Re: [PATCH v7 3/9] PCI: endpoint: functions/pci-epf-test: Add second entry

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Tue, May 01, 2018 at 05:53:59PM +0530, Kishon Vijay Abraham I wrote:
> Hi Lorenzo,
> 
> On Tuesday 01 May 2018 05:24 PM, Lorenzo Pieralisi wrote:
> > On Tue, May 01, 2018 at 03:37:47PM +0530, Kishon Vijay Abraham I wrote:
> >> Hi Lorenzo,
> >>
> >> On Thursday 26 April 2018 10:26 PM, Lorenzo Pieralisi wrote:
> >>> On Tue, Apr 24, 2018 at 02:44:40PM +0100, Gustavo Pimentel wrote:
> >>>> Adds a seconds entry on the pci_epf_test_ids structure that disables the
> >>>
> >>> "Add a second entry to..."
> >>>
> >>>> linkup_notifier parameter on driver for the designware EP.
> >>>>
> >>>> This allows designware EPs that doesn't have linkup notification signal
> >>>> to work with pcitest.
> >>>>
> >>>> Updates the binding documentation accordingly.
> >>>
> >>> Valid for all the series: use imperative sentences.
> >>>
> >>> eg:
> >>>
> >>> "Update the binding documentation accordingly".
> >>>
> >>> not
> >>>
> >>> "Updates the binding documentation accordingly".
> >>>
> >>>> Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
> >>>> Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
> >>>> ---
> >>>> Change v2->v3:
> >>>>  - Added second entry in pci_epf_test_ids structure.
> >>>>  - Remove test_reg_bar field assignment on second entry.
> >>>> Changes v3->v4:
> >>>>  - Nothing changed, just to follow the patch set version.
> >>>> Changes v4->v5:
> >>>>  - Changed pci_epf_test_cfg2 to pci_epf_test_designware.
> >>>> Changes v5->v6:
> >>>>  - Changed name field from pci_epf_test_designware to pci_epf_test_dw.
> >>>> Changes v6->v7:
> >>>>  - Changed variable name from data_cfg2 to data_linkup_notifier_disabled.
> >>>>
> >>>>  Documentation/PCI/endpoint/function/binding/pci-test.txt | 2 ++
> >>>>  drivers/pci/endpoint/functions/pci-epf-test.c            | 8 ++++++++
> >>>>  2 files changed, 10 insertions(+)
> >>>>
> >>>> diff --git a/Documentation/PCI/endpoint/function/binding/pci-test.txt b/Documentation/PCI/endpoint/function/binding/pci-test.txt
> >>>> index 3b68b95..dc39f47 100644
> >>>> --- a/Documentation/PCI/endpoint/function/binding/pci-test.txt
> >>>> +++ b/Documentation/PCI/endpoint/function/binding/pci-test.txt
> >>>> @@ -1,6 +1,8 @@
> >>>>  PCI TEST ENDPOINT FUNCTION
> >>>>  
> >>>>  name: Should be "pci_epf_test" to bind to the pci_epf_test driver.
> >>>> +name: Should be "pci_epf_test_dw" to bind to the pci_epf_test driver
> >>>> +      with a custom configuration for the designware EP.
> >>>
> >>> The link between the "name" and the device created is quite obscure and
> >>> reading pci-test-howto.txt certainly does not clarify it.
> >>>
> >>> In pci-test-howto.txt an explanation should be added to the configs
> >>> device creation paragraph to clarify it.
> >>>
> >>>>  Configurable Fields:
> >>>>  vendorid	 : should be 0x104c
> >>>> diff --git a/drivers/pci/endpoint/functions/pci-epf-test.c b/drivers/pci/endpoint/functions/pci-epf-test.c
> >>>> index 7cef851..4ab463b 100644
> >>>> --- a/drivers/pci/endpoint/functions/pci-epf-test.c
> >>>> +++ b/drivers/pci/endpoint/functions/pci-epf-test.c
> >>>> @@ -459,10 +459,18 @@ static int pci_epf_test_bind(struct pci_epf *epf)
> >>>>  	return 0;
> >>>>  }
> >>>>  
> >>>> +static const struct pci_epf_test_data data_linkup_notifier_disabled = {
> >>>> +	.linkup_notifier = false
> >>>> +};
> >>>> +
> >>>>  static const struct pci_epf_device_id pci_epf_test_ids[] = {
> >>>>  	{
> >>>>  		.name = "pci_epf_test",
> >>>>  	},
> >>>> +	{
> >>>> +		.name = "pci_epf_test_dw",
> >>>> +		.driver_data = (kernel_ulong_t)&data_linkup_notifier_disabled,
> >>>> +	},
> >>>>  	{},
> >>>
> >>> Should not this be a property derived from the controller compatible
> >>> property instead of the test device name written in configfs ?
> >>
> >> pci_epf_test is an independent driver on its own that operates in a layer above
> >> the controller driver. So it does not get the controller compatible (which is
> >> used in controller drivers like pcie-designware-plat.c). pci_epf_test uses
> >> "pci_epf_device_id" which is _similar_ to "of_device_id" used by platform drivers.
> > 
> > I understand that, the problem is that the independent driver depends on
> > features of the related controller driver as this patch shows. This
> > patch basically says that if you use a specific path in configfs (that
> > includes pci_epf_test_dw) your device has specific HW features (eg
> > linkup notifier above), that obviously depends on the platform HW not on
> > the string you use in configfs.
> > 
> > What I am questioning is a) if I understand this right and b) whether
> > this is the right approach.
> 
> Your understanding is right. Ideally pci-epf-test driver shouldn't have any HW
> specific configuration. But different HW have different configurations and
> pci-epf-test should be informed of the configuration the HW supports.

I am honestly very confused. First off, I do not understand what this
patch really does (or better what DW can't do so that it needs a
specific configfs string and therefore a specific configuration).

What's the purpose of the linkup notifier ? What does it mean that
the DW HW can't handle it ?

Are we referring to the pci_epf_linkup() function ?

If it is a HW configuration (ie the DW HW does not have a signal to
report that the link is up ?) its enablement must depend on HW
controller properties not configfs entries, I do not like what this
patch does (probably because I am confused and I do not understand it).

Please let me know your thoughts on this, thanks.

Lorenzo

> 
> configfs is just one way of creating epf_device and it was mainly added since
> pci-epf-test cannot have a dt entry because it doesn't represent anything in
> the HW.
> 
> The other option was to have a callback to EPC driver to get the features it
> supports. But a particular feature that is required might be specific to a EPF
> driver.
> 
> I find the driver_data approach in pci_epf_device_id to be more clean.
> 
> Thanks
> Kishon