Re: [PATCH] PCI: enable extended tags support for PCIe endpoints

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Nov 10, 2016 at 01:35:41PM -0500, Sinan Kaya wrote:
> On 9/24/2016 10:10 PM, Sinan Kaya wrote:
> > Each PCIe device can issue up to 32 transactions at a time by default.
> > Each transaction is tracked by a tag number on the bus. 32 outstanding
> > transactions is not enough for some performance critical applications
> > especially when a lot of small sized frames are transmitted.
> > 
> > Extended tags support increases this number to 256. Devices not
> > supporting extended tags tie-off this field to 0. According to ECN, it
> > is safe to enable this feature for all PCIe endpoints.
> > 
> > Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
> > ---
> >  drivers/pci/probe.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > index 93f280d..2424f38 100644
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -1505,12 +1505,19 @@ static void program_hpp_type2(struct pci_dev *dev, struct hpp_type2 *hpp)
> >  	 */
> >  }
> >  
> > +static int pci_configure_extended_tags(struct pci_dev *dev)
> > +{
> 
> I should have checked the capability here before trying to enable it. 
> I'll post a follow up patch on this. 
> 
> Is there any other feedback?

If this were completely safe to enable for every device that supported
it, why would there be an enable bit in Device Control?

I don't know anything about extended tags, but it worries me a little
when there's a "go-fast" switch and no explanation about when and why
we might need to go slow.

> > +	return pcie_capability_set_word(dev, PCI_EXP_DEVCTL,
> > +					 PCI_EXP_DEVCTL_EXT_TAG);
> > +}
> > +
> >  static void pci_configure_device(struct pci_dev *dev)
> >  {
> >  	struct hotplug_params hpp;
> >  	int ret;
> >  
> >  	pci_configure_mps(dev);
> > +	pci_configure_extended_tags(dev);
> >  
> >  	memset(&hpp, 0, sizeof(hpp));
> >  	ret = pci_get_hp_params(dev, &hpp);
> > 
> 
> 
> -- 
> Sinan Kaya
> Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
> Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
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