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

[Bharat Kumar Gogada]

Hi Bjorn, can you comment on this. Marc has also replied for query on irq_dispose_mapping().

> 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.
> 
> > > > > +		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.
> 
> >
> > 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.
> >
> > > > > +			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.
> 
> > > > > +		}
> > > > > +	} while (status);
> Bharat
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