linux-kernel - Re: [PATCH v4 1/2] PCI: pci-host-generic: add support for Synopsys DesignWare RC in ECAM mode

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Date:   Tue, 10 Oct 2017 10:04:03 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Will Deacon <will.deacon@....com>,
        linux-pci <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Leif Lindholm <leif.lindholm@...aro.org>,
        Graeme Gregory <graeme.gregory@...aro.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Herring <robh@...nel.org>
Subject: Re: [PATCH v4 1/2] PCI: pci-host-generic: add support for Synopsys
 DesignWare RC in ECAM mode

On 10 October 2017 at 01:13, Bjorn Helgaas <helgaas@...nel.org> wrote:
> On Mon, Oct 09, 2017 at 05:46:07PM +0100, Will Deacon wrote:
>> On Fri, Oct 06, 2017 at 06:21:59PM -0500, Bjorn Helgaas wrote:
>> > On Fri, Oct 06, 2017 at 05:39:18PM +0100, Ard Biesheuvel wrote:
>> > > diff --git a/drivers/pci/host/pci-host-generic.c b/drivers/pci/host/pci-host-generic.c
>> > > index 7d709a7e0aa8..01e81a30e303 100644
>> > > --- a/drivers/pci/host/pci-host-generic.c
>> > > +++ b/drivers/pci/host/pci-host-generic.c
>> > > @@ -35,6 +35,43 @@ static struct pci_ecam_ops gen_pci_cfg_cam_bus_ops = {
>> > >   }
>> > >  };
>> > >
>> > > +static int pci_dw_ecam_config_read(struct pci_bus *bus, u32 devfn, int where,
>> > > +                            int size, u32 *val)
>> > > +{
>> > > + struct pci_config_window *cfg = bus->sysdata;
>> > > +
>> > > + /*
>> > > +  * The Synopsys DesignWare PCIe controller in ECAM mode will not filter
>> > > +  * type 0 config TLPs sent to devices 1 and up on its downstream port,
>> > > +  * resulting in devices appearing multiple times on bus 0 unless we
>> > > +  * filter out those accesses here.
>> > > +  */
>> > > + if (bus->number == cfg->busr.start && PCI_SLOT(devfn) > 0)
>> > > +         return PCIBIOS_DEVICE_NOT_FOUND;
>> >
>> > I think we should return 0xffffffff data here, as we do in other
>> > similar accessors (dw_pcie_rd_conf(), altera_pcie_cfg_read(),
>> > rockchip_pcie_rd_conf()).
>> >
>> > Actually, xilinx-nwl has a nice trick: it implements
>> > nwl_pcie_map_bus(), which returns NULL for invalid devices.  Then it
>> > can use the generic accessors, and pci_generic_config_read() already
>> > fills in ~0 for this case.
>> >
>> > What do you think of the following?  I put it on my pci/host-generic
>> > branch for now (pending your response and an ack from Will).
>>
>> I'd probably just inline the device check in pci_dw_ecam_map_bus directly,
>> but either way:
>>
>> Acked-by: Will Deacon <will.deacon@....com>
>
> Thanks.  The reason I split out pci_dw_valid_device() is because there are
> several other *_valid_device() functions in other drivers that do basically
> the same thing, and I think it's useful to be able to see that they're all
> similar, as opposed to being buried in the config accessor or map
> functions, which vary a little more.
>
>> I'm assuming this is so small that there's no point in having a Kconfig
>> option for these guys that depends on the generic host driver?
>
> Good question; I don't care either way.

In this case, may I suggest we leave the patch as is? Neither of you
seem to mind, and it will make /my/ life much easier, given that I
don't have to chase distros to get this driver enabled in their kernel
configs (unless we make if 'def_bool y', in which case it makes even
less sense to add the Kconfig option in the first place).