Re: [PATCH v11] PCI: Xilinx-NWL-PCIe: Added support for Xilinx NWL PCIe Host Controller

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Dec 21, 2015 at 05:23:47AM +0000, Bharat Kumar Gogada wrote:
> Hi Bjorn, can you comment on this. Marc has also replied for query on irq_dispose_mapping().

I'm not sure exactly what you want me to comment on.

> > Subject: RE: [PATCH v11] PCI: Xilinx-NWL-PCIe: Added support for Xilinx NWL
> > PCIe Host Controller
> > 
> > > Subject: Re: [PATCH v11] PCI: Xilinx-NWL-PCIe: Added support for
> > > Xilinx NWL PCIe Host Controller
> > >
> > > [+cc Marc for irq_dispose_mapping() question]
> > >
> > > On Thu, Dec 10, 2015 at 02:10:34PM +0000, Bharat Kumar Gogada wrote:
> > > I'm trying to figure out what the difference is between these two
> > > checks and why you have both of them:
> > >
> > > > +	if (bus->number == pcie->root_busno && devfn > 0)
> > > > +	if (bus->primary == pcie->root_busno && devfn > 0)
> > >
> > > If I understand correctly, pcie->root_busno is the bus number of the
> > > Root Port device (likely 00).  I think the "bus->number ==
> > > pcie->root_busno && devfn > 0" check means that the Root Port, e.g.,
> > > 00:00.0, is the only device allowed on bus 00.  Often a Root Complex
> > > contains several Root Ports and other integrated devices that typically are
> > on bus 00.
> > > But in your case, I think you're saying there is only the single Root
> > > Port and no other devices.
> > >
> > > I think that first check takes care of everything on bus 00, so I'm
> > > trying to figure out what the second check is for.  Assume your Root
> > > Port is device
> > > 00:00.0 and it is a bridge to [bus 01-ff].  Then we have two pci_bus
> > > structs with these values:
> > >
> > >   bus->number = 00
> > >   bus->primary = 00
> > >   bus->busn_res = [bus 00-ff]
> > >
> > >   bus->number = 01
> > >   bus->primary = 00
> > >   bus->busn_res = [bus 01-ff]
> > >
> > > Because of the first check, 00:00.0 is the only possible device on bus
> > > 00, and because of the second check, 01:00.0 is the only possible device on
> > bus 01.
> > > Therefore, you don't support a multifunction device connected to the
> > > Root Port.  Right?
> > >
> > We support multifunction devices also, so this check should not be there, will
> > remove this check in next patch.

It looks like you're planning to change this.

> > > > > > +		return false;
> > > > > > +
> > > > > > +	return true;
> > > > > > +}
> > > > > > + * nwl_setup_sspl - Set Slot Power limit
> > > > > > + *
> > > > > > + * @pcie: PCIe port information  */ static int
> > > > > > +nwl_setup_sspl(struct nwl_pcie *pcie)
> > > > >
> > > > The Set_Slot_Power_Limit Message includes a one DW data payload. The
> > > > data payload is copied from the Slot Capabilities register of the
> > > > Downstream Port and is written into the Device Capabilities register
> > > > of the Upstream Port on the other side of the Link. Bits 9:8 of the
> > > > data payload map to the Slot Power Limit Scale field and bits 7:0
> > > > map to the Slot Power Limit Value field. Bits 31:10 of the data
> > > > payload must be set to all 0's by the Transmitter and ignored by the
> > Receiver.
> > >
> > > > This Message is sent automatically by the Downstream Port (of a Root
> > > > Complex or a Switch) when one of the following events occurs:
> > > > -> On a Configuration Write to the Slot Capabilities register (see
> > > > Section 7.8.9) when the Data Link Layer reports DL_Up status.
> > >
> > > I interpret this as meaning "the *hardware* automatically sends a
> > > Set_Slot_Power_Limit Message."  There's no mention of software doing
> > > anything other than the configuration write.
> > >
> > > If your hardware doesn't do that, I think it's a defect.  It's fine to
> > > work around it, but we should have a comment to that effect so people
> > > don't copy the code to new drivers that don't need it.
> > 
> > Our hardware is not capable of doing it, so we are doing it software. Yes I will
> > add some comments.

And add a comment here.

> > > It's a little strange that 7.8.9 talks about writing to this register
> > > when all of its fields are HwInit and supposedly read-only.  I had
> > > assumed devices would use strapping or implementation-specific
> > > registers to set the Slot Power values, but maybe some devices use direct
> > writes to Slot Capabilities instead.
> > >
> > > BTW, I noticed a related lspci bug: it didn't decode the Capture Slot
> > > Power Limit in Device Capabilities of Endpoints.  I posted a fix for that
> > separately.
> > >
> > > The Slot Power Limit (in Slot Capabilities) indicates how much power
> > > the slot can supply to a downstream device.  That's a function of the
> > > platform design, so it seems like this is something you want to set
> > > via DT or some other mechanism that knows about the platform.
> > > Intercepting all config writes and updating it with whatever the
> > > caller supplies doesn't sound wise.  The value might be coming from
> > > setpci or some other source with no knowledge of the platform.
> > 
> > Agreed, but this is what can be done, it is difficult to determine who does
> > what.

Your driver is based on DT.  What prevents you from adding a DT property
that shows the slot power capability?

> > > > > > +			status = nwl_bridge_readl(pcie,
> > TX_PCIE_MSG)
> > > > > > +						  & MSG_DONE_BIT;
> > > > > > +			if (status) {
> > > > > > +				status = nwl_bridge_readl(pcie,
> > > > > TX_PCIE_MSG)
> > > > > > +						  &
> > MSG_DONE_STATUS_BIT;
> > >
> > > > > It's not clear to me whether you need to re-read TX_PCIE_MSG here.
> > > >
> > > > MSG_DONE_BIT qualifies MSG_DONE_STATUS_BIT; so value of
> > > > MSG_DONE_STATUS_BIT is valid only when MSG_DONE_BIT = 1
> > >
> > > That doesn't answer the question of whether another read is required.
> > > In fact, I would argue that if MSG_DONE_STATUS_BIT is only valid when
> > > MSG_DONE_BIT = 1, you *should* only do one read, because you want to
> > > capture both bits simultaneously so you know they're consistent, e.g.,
> > >
> > >   status = nwl_bridge_readl(pcie, TX_PCIE_MSG);
> > >   if (status & MSG_DONE_BIT) {
> > >     if (status & MSG_DONE_STATUS_BIT)
> > >       ...
> > >   }
> > >
> > > If you read the register twice, you always have to worry about what
> > > changes MSG_DONE_BIT, and how you guarantee that the second read
> > > happens before MSG_DONE_BIT changes.
> > >
> > Agreed, will do it in this way, once will also confirm with IP owner regarding
> > both bits being updated parallel.

It sounds like you're working on resolving this.

Did I miss a question for me?

Bjorn