Re: [PATCH v3 1/5] MdeModulePkg: introduce non-discoverable device protocol

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 18 Nov 2016, at 14:39, Ni, Ruiyu <ruiyu.ni@intel.com> wrote:

>>>>>>>>> 1. Can you add "PCI" keyword into the protocol name?
>>>>>>>>> e.g.: EDKII_NON_DISCOVERABLE_PCI_DEVICE_PROTOCOL_GUID
>>>>>>>>> 
>>>>>>>> 
>>>>>>>> No. This protocol does not describe pci devices, and it is a peculiarity of the
>>>>>>>> edk2 driver stack that some non-pci devices can only be driven by pci drivers.
>>>>>>>> 
>>>>>>>> in other words, pci is part of the /driver/ side, and it is perfectly possible for,
>>>>>>>> e.g., a non-discoverable ahci device to be driven by a different non-pci driver
>>>>>>>> in the future.
>>>>>>>> 
>>>>>>> 
>>>>>>> I see. So some types of devices are handled by the current
>>>>>>> NonDiscoveablePciDevice driver, and some other types of devices may be
>>>>>>> handled by a future NonDiscoverableXXXDevice driver.
>>>>>>> Now since the AHCI type is already handled by the NonDiscoverablePciDevice
>>>>>>> driver, when there is a new NonDiscoverableXXXDevice driver, how can the two
>>>>>>> know whether it should manage the AHCI type device or not?
>>>>>> 
>>>>>> Good question. But how does the UEFI driver model deal with that? What happens if i have two drivers that both support
>>>> the
>>>>>> Ahci Pci class codes?
>>>>> PCI CFG header contains VendorID/DeviceID fields which can be used to distinguish
>>>>> them.
>>>>> 
>>>> 
>>>> No, that is not what I mean.
>>>> 
>>>> Your question is how we should deal with multiple drivers that
>>>> support, for instance, the AHCI non-discoverable device type. My
>>>> answer is that this is not any different from a platform configuration
>>>> that has more than one PCI I/O based driver that supports the AHCI PCI
>>>> class codes. The UEFI driver model has priority rules and protocols to
>>>> decide which driver gets precedence. I don't see how it should be any
>>>> different here.
>>> 
>>> I see they are different. Based on PciIo, the *HCI drivers can query
>>> additional information from PCI CFG header, instead of just using
>>> the PCI class code.
>>> 
>>> But with the NonDiscoverableDevice protocol, there is no additional
>>> information can help the *HCI drivers decide which to manage.
>>> 
>>> I don't see any practical negative point which prevents degrading
>>> NonDiscoverableDevice protocol to NonDiscoverable*Pci*Protocol.
>>> After all, as I said, all *HCI drivers are based on PciIo.
>>> 
>> 
>> Yes the *drivers* are based on PCI. But that does not make the
>> *devices* PCI devices. That is the whole problem we are trying to deal
>> with. So describing the non-PCI devices as PCI devices is incorrect
>> imo. The fact that we will use PCI drivers to drive non-PCI devices is
>> an implementation detail of EDK2, and is a property of the *driver*
>> side not the *device* side. So using PCI class codes etc to wire up
>> the correct driver should be local to the driver, and not pollute the
>> description of the device.
>> 
>> For example, if we would ever split the AHCI driver into a AHCI part
>> and a PCI part (which I know is unlikely to occur), I would want the
>> non-PCI AHCI driver to be used with the same protocol. Perhaps that
>> means we need a protocol for each type of device rather than an enum?
>> In any case, putting PCI-specific metadata into the device description
>> makes the situation worse, because now both the *device* and the
>> *driver* side are forced to use PCI internals to describe devices that
>> have nothing to do with PCI
> 
> If I understand correctly, you want the protocol producer can simply
> produce such protocol without the knowledge of PCI. I agree!
> But we do need to make the protocol definition stable enough. I do not
> like to see the enum type being extended in future to support more types
> of devices.
> 1. Can you use different GUIDs for different types of devices?

Yes that seems like a reasonable approach, in the spirit of EDK2 :-)

> 2. As I replied as comment #2 to patch 3/5, do you have better way to
> deal with the SDHCI Host controller driver access?


I need to think about this