linux-kernel - Re: [PATCH v8 4/6] PCI: Allow extend_bridge_window() to shrink resource if necessary

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Message-ID: <20191023093932.GS2819@lahna.fi.intel.com>
Date:   Wed, 23 Oct 2019 12:39:32 +0300
From:   "mika.westerberg@...ux.intel.com" <mika.westerberg@...ux.intel.com>
To:     Nicholas Johnson <nicholas.johnson-opensource@...look.com.au>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "corbet@....net" <corbet@....net>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "logang@...tatee.com" <logang@...tatee.com>
Subject: Re: [PATCH v8 4/6] PCI: Allow extend_bridge_window() to shrink
 resource if necessary

On Wed, Oct 23, 2019 at 09:16:10AM +0000, Nicholas Johnson wrote:
> On Tue, Oct 08, 2019 at 03:09:07PM +0300, mika.westerberg@...ux.intel.com wrote:
> > On Fri, Jul 26, 2019 at 12:54:22PM +0000, Nicholas Johnson wrote:
> > > Remove checks for resource size in extend_bridge_window(). This is
> > > necessary to allow the pci_bus_distribute_available_resources() to
> > > function when the kernel parameter pci=hpmemsize=nn[KMG] is used to
> > > allocate resources. Because the kernel parameter sets the size of all
> > > hotplug bridges to be the same, there are problems when nested hotplug
> > > bridges are encountered. Fitting a downstream hotplug bridge with size X
> > > and normal bridges with size Y into parent hotplug bridge with size X is
> > > impossible, and hence the downstream hotplug bridge needs to shrink to
> > > fit into its parent.
> > 
> > Maybe you could show the topology here which needs shrinking.
> > 
> > > Add check for if bridge is extended or shrunken and adjust pci_dbg to
> > > reflect this.
> > > 
> > > Reset the resource if its new size is zero (if we have run out of a
> > > bridge window resource). If it is set to zero size and left, it can
> > > cause significant problems when it comes to enabling devices.
> > 
> > Same comment here about explaining the "significant problems".
> I have in the past, but because the problems are very hard to describe succinctly, it just turns into a 
> nightmare. I can try to do it again.
> 
> > > 
> > > Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@...look.com.au>
> > > ---
> > >  drivers/pci/setup-bus.c | 16 +++++++++++-----
> > >  1 file changed, 11 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> > > index a072781ab..7e1dc892a 100644
> > > --- a/drivers/pci/setup-bus.c
> > > +++ b/drivers/pci/setup-bus.c
> > > @@ -1823,13 +1823,19 @@ static void extend_bridge_window(struct pci_dev *bridge, struct resource *res,
> > 
> > Since it is also shrinking now maybe name it adjust_bridge_window() instead?
> I am happy to do this.
> 
> If we can drop the pci_dbg() calls, then I might be able to drop this 
> function entirely. During the development of this patch, that is exactly 
> what I did. How important are the pci_dbg calls to you?

Well they are still useful when debugging resource allocation issues
(and they match similar we do when extending number of buses). I would
like to keep them if possible.