Re: [PATCH v3 3/3] PCI: pciehp: Add dmi table for in-band presence disabled

[Alex G.]

On 10/21/19 1:19 PM, Stuart Hayes wrote:
> On 10/21/19 8:47 AM, Mika Westerberg wrote:
> > On Thu, Oct 17, 2019 at 03:32:56PM -0400, Stuart Hayes wrote:
> > > Some systems have in-band presence detection disabled for hot-plug PCI
> > > slots, but do not report this in the slot capabilities 2 (SLTCAP2) register.
> > > On these systems, presence detect can become active well after the link is
> > > reported to be active, which can cause the slots to be disabled after a
> > > device is connected.
> > >
> > > Add a dmi table to flag these systems as having in-band presence disabled.
> > >
> > > Signed-off-by: Stuart Hayes <stuart.w.hayes@xxxxxxxxx>
> > > ---
> > >   drivers/pci/hotplug/pciehp_hpc.c | 14 ++++++++++++++
> > >   1 file changed, 14 insertions(+)
> > >
> > > diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
> > > index 02eb811a014f..4d377a2a62ce 100644
> > > --- a/drivers/pci/hotplug/pciehp_hpc.c
> > > +++ b/drivers/pci/hotplug/pciehp_hpc.c
> > > @@ -14,6 +14,7 @@
> > >   
> > >   #define dev_fmt(fmt) "pciehp: " fmt
> > >   
> > > +#include <linux/dmi.h>
> > >   #include <linux/kernel.h>
> > >   #include <linux/types.h>
> > >   #include <linux/jiffies.h>
> > > @@ -26,6 +27,16 @@
> > >   #include "../pci.h"
> > >   #include "pciehp.h"
> > >   
> > > +static const struct dmi_system_id inband_presence_disabled_dmi_table[] = {
> > > +	{
> > > +		.ident = "Dell System",
> > > +		.matches = {
> > > +			DMI_MATCH(DMI_OEM_STRING, "Dell System"),
> >
> > Sorry if this has been discussed previously already but isn't this going
> > to apply on all Dell systems, not just the affected ones? Is this the
> > intention?
>
> Yes, that is the intention. Applying this just makes the hotplug code wait for
> the presence detect bit to be set before proceeding, which ideally wouldn't hurt
> anything--for devices that don't have inband presence detect disabled, presence
> detect should already be up when the code in patch 2/3 starts to wait for it.
>
> The only issue should be with broken hotplug implementations that don't ever
> bring presence detect active (these apparently exist)--but even those would still
> work, they would just take an extra second to come up.
>
> On the other hand, a number of Dell systems have (and will have) NVMe
> implementations that have inband presence detect disabled (but they won't have
> the new bit implemented to report that), and they don't work correctly without
> this.

I think it's clearer if this is explained in a comment. That it doesn't 
break anything, and we're okay this applies to all hotplug ports, even 
those that are not in front of an NVMe backplane.

Alex