Re: [dpdk-dev] [PATCH] pci: Add the class_id support in pci probe

[Panu Matilainen <pmatilai@redhat.com>]

On 01/28/2016 11:38 PM, Thomas Monjalon wrote:
> 2016-01-13 14:22, Panu Matilainen:
>> On 01/13/2016 01:55 PM, Bruce Richardson wrote:
>>> On Thu, Dec 31, 2015 at 09:12:14AM -0800, Stephen Hemminger wrote:
>>>> On Tue, 29 Dec 2015 10:53:26 +0800
>>>> Ziye Yang <ziye.yang@intel.com> wrote:
>>>>
>>>>> This patch is used to add the class_id support
>>>>> for pci_probe since some devices need the class_info
>>>>> (class_code, subclass_code, programming_interface)
>>>>>
>>>>> Signed-off-by: Ziye Yang <ziye.yang@intel.com>
>>>>
>>>> Since rte_pci is exposed to application this breaks the ABI.
>>>
>>> But applications are not going to be defining rte_pci_ids values internally, are
>>> they? That is for drivers to use. Is this really an ABI breakage for applications that we
>>> need to be concerned about?
>>
>> There might not be applications using it but drivers are ABI consumers
>> too - think of 3rd party drivers and such.
>
> Drivers are not ABI consumers in the sense that ABI means
> Application Binary Interface.
> We are talking about drivers interface here.
> When establishing the ABI policy we were discussing about applications only.

Generally speaking an ABI is an interface between two program (or 
software if you like) modules, its not specific to "applications". 
Looking at http://dpdk.org/doc/guides/contributing/versioning.html I see 
it does only talk about applications, but an ABI consumer can also be 
another library. A driver calling rte_malloc() is just as much 
librte_eal ABI consumer as anything else.

Now, I understand that drivers use and need interface(s) that 
applications have no use for or simply cannot use, and those interfaces 
could be subject to different policies. As an extreme example, the Linux 
kernel has two isolated ABIs, one is the userland system call interface 
which is guaranteed to stay forever and the other is kernel module 
interface, guarantees nothing at all.

In DPDK the difference is far muddier than that since all the interfaces 
live in common, versioned userland DSOs. So if there are two different 
interfaces following two different policies, it's all the more important 
to clearly document them. One simple way could be using a different 
prefix than rte_.

> I agree we must allow 3rd party drivers but there is no good reason
> to try to upgrade DPDK without upgrading/porting the external drivers.
> If someone does not want to release its driver and keep upgrading DPDK,
> it is acceptable IMHO to force an upgrade of its driver.

Note that I've no particular sympathy for 3rd party drivers as such. 
What I *do* care about is that breakage is made explicit, as in drivers 
built for an incompatible version refuse to load at all, instead of 
silently corrupting memory etc.

	- Panu -