linux-kernel - Re: [PATCH v6 3/4] PCI: brcmstb: Wait for Raspberry Pi's firmware when present

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Message-ID: <47c543e2144d5247743548b00d1931e9fc217f43.camel@suse.de>
Date:   Thu, 02 Apr 2020 16:27:23 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Florian Fainelli <f.fainelli@...il.com>,
        bcm-kernel-feedback-list@...adcom.com,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Andrew Murray <amurray@...goodpenguin.co.uk>,
        linux-usb@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, gregkh@...uxfoundation.org,
        tim.gover@...pberrypi.org, linux-pci@...r.kernel.org,
        wahrenst@....net, sergei.shtylyov@...entembedded.com
Subject: Re: [PATCH v6 3/4] PCI: brcmstb: Wait for Raspberry Pi's firmware
 when present

Hi Bjorn,

On Wed, 2020-04-01 at 15:41 -0500, Bjorn Helgaas wrote:
> On Tue, Mar 24, 2020 at 07:28:11PM +0100, Nicolas Saenz Julienne wrote:
> > xHCI's PCI fixup, run at the end of pcie-brcmstb's probe, depends on
> 
> Is there a function name for this fixup that you can mention?

Yes, rpi_firmware_init_vl805(), I'll update the description.

> > RPi4's VideoCore firmware interface to be up and running. It's possible
> > for both initializations to race, so make sure it's available prior to
> > starting.
> 
> I guess "both initializations" means brcm_pcie_probe() and something
> else?  It'd be nice to include that function name here, too.

Noted, I'll be more explicit on the next version of the series. More in depth
explanation below.

> > Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
> > Reviewed-by: Florian Fainelli <f.fainelli@...il.com>
> > ---
> >  drivers/pci/controller/pcie-brcmstb.c | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> > 
> > diff --git a/drivers/pci/controller/pcie-brcmstb.c
> > b/drivers/pci/controller/pcie-brcmstb.c
> > index 3a10e678c7f4..a3d3070a5832 100644
> > --- a/drivers/pci/controller/pcie-brcmstb.c
> > +++ b/drivers/pci/controller/pcie-brcmstb.c
> > @@ -28,6 +28,8 @@
> >  #include <linux/string.h>
> >  #include <linux/types.h>
> >  
> > +#include <soc/bcm2835/raspberrypi-firmware.h>
> > +
> >  #include "../pci.h"
> >  
> >  /* BRCM_PCIE_CAP_REGS - Offset for the mandatory capability config regs */
> > @@ -917,11 +919,24 @@ static int brcm_pcie_probe(struct platform_device
> > *pdev)
> >  {
> >  	struct device_node *np = pdev->dev.of_node, *msi_np;
> >  	struct pci_host_bridge *bridge;
> > +	struct device_node *fw_np;
> >  	struct brcm_pcie *pcie;
> >  	struct pci_bus *child;
> >  	struct resource *res;
> >  	int ret;
> >  
> > +	/*
> > +	 * We have to wait for the Raspberry Pi's firmware interface to be up
> > +	 * as some PCI fixups depend on it.
> 
> It'd be nice to know the nature of this dependency between the
> firmware interface and the fixups.  This may be useful for future
> maintenance.  E.g., if PCI config access doesn't work until the
> firmware interface is up, that would affect almost everything.  But
> you say "some PCI fixups", so I suppose the actual dependency is
> probably something else.

Sorry it wasn't clear enough, I'll redo this comment. Also note that the PCIe
bus and the XHCI chip are hardwired, so that's the only device that'll ever be
available on the bus.

VIA805's XHCI firmware has to be loaded trough RPi's firmware mailbox in
between the PCIe bus probe and the subsequent USB probe. Note that a PCI reset
clears the firmware. The only mechanism available in between the two operations
are PCI Fixups. These are limited in their own way, as I can't return
-EPROBE_DEFER if the firmware interface isn't available yet. Hence the need for
an explicit dependency between pcie-brcmstb and raspberrypi's firmware mailbox
device.

Your concern here showcases this series' limitations. From a high level
perspective it's not clear to me who should be responsible for downloading the
firmware. And I get the feeling I'm abusing PCI fixups. I haven't found any
smart way to deal with this three way dependency of platform/non-platform
devices. I even looked into adding -EPROBE_DEFER support to fixups, but I fear
that would entail moving them into the core device definition.

Regards,
Nicolas


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