[U-Boot] [PATCH V2 2/2] pci: Update documentation to make 'compatible' string optional

[Marek Vasut]

On 09/02/2018 03:07 AM, Simon Glass wrote:
> Hi,
> 
> On 1 September 2018 at 16:41, Marek Vasut <marek.vasut at gmail.com> wrote:
>> On 09/01/2018 11:45 PM, Simon Glass wrote:
>>> Hi Marek,
>>>
>>> On 30 August 2018 at 04:20, Marek Vasut <marek.vasut at gmail.com> wrote:
>>>> On 08/30/2018 02:29 AM, Simon Glass wrote:
>>>>> Hi Marek,
>>>>
>>>> Hi,
>>>>
>>>>> On 24 August 2018 at 12:27, Marek Vasut <marek.vasut at gmail.com> wrote:
>>>>>> Reword the documentation to make it clear the compatible string is now
>>>>>> optional, yet still matching on it takes precedence over PCI IDs and
>>>>>> PCI classes.
>>>>>>
>>>>>> Signed-off-by: Marek Vasut <marek.vasut+renesas at gmail.com>
>>>>>> Cc: Simon Glass <sjg at chromium.org>
>>>>>> Cc: Tom Rini <trini at konsulko.com>
>>>>>> ---
>>>>>> V2: New patch
>>>>>> ---
>>>>>>  doc/driver-model/pci-info.txt | 14 +++++++++-----
>>>>>>  1 file changed, 9 insertions(+), 5 deletions(-)
>>>>>>
>>>>>> diff --git a/doc/driver-model/pci-info.txt b/doc/driver-model/pci-info.txt
>>>>>> index e1701d1fbc..14364c5c75 100644
>>>>>> --- a/doc/driver-model/pci-info.txt
>>>>>> +++ b/doc/driver-model/pci-info.txt
>>>>>> @@ -34,11 +34,15 @@ under that bus.
>>>>>>  Note that this is all done on a lazy basis, as needed, so until something is
>>>>>>  touched on PCI (eg: a call to pci_find_devices()) it will not be probed.
>>>>>>
>>>>>> -PCI devices can appear in the flattened device tree. If they do this serves to
>>>>>> -specify the driver to use for the device. In this case they will be bound at
>>>>>> -first. Each PCI device node must have a compatible string list as well as a
>>>>>> -<reg> property, as defined by the IEEE Std 1275-1994 PCI bus binding document
>>>>>> -v2.1. Note we must describe PCI devices with the same bus hierarchy as the
>>>>>> +PCI devices can appear in the flattened device tree. If they do, their node
>>>>>> +often contains extra information which cannot be derived from the PCI IDs or
>>>>>> +PCI class of the device. Each PCI device node must have a <reg> property, as
>>>>>> +defined by the IEEE Std 1275-1994 PCI bus binding document v2.1. Compatible
>>>>>> +string list is optional and generally not needed, since PCI is discoverable
>>>>>> +bus, albeit there are justified exceptions. If the compatible string is
>>>>>> +present, matching on it takes precedence over PCI IDs and PCI classes.
>>>>>> +
>>>>>> +Note we must describe PCI devices with the same bus hierarchy as the
>>>>>>  hardware, otherwise driver model cannot detect the correct parent/children
>>>>>>  relationship during PCI bus enumeration thus PCI devices won't be bound to
>>>>>>  their drivers accordingly. A working example like below:
>>>>>> --
>>>>>> 2.16.2
>>>>>>
>>>>>
>>>>> Are we really saying that compatible strings are 'generally not needed'?
>>>>
>>>> Yes, PCI is a discoverable bus.
>>>>
>>>>> device tree pci supplement 2.1 talks about some new formats for the
>>>>> compatible string, but doesn't say it is not needed. I much prefer a
>>>>> compatible string since it is easy to find the driver in the source
>>>>> code.
>>>>
>>>> But it duplicates (badly) what the PCI IDs and classes are used for
>>>> since PCI's inception.
>>>>
>>>>> Can way say that a compatible string is preferred, but in extremis you
>>>>> can avoid it by...
>>>>
>>>> No, see above, PCI is discoverable by design.
>>>
>>> I feel that these two things are orthogonal.
>>>
>>> You can probe the bus and find a device. That is the 'discoverable' part.
>>>
>>> But it is not automatically configurable. If it it were, there would
>>> be no DT node and no settings in the DT for the device. But from your
>>> patch, in some cases we need more information, and the DT node
>>> provides that.
>>
>> Pretty much, you can have stuff on the PCI card which needs extra info.
>>
>>> So to get the settings to pass to the driver, you have to find the
>>> device-tree node to use for the device. The only way U-Boot supports
>>> is to use the 'reg' property, which specifies the PCI address. (We
>>> don't support a compatible string starting with "pci...". We could
>>> support that, but it is more code for essentially the same purpose.)
>>
>> Yes
>>
>>> So we are not talking about the discoverability, which is already
>>> supported by U-Boot. We are talking about the configuration of the
>>> device, via settings passed to the driver.
>>
>> Yes
>>
>>> In fact the only issue here is whether to require a compatible string
>>> for PCI nodes or allow matching solely based on the 'reg' property. Is
>>> the latter widely used in Linux? Presumably not on x86, which doesn't
>>> even use DT.
>>
>> I only see the compatible string used for bridges, the rest of the
>> subdevices match on reg property.
> 
> Where are you looking?

Roughly 'git grep -A 10 pci arch/arm*/boot/dts' in Linux

-- 
Best regards,
Marek Vasut