Date:   Mon, 4 Dec 2017 18:20:13 +0000
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Cyrille Pitchen <cyrille.pitchen@...e-electrons.com>
Cc:     bhelgaas@...gle.com, kishon@...com, linux-pci@...r.kernel.org,
        adouglas@...ence.com, stelford@...ence.com, dgary@...ence.com,
        kgopi@...ence.com, eandrews@...ence.com,
        thomas.petazzoni@...e-electrons.com, sureshp@...ence.com,
        nsekhar@...com, linux-kernel@...r.kernel.org, robh@...nel.org,
        devicetree@...r.kernel.org, ard.biesheuvel@...aro.org
Subject: Re: [PATCH 3/5] PCI: cadence: Add host driver for Cadence PCIe
 controller

[+Ard]

Hi Cyrille,

On Sun, Dec 03, 2017 at 09:44:46PM +0100, Cyrille Pitchen wrote:

[...]

> >> +cdns_pci_map_bus(struct pci_bus *bus, unsigned int devfn, int where)
> >> +{
> >> +     struct pci_host_bridge *bridge = pci_find_host_bridge(bus);
> >> +     struct cdns_pcie_rc *rc = pci_host_bridge_priv(bridge);
> >> +     struct cdns_pcie *pcie = &rc->pcie;
> >> +     unsigned int busn = bus->number;
> >> +     u32 addr0, desc0;
> >> +
> >> +     if (busn < rc->bus_range->start || busn > rc->bus_range->end)
> >> +             return NULL;
> > 
> > It does not hurt but I wonder whether you really need this check.
> >
> 
> I can remove it.
>  
> >> +     if (busn == rc->bus_range->start) {
> >> +             if (devfn)
> > 
> > I suspect I know why you need this check but I ask you to explain it
> > anyway if you do not mind please.
> >
> 
> If I have understood correctly, Cadence team told me that only the root
> port is available on the first bus through device 0, function 0.
> No other device/function should connected on this bus, all other devices
> are behind at least one PCI bridge.
> 
> I can add a comment here to explain that.

That's understood, the question is what happens if you do scan devfn != 0.

> >> +                     return NULL;
> >> +
> >> +             return pcie->reg_base + (where & 0xfff);
> >> +     }
> >> +
> >> +     /* Update Output registers for AXI region 0. */
> >> +     addr0 = CDNS_PCIE_AT_OB_REGION_PCI_ADDR0_NBITS(12) |
> > 
> > Ok, so for every config access you reprogram addr0 to reflect the
> > correct bus|devfn ID in the PCI bus TLP corresponding to an ECAM address
> > in CPU physical address space, is my understanding correct ?
> >
> 
> The idea is to able to use only a 4KB memory area at a fixed address in the
> space allocated for the PCIe controller in the AXI bus. I guess the plan is
> to leave more space on the AXI bus to map all other PCIe devices.
> 
> This is just my guess. Anyway one purpose of this driver was actually to
> perform all PCI configuration space accesses through this single 4KB memory
> area in the AXI bus, changing the mapping dynamically to reach the relevant
> PCI device. 

Thank you for explaining - that matches my understanding.

> >> +             CDNS_PCIE_AT_OB_REGION_PCI_ADDR0_DEVFN(devfn) |
> >> +             CDNS_PCIE_AT_OB_REGION_PCI_ADDR0_BUS(busn);
> >> +     cdns_pcie_writel(pcie, CDNS_PCIE_AT_OB_REGION_PCI_ADDR0(0), addr0);
> >> +
> >> +     /* Configuration Type 0 or Type 1 access. */
> >> +     desc0 = CDNS_PCIE_AT_OB_REGION_DESC0_HARDCODED_RID |
> >> +             CDNS_PCIE_AT_OB_REGION_DESC0_DEVFN(0);
> >> +     /*
> >> +      * The bus number was already set once for all in desc1 by
> >> +      * cdns_pcie_host_init_address_translation().
> >> +      */
> >> +     if (busn == rc->bus_range->start + 1)
> >> +             desc0 |= CDNS_PCIE_AT_OB_REGION_DESC0_TYPE_CONF_TYPE0;
> >> +     else
> >> +             desc0 |= CDNS_PCIE_AT_OB_REGION_DESC0_TYPE_CONF_TYPE1;
> > 
> > I would like to ask you why you have to do it here and the root port
> > does not figure it out by itself, I do not have the datasheet so I am
> > just asking for my own information.
> 
> PCI configuration space registers of the root port can only be read through
> the APB bus at offset 0:
> ->reg_base + (where & 0xfff)
> 
> They are internal registers of the PCIe controller so no TLP on the PCIe bus.
> 
> However to access the PCI configuration space registers of any other device,
> the PCIe controller builds then sends a TLP on the PCIe bus using the offset
> in the 4KB AXI area as the offset of the register in the PCI configuration
> space:
> ->cfg_base + (where & 0xfff)
> 
> > 
> >> +     cdns_pcie_writel(pcie, CDNS_PCIE_AT_OB_REGION_DESC0(0), desc0);
> >> +
> >> +     return rc->cfg_base + (where & 0xfff);
> >> +}
> >> +
> >> +static struct pci_ops cdns_pcie_host_ops = {
> >> +     .map_bus        = cdns_pci_map_bus,
> >> +     .read           = pci_generic_config_read,
> >> +     .write          = pci_generic_config_write,
> >> +};
> >> +
> >> +static const struct cdns_pcie_rc_data cdns_pcie_rc_data = {
> >> +     .max_regions    = 32,
> >> +     .vendor_id      = PCI_VENDOR_ID_CDNS,
> >> +     .device_id      = 0x0200,
> >> +     .no_bar_nbits   = 32,
> >> +};
> > 
> > Should (some of) these parameters be retrieved through a DT binding ?
> >
> 
> Indeed, maybe we get max_regions and no_bar_nbits from the DT.
> 
> About the vendor and device IDs, I don't know which would be the best
> choice between some dedicated DT properties or associating a custom
> structure as above to the 'compatible' string.
> 
> Honestly, I don't have any strong preference, please just tell me what
> you would prefer :)

I think it is best to ask DT maintainers (in CC) POV on this, they
certainly have a more comprehensive view than mine on the subject - I
have just noticed that _some_ data can be retrieved through DT therefore
I raised the point - either through different compatible strings or
some IP specific properties.

> >> +static const struct of_device_id cdns_pcie_host_of_match[] = {
> >> +     { .compatible = "cdns,cdns-pcie-host",
> >> +       .data = &cdns_pcie_rc_data },
> >> +
> >> +     { },
> >> +};
> >> +
> >> +static int cdns_pcie_parse_request_of_pci_ranges(struct device *dev,
> >> +                                              struct list_head *resources,
> >> +                                              struct resource **bus_range)
> >> +{
> >> +     int err, res_valid = 0;
> >> +     struct device_node *np = dev->of_node;
> >> +     resource_size_t iobase;
> >> +     struct resource_entry *win, *tmp;
> >> +
> >> +     err = of_pci_get_host_bridge_resources(np, 0, 0xff, resources, &iobase);
> >> +     if (err)
> >> +             return err;
> >> +
> >> +     err = devm_request_pci_bus_resources(dev, resources);
> >> +     if (err)
> >> +             return err;
> >> +
> >> +     resource_list_for_each_entry_safe(win, tmp, resources) {
> >> +             struct resource *res = win->res;
> >> +
> >> +             switch (resource_type(res)) {
> >> +             case IORESOURCE_IO:
> >> +                     err = pci_remap_iospace(res, iobase);
> >> +                     if (err) {
> >> +                             dev_warn(dev, "error %d: failed to map resource %pR\n",
> >> +                                      err, res);
> >> +                             resource_list_destroy_entry(win);
> >> +                     }
> >> +                     break;
> >> +             case IORESOURCE_MEM:
> >> +                     res_valid |= !(res->flags & IORESOURCE_PREFETCH);
> >> +                     break;
> >> +             case IORESOURCE_BUS:
> >> +                     *bus_range = res;
> >> +                     break;
> >> +             }
> >> +     }
> >> +
> >> +     if (res_valid)
> >> +             return 0;
> >> +
> >> +     dev_err(dev, "non-prefetchable memory resource required\n");
> >> +     return -EINVAL;
> > 
> > Nit, I prefer you swap these two as it is done in pci-aardvark.c:
> > 
> >         if (!res_valid) {
> >                 dev_err(dev, "non-prefetchable memory resource required\n");
> >                 return -EINVAL;
> >         }
> > 
> >         return 0;
> > 
> > but as per previous replies this function can be factorized in
> > core PCI code - I would not bother unless you are willing to write
> > the patch series that does the refactoring yourself :)
> > 
> >> +}
> >> +
> >> +static int cdns_pcie_host_init_root_port(struct cdns_pcie_rc *rc)
> >> +{
> >> +     const struct cdns_pcie_rc_data *data = rc->data;
> >> +     struct cdns_pcie *pcie = &rc->pcie;
> >> +     u8 pbn, sbn, subn;
> >> +     u32 value, ctrl;
> >> +
> >> +     /*
> >> +      * Set the root complex BAR configuration register:
> >> +      * - disable both BAR0 and BAR1.
> >> +      * - enable Prefetchable Memory Base and Limit registers in type 1
> >> +      *   config space (64 bits).
> >> +      * - enable IO Base and Limit registers in type 1 config
> >> +      *   space (32 bits).
> >> +      */
> >> +     ctrl = CDNS_PCIE_LM_BAR_CFG_CTRL_DISABLED;
> >> +     value = CDNS_PCIE_LM_RC_BAR_CFG_BAR0_CTRL(ctrl) |
> >> +             CDNS_PCIE_LM_RC_BAR_CFG_BAR1_CTRL(ctrl) |
> >> +             CDNS_PCIE_LM_RC_BAR_CFG_PREFETCH_MEM_ENABLE |
> >> +             CDNS_PCIE_LM_RC_BAR_CFG_PREFETCH_MEM_64BITS |
> >> +             CDNS_PCIE_LM_RC_BAR_CFG_IO_ENABLE |
> >> +             CDNS_PCIE_LM_RC_BAR_CFG_IO_32BITS;
> >> +     cdns_pcie_writel(pcie, CDNS_PCIE_LM_RC_BAR_CFG, value);
> >> +
> >> +     /* Set root port configuration space */
> >> +     if (data->vendor_id != 0xffff)
> >> +             cdns_pcie_rp_writew(pcie, PCI_VENDOR_ID, data->vendor_id);
> >> +     if (data->device_id != 0xffff)
> >> +             cdns_pcie_rp_writew(pcie, PCI_DEVICE_ID, data->device_id);
> >> +
> >> +     cdns_pcie_rp_writeb(pcie, PCI_CLASS_REVISION, 0);
> >> +     cdns_pcie_rp_writeb(pcie, PCI_CLASS_PROG, 0);
> >> +     cdns_pcie_rp_writew(pcie, PCI_CLASS_DEVICE, PCI_CLASS_BRIDGE_PCI);
> >> +
> >> +     pbn = rc->bus_range->start;
> >> +     sbn = pbn + 1; /* Single root port. */
> >> +     subn = rc->bus_range->end;
> >> +     cdns_pcie_rp_writeb(pcie, PCI_PRIMARY_BUS, pbn);
> >> +     cdns_pcie_rp_writeb(pcie, PCI_SECONDARY_BUS, sbn);
> >> +     cdns_pcie_rp_writeb(pcie, PCI_SUBORDINATE_BUS, subn);
> > 
> > Again - I do not have the datasheet for this device therefore I would
> > kindly ask you how this works; it seems to me that what you are doing
> > here is done through normal configuration cycles in an ECAM compliant
> > system to program the RP PRIMARY/SECONDARY/SUBORDINATE bus - I would
> > like to understand why this code is needed.
> >
> 
> I will test without those lines to test whether I can remove them.
> 
> At first, the PCIe controller was tested by Cadence team: there was code
> in their bootloader to initialize the hardware (building the AXI <-> PCIe
> mappings, ...): the bootloader used to set the primary, secondary and
> subordinate bus numbers in the root port PCI config space.
> 
> Also there was a hardware trick to redirect accesses of the lowest
> addresses in the AXI bus to the APB bus so the PCI configuration space of
> the root port could have been accessed from the AXI bus too.
> 
> The AXI <-> PCIe mapping being done by the bootloader and the root port
> config space being accessible from the AXI bus, it was possible to use
> the pci-host-generic driver.

That's what I was getting at. Ard (CC'ed) implemented a firmware set-up
(even though it was for a different IP but maybe it applies here) that
allows the kernel to use the pci-host-generic driver to initialize the
PCI controller:

https://marc.info/?l=linux-pci&m=150360022626351&w=2

I want to understand if there is an IP initialization sequence whereby
this IP can be made to work in an ECAM compliant way and therefore
reuse (most of) the pci-host-generic driver code.

> However, the hardware trick won't be included in the final design since
> Cadence now wants to perform all PCI configuration space accesses through
> a small 4KB window at a fixed address on the AXI bus.

I would like to understand what the HW "trick" (if you can disclose it)
was, because if there is a chance to reuse the pci-host-generic driver
for this IP I want to take it (yes it may entail some firmware set-up in
the bootloader) - was it a HW trick or a specific IP SW configuration ?

> Also, we now want all initialisations to be done by the linux driver
> instead of the bootloader.

That's a choice, I do not necessarily agree with it and I think we
should aim for more standardization on the PCI host bridge set-up
at firmware->kernel handover on DT platforms.

> I simply moved all those initialisations from the bootloader to the linux
> driver but actually there is a chance that I can remove the 3 writes to
> the PCI_*_BUS registers.

I asked because I do not have this IP documentation so I rely on you to
provide the correct initialization sequence and an explanation for it,
I think I understand now the initialization sequence a bit more but it
would be good to get to the bottom of it.

Thank you,
Lorenzo

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