Re: [PATCH 2/3] QIB: Removing usage of pcie_set_mps()

["Jiang, Dave" <dave.jiang@intel.com>]

On Mon, 2015-08-17 at 17:30 -0500, Bjorn Helgaas wrote:
> [+cc Mike, linux-rdma]
> 
> On Wed, Jul 29, 2015 at 04:18:54PM -0600, Keith Busch wrote:
> > From: Dave Jiang <dave.jiang@intel.com>
> > 
> > This is in perperation of un-exporting the pcie_set_mps() function
> > symbol. A driver should not be changing the MPS as that is the
> > responsibility of the PCI subsystem.
> 
> Please explain the implications of removing this code.  Does this 
> affect
> performance of the device?  If so, how do we get that performance 
> back?

Honestly I don't know. But at the same time I think the driver
shouldn't be touching the MPS at all. Shouldn't that be left to the
PCIe subsystem and rely on the PCIe subsystem to set this to a sane
value? 

> 
> I also cc'd the QIB maintainers for you:
> 
>   QIB DRIVER
>   M:      Mike Marciniszyn <infinipath@intel.com>
>   L:      linux-rdma@vger.kernel.org
>   F:      drivers/infiniband/hw/qib/
> 
> > Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> > ---
> >  drivers/infiniband/hw/qib/qib_pcie.c |   27 +---------------------
> > -----
> >  1 file changed, 1 insertion(+), 26 deletions(-)
> > 
> > diff --git a/drivers/infiniband/hw/qib/qib_pcie.c 
> > b/drivers/infiniband/hw/qib/qib_pcie.c
> > index 4758a38..b8a2dcd 100644
> > --- a/drivers/infiniband/hw/qib/qib_pcie.c
> > +++ b/drivers/infiniband/hw/qib/qib_pcie.c
> > @@ -557,12 +557,11 @@ static void qib_tune_pcie_coalesce(struct 
> > qib_devdata *dd)
> >   */
> >  static int qib_pcie_caps;
> >  module_param_named(pcie_caps, qib_pcie_caps, int, S_IRUGO);
> > -MODULE_PARM_DESC(pcie_caps, "Max PCIe tuning: Payload (0..3), 
> > ReadReq (4..7)");
> > +MODULE_PARM_DESC(pcie_caps, "Max PCIe tuning: ReadReq (4..7)");
> >  
> >  static void qib_tune_pcie_caps(struct qib_devdata *dd)
> >  {
> >  	struct pci_dev *parent;
> > -	u16 rc_mpss, rc_mps, ep_mpss, ep_mps;
> >  	u16 rc_mrrs, ep_mrrs, max_mrrs;
> >  
> >  	/* Find out supported and configured values for parent 
> > (root) */
> > @@ -575,30 +574,6 @@ static void qib_tune_pcie_caps(struct 
> > qib_devdata *dd)
> >  	if (!pci_is_pcie(parent) || !pci_is_pcie(dd->pcidev))
> >  		return;
> >  
> > -	rc_mpss = parent->pcie_mpss;
> > -	rc_mps = ffs(pcie_get_mps(parent)) - 8;
> > -	/* Find out supported and configured values for endpoint 
> > (us) */
> > -	ep_mpss = dd->pcidev->pcie_mpss;
> > -	ep_mps = ffs(pcie_get_mps(dd->pcidev)) - 8;
> > -
> > -	/* Find max payload supported by root, endpoint */
> > -	if (rc_mpss > ep_mpss)
> > -		rc_mpss = ep_mpss;
> > -
> > -	/* If Supported greater than limit in module param, limit 
> > it */
> > -	if (rc_mpss > (qib_pcie_caps & 7))
> > -		rc_mpss = qib_pcie_caps & 7;
> > -	/* If less than (allowed, supported), bump root payload */
> > -	if (rc_mpss > rc_mps) {
> > -		rc_mps = rc_mpss;
> > -		pcie_set_mps(parent, 128 << rc_mps);
> > -	}
> > -	/* If less than (allowed, supported), bump endpoint 
> > payload */
> > -	if (rc_mpss > ep_mps) {
> > -		ep_mps = rc_mpss;
> > -		pcie_set_mps(dd->pcidev, 128 << ep_mps);
> > -	}
> > -
> >  	/*
> >  	 * Now the Read Request size.
> >  	 * No field for max supported, but PCIe spec limits it to 
> > 4096,