Re: [PATCH v3] PCI: endpoint: pci-epf-vntb: Track link state from both sides

[Koichiro Den]

On Thu, Oct 15, 2026 at 11:44:46AM +0530, Manivannan Sadhasivam wrote:
> On Wed, Oct 14, 2026 at 04:14:48PM +0900, Koichiro Den wrote:
> > The control-region link status is currently updated only by
> > COMMAND_LINK_UP and COMMAND_LINK_DOWN from the HOST. The virtual
> > NTB link callbacks are empty. Consequently, the HOST can see the link as
> > up before the VHOST has enabled it, and ntb_link_disable() on the VHOST
> > leaves LINK_STATUS_UP set without notifying either NTB client.
> > 
> > ntb_netdev can hide this device-level state bug because ntb_transport uses
> > a separate per-QP LINK_DOWN_FLAG message. Use ntb_tool to observe the
> > device link state directly:
> > 
> > After bringing both sides up, start one waiter on each side:
> > 
> >   HOST# echo N > /sys/kernel/debug/ntb_tool/<H-device>/peer0/link_event
> >   VHOST# echo N > /sys/kernel/debug/ntb_tool/<V-device>/peer0/link_event
> > 
> > While both waiters are blocked, disable the link on the VHOST:
> > 
> >   VHOST# echo N > /sys/kernel/debug/ntb_tool/<V-device>/link
> > 
> > Without this patch, the link-disable command succeeds, but peer0/link
> > remains Y on both sides and neither waiter returns.
> > 
> > Track HOST and VHOST enablement separately and report the effective link
> > as up only when both sides have enabled it, as pci-epf-ntb does for its
> > two physical hosts. Both NTB clients observe this effective state, so
> > notify both sides only when it changes, including the side whose request
> > completes the transition. Serialize the two paths because HOST commands
> > run from delayed work while the VHOST callbacks may run concurrently.
> > 
> > With this patch, peer0/link changes to N on both sides and both waiters
> > return.
> > 
> > Fixes: e35f56bb0330 ("PCI: endpoint: Support NTB transfer between RC and EP")
> > Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
> > ---
> > Changes in v3:
> >   - Rebase onto the latest pci/endpoint.
> >   - Drop redundant dma_wmb(). (Frank)
> >   - Retest using ntb_tool.
> >   - Link to v2: https://lore.kernel.org/r/20260904162049.3432464-1-den@xxxxxxxxxxxxx/
> > 
> >  drivers/pci/endpoint/functions/pci-epf-vntb.c | 90 +++++++++++++++----
> >  1 file changed, 73 insertions(+), 17 deletions(-)
> > 
> > diff --git a/drivers/pci/endpoint/functions/pci-epf-vntb.c b/drivers/pci/endpoint/functions/pci-epf-vntb.c
> > index 07ba338732d4..a11f9260ee86 100644
> > --- a/drivers/pci/endpoint/functions/pci-epf-vntb.c
> > +++ b/drivers/pci/endpoint/functions/pci-epf-vntb.c
> > @@ -41,6 +41,7 @@
> >  #include <linux/delay.h>
> >  #include <linux/io.h>
> >  #include <linux/module.h>
> > +#include <linux/mutex.h>
> >  #include <linux/slab.h>
> >  
> >  #include <linux/pci-ep-msi.h>
> > @@ -147,7 +148,10 @@ struct epf_ntb {
> >  	u16 vntb_pid;
> >  	u16 vntb_vid;
> >  
> > -	bool linkup;
> > +	/* Serialize HOST and VHOST link state changes. */
> > +	struct mutex link_lock;
> > +	bool host_linkup;
> > +	bool vhost_linkup;
> >  
> >  	/*
> >  	 * True when doorbells are interrupt-driven (MSI or embedded), false
> > @@ -183,24 +187,54 @@ static struct pci_epf_header epf_ntb_header = {
> >  	.interrupt_pin	= PCI_INTERRUPT_INTA,
> >  };
> >  
> > +static void epf_ntb_update_link(struct epf_ntb *ntb)
> > +{
> > +	u16 link_status = READ_ONCE(ntb->reg->link_status);
> > +
> > +	/* The link is usable only after both sides have enabled it. */
> 
> I see a bit of ambiguity here and it is confusing. This comment says that both
> host and vhost has to enable the link first. Yet, the variables are named as
> '{host/vhost}_linkup'. Also, the functions setting these variables are named
> as:
> 
> 	epf_ntb_link_up()
> 	vntb_epf_set_link() 
> 
> Even though these two are doing the same thing but on different side, these two
> functions are not symmetric. This needs to be fixed up.

That's indeed a fair point. How about the following renames?

       v3 (this rev)               v4 (next rev)
  --   --------------------        ---------------------------
  #1   epf_ntb::host_linkup   =>   epf_ntb::host_link_enabled
  #2   epf_ntb::vhost_linkup  =>   epf_ntb::vhost_link_enabled
  #3   epf_ntb_link_up()      =>   epf_ntb_set_link_enabled()
  #4   vntb_epf_set_link()    =>   vntb_epf_set_link_enabled()

We might as well rename the 'link_up' argument to 'enabled' in both functions.

note:

  #3 : This is an existing function, but with this patch it records HOST
       enablement, which does not necessarily mean the link is up. So I think
       renaming it in this patch makes sense.

  #4 : For symmetry, I realized that the VHOST helper could use the same return
       pattern:

       if (notify)
           return epf_ntb_notify_link(ndev);
       return 0;

If no objections, I'll respin accordingly.
Thanks for the review!

Best regards,
Koichiro Den

> 
> - Mani
> 
> -- 
> மணிவண்ணன் சதாசிவம்