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

[Simon Glass]

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?

Regards,
Simon