm sending this for review only, the code itself is untested.
>>>>
>>>> I haven't entirely made up my mind on the fourth patch. It could be
>>>> dropped, as it uses the API for a somewhat different purpose.
>>>>
>>>
>>> Sorry for not reviewing this earlier, but I'm traveling this week to
>>> China, and I was having some troubles with the Internet. Btw, I don't
>>> have my notebook here (just a chromebook without the media tree).
>>> So, please consider this as just a preliminary review.
>>>
>>> I won't be comment this series patch by patch, because it is really
>>> painful to do it while here with this Internet.
>>>
>>> Also, I want to discuss the patch series as a hole.
>>>
>>>   From what we've agreed last week, we won't be using a separate ID
>>> range for the entity ID, but this patch series is actually adding
>>> it, and, IMHO, using a confusing nomenclature: instead of calling the
>>> entity ID range as "entity_id" at the media_device struct, you're
>>> now calling it "low_id". That sounds confusing to me. If you think
>>> we should keep a separate range for entities, calling it as
>>> "entity_id" is clearer.
>>
>> It's *not* the entity ID. It's a number used internally to keep track of
>> the entities, and only used for this purpose, nothing else. If you look
>> at the patch, the number of places where low_id is used is very limited.
>> Individual drivers do not and must not access the low_id field.
>>
>> The underlying algorithm for keeping track of entities does not change,
>> but that could be changed later on without affecting the users of the
>> alrogithm. --- See patch 3.
>>
>> There are two main reasons for this:
>>
>> 1. No need to implement a new algorithm which would be less efficient
>> but could cope in cases we do not have yet; this can be done later on,
>> as patch 3 adds an API to access the information without making
>> assumptions on the implementation.
>
> If this is an internal number used only by the graph traversal
> algorithm, then the implementation doesn't seem correct. I mean:
> such number should be generated internally when starting the
> graph traversal algorithm, and it would be better to store such
> graph-traversal internal algorithm data on a separate struct.

In that case the number would become enumeration specific. I have no 
problem in that, but if a single entity specific number works for the 
purpose, I think that could be used as well. Using something else than a 
constant value per entity quickly will require memory allocation, and 
graph traversal is performance critical in many cases.

>
>> 2. It does solve the problem. The graph object IDs of the entities can
>> be large without adversely affecting the functionality of existing drivers.
>
> Right now, with just those 9 patches, it doesn't ;)
>
> I mean: if I apply your patches after the MC next gen ones and try to use the
> graph traversal, it will do the wrong thing for hybrid TV cards, as the number
> of entities there are more than 64. Ok, easy to fix after this series by
> just changing the value of MEDIA_ENTITY_MAX_LOW_ID, but this will only
> work while we don't implement dynamic support.

Fair enough. Still, it didn't work before either: the graph traversal 
algorithm depends on entity ID not exceeding MEDIA_ENTITY_ENUM_MAX_ID 
(which this set renames).

I still consider that a separate problem.

>
>>>
>>> Also, you said at the low_id comment that if an entity is
>>> unregistered and then re-registered, it would preserve the same
>>
>> I never claimed that, and the patchset does not implement that either.
>
> That's what I understood from this comment:
>
> 	Rename the macro as it no longer is a maximum ID that an entity can have,
> 	but a low ID which is used for internal purposes of enumeration. This is
> 	the maximum number of concurrent entities that may exist in a media
> 	device.
>
> If this is the "max number of concurrent entities", you need to reassign
> those numbers when entities are removed.
>
> If this is not what you're meaning, then fix the patch description
> to let it clearer.
>
> I guess then that "low ID" is actually an upper bound for that
> graph-traversal only control number. We should really not use the word "ID"
> here, as this is not an ID. it is just some index/control number that will
> be granted to be below some upper bound.

Index sounds good to me, it's indeed clearer. I didn't like "low" 
either, it just happens to be that way right now. With a different 
algorithm, I think we can get rid of the entire field --- but it will 
have to be replaced by something that requires memory allocation.

...

>>>
>>> - some other mechanism would be available for drivers that
>>>     would support dynamic entity creation.
>>>
>>> So, I don't see how this would solve the problems that we
>>> discussed at the last week IRC chats.
>>>
>>> Am I missing something?
>>
>> This set indeed solves a problem, and that problem was unrestricting the
>> graph object ID of the entities. There are other problems remaining
>> before entities can be e.g. unregistered one by one, but they are
>> separate problems.
>
> That's not the way I see it ;) I mean, for the current MC code, this patch
> series is not needed, as all drivers right now have less than 64 entities.

In the last IRC meeting we agreed to have a single ID range for graph 
object IDs. Now that ID will be given to all graph objects, 64 will be 
reached much sooner than it used to, and it probably happens on at least 
some existing drivers as well.

>
> This need only starts after/during the MC next gen patches, in order
> to address two changes that come on this series:
>
> - having an arbitrary entity ID number that will be a way bigger
>    than the current upper bound (the number of entities);

It's indeed *the number of entities*, not entity ID.

>
> - support more than 64 entities.
>
> So, I would be expecting a patch series that would either:
>
> 1) change the graph traversal algorithms to not depend on some
> upper bound limit;
>
> 	or:
>
> 2) dynamically create whatever internal index number and to
> dynamically determine the upper bound limit that would be needed for the
> graph traversal algorithm to work, allocating those data when the graph
> traversal algorithm starts and freeing them when the graph traversal
> stops, and/or when the media device is unregistered.

Even with 4 k entities, the memory required by that algorithm to track 
the entities would be just 512 bytes + 16 * depth. I wonder if a more 
sophisticated algorithm would be able to operate with less or as 
efficiently.

The memory would need to come from elsewhere than from the stack obviously.

>
>>
>>>
>>> Regards,
>>> Mauro
>>>
>>> PS.: sparse already complains on two places at the media-entity where
>>> bitmaps are declared at the stack. With max entities equal to 64,
>>> that's not an issue, but if we change to a higher number, those will
>>> need to be dynamically allocated, in order to avoid stack overflows.
>>> I didn't see any patches touching that.
>>
>> I agree. The aim of the set was not to increase the number of concurrent
>> entities. That is a separate problem which can be solved later on once
>> we have a use case for it.
>>
>
> I reviewed that code. The problem there is actually unrelated to what
> this patch series is trying to solve, as the problem there is due to the
> number of PADs (and not on the number of entities). I mean about this
> code at media_entity_pipeline_start():
>
> 	while ((entity = media_entity_graph_walk_next(&graph))) {
> 		DECLARE_BITMAP(active, entity->num_pads);
> 		DECLARE_BITMAP(has_no_links, entity->num_pads);
>
> That limits the max number of pads, as a big number would cause a
> Linux stack overflow.
>
> Those bitmaps should likely be moved to struct media_pipeline or
> struct media_device and be either dynamically created/removed or
> having them relying on a max number of pads.

How many pads are we looking forwards to have?

I don't think we have a fixed limit as such, but AFAIR there's no driver 
using more than around five at the moment, so there was really no need 
to worry about it.

I guess it'd be good to have a maximum though just to be on the safe side.

-- 
Regards,

Sakari Ailus
sakari.ailus@linux.intel.com