[dpdk-dev] [PATCH 00/13] Introducing EAL Bus-Device-Driver Model

[Shreyansh Jain]

On Wednesday 07 December 2016 05:47 PM, David Marchand wrote:
> Hello Shreyansh,
>
> On Wed, Dec 7, 2016 at 10:55 AM, Shreyansh Jain <shreyansh.jain at nxp.com> wrote:
>> On Wednesday 07 December 2016 02:22 AM, David Marchand wrote:
>>>> 0002~0003: Introducing the basic Bus model and associated test case
>>>> 0005:      Support insertion of device rather than addition to tail
>>>
>>>
>>> Patch 2 and 5 could be squashed.
>>
>>
>> I deliberately kept them separate. I intent to extend the Patch 5 for
>> hotplugging. But, if I don't end up adding support for that in this series,
>> I will merge these two.
>
> Fine.
>
>
>>> The constructor priority stuff seems unneeded as long as we use
>>> explicit reference to a global (or local, did not check) bus symbol
>>> rather than a runtime lookup.
>>
>>
>> I didn't understand your point here.
>> IMO, constructor priority (or some other way to handle this) is important. I
>> faced this issue while verifying it at my end when the drivers were getting
>> registered before the bus.
>>
>> Can you elaborate more on '..use explicit reference to a global...'?
>
> The drivers register themselves to a bus using this bus specific api.
>
> For pci, this is rte_eal_pci_register().
> The pci_bus object must be moved to eal_common_pci.c (we can stil
> internally expose for bsd / linux specific implementations).
> Then, rte_eal_pci_register() can add the pci driver to the pci_bus
> drivers list even if this pci_bus object is not registered yet to the
> buses list.

So, in eal_common_bus.c

--->8---

struct rte_bus *global_ptr_to_pci_bus = NULL;

struct rte_bus pci_bus = { ... };

rte_eal_pci_register() {
     if (global_ptr_to_pci_bus == NULL)
         rte_eal_bus_register(&pci_bus)
     else
        // continue as if PCI bus is registered
}

--->8---

so, no RTE_REGISTER_BUS()?

If yes, then RTE_REGISTER_BUS() should also check for an existing 
registration for duplication.

I was banking on a model where bus handlers (or bus drivers) are 
independent entities, just like PMDs. So, we have a bus XYZ without any 
drivers necessarily based on it.

By making registration dependent on driver registration, it becomes 
implicit that buses don't exist without drivers.
I am not in favor of this - or maybe I lack enough reason for this 
(about how it will make framework/PMD life better).

>
> And no constructor order issue ?
>
>
>>>
>>>
>>>> 0004:      Add scan and match callbacks for the Bus and updated test case
>>>
>>>
>>> Why do you push back the bus object in the 'scan' method ?
>>> This method is bus specific which means that the code "knows" the
>>> object registered with the callback.
>>
>>
>> This 'knows' is the grey area for me.
>> The bus (for example, PCI) after scanning needs to call
>> rte_eal_bus_add_device() to link the device in bus's device_list.
>>
>> Two options:
>> 1. Have a global reference to "pci" bus (rte_bus) somewhere in eal_pci.c
>> 2. Call rte_eal_get_bus() every time someone needs the reference.
>> 3. C++ style, 'this->'.
>>
>> I have taken the 3rd path. It simplifies my code to not assume a handle as
>> well as not allow for reference fetch calls every now and then.
>>
>> As a disadvantage: it means passing this as argument - and some cases
>> maintaining it as __rte_unused.
>>
>> Taking (1) or (2) is not advantageous than this approach.
>
> 1) is the simplest one.
>
> When you write a pci_scan method and embed it in you pci_bus object,
> but this pci_scan method still wonders which bus object it is supposed
> to work on, this is a bit like Schizophrenia ;-).

:)
This now is linked to the above issue of constructor priority and having 
a global bus reference. I don't personally prefer it.
I will still give this a serious thought, though.

>
>
>>> Is is that you want to have a single scan method used by multiple buses ?
>>
>>
>> Yes, but only as a use case. For example, platform devices are of various
>> types - what if we have a south-bound bus over a platform bus. In which
>> case, a hierarchical bus layout is possible.
>> But, this is far-fetched idea for now.
>
> Well, if you have no usecase at the moment, let's keep it simple, please.
>

Ok.

>
>>>
>>>> 0006:      Integrate bus scan/match with EAL, without any effective
>>>> driver
>>>
>>>
>>> Hard to find a right balance in patch splittng, but patch 4 and 6 are
>>> linked, I would squash them into one.
>>
>>
>> Yes, it is hard and sometimes there is simply no strong rationale for
>> splitting or merging. This is one of those cases.
>> My idea was that one patch _only_ introduces Bus services (structures,
>> functions etc) and another should enable the calls to it from EAL.
>> In that sense, I still think 4 and 6 should remain separate, may be
>> consecutive, though.
>
> Ok, will see in next version of the patchset.

Is there anything specific that you are looking for in patchset v2?
I was thinking of:
0. fixing BSD compilation issue reported by CI
1. improving the test_pci.c
2. hotplugging
3. trying to move PCI to drives/bus/pci/linux/* and resolving how 
drivers link to it, and how EAL resources like devargs are consumed.

Anything else?

>
>
>>>
>>>> 0007:      rte_pci_driver->probe replaced with rte_driver->probe
>>>
>>>
>>> This patch is too big, please separate in two patches: eal changes
>>> then ethdev/driver changes.
>>
>>
>> I don't think that can be done. One change is incomplete without the other.
>>
>> Changes to all files are only for rte_pci_driver->probe to rte_driver->probe
>> movement. EAL changes is to allow rte_eth_dev_pci_probe function after such
>> a change as rte_driver->probe has different arguments as compared to
>> rte_pci_driver->probe. The patches won't compile if I split.
>>
>> Am I missing something?
>>>
>>> Why do you push back the driver object in the 'probe' method ? (idem
>>> rte_bus->scan).
>>
>>
>> I am assuming you are referring to rte_driver->probe().
>> This is being done so that implementations (specific to drivers on a
>> particular bus) can start extracting the rte_xxx_driver, if need be.
>>
>> For example, for e1000/em_ethdev.c, rte_driver->probe() have been set to
>> rte_eth_dev_pci_probe() which requires rte_pci_driver to work with. In
>> absence of the rte_driver object, this function cannot call
>> rte_pci_driver->probe (for example) for driver specific operations.
>
> Sorry, I am thinking a step ahead with eth_driver out of the picture.
> But once eth_driver disappears, I can see no reason to keep this
> driver in the probe method (Schizophrenia again).

When eth_driver disappears, i was thinking of accomodating the 
eth_dev_init into the rte_pci_driver->probe/init.
But, this is still a nascent thought.
I am yet to start working on eth_driver.

>
>
>