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

[Marek Vasut]

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.

-- 
Best regards,
Marek Vasut