Re: [PATCH v2] PCI: cadence: Fix Gen2 Link Retraining process

[Bjorn Helgaas]

On Wed, May 10, 2023 at 06:47:46PM +0530, Siddharth Vadapalli wrote:
> On 09-05-2023 23:54, Bjorn Helgaas wrote:
> > On Tue, May 09, 2023 at 12:37:31PM +0530, Siddharth Vadapalli wrote:
> >> On 09/05/23 02:44, Bjorn Helgaas wrote:
> >>> On Wed, Mar 15, 2023 at 12:38:00PM +0530, Siddharth Vadapalli wrote:
> >>>> The Link Retraining process is initiated to account for the Gen2 defect in
> >>>> the Cadence PCIe controller in J721E SoC. The errata corresponding to this
> >>>> is i2085, documented at:
> >>>> https://www.ti.com/lit/er/sprz455c/sprz455c.pdf
> >>>>
> >>>> The existing workaround implemented for the errata waits for the Data Link
> >>>> initialization to complete and assumes that the link retraining process
> >>>> at the Physical Layer has completed. However, it is possible that the
> >>>> Physical Layer training might be ongoing as indicated by the
> >>>> PCI_EXP_LNKSTA_LT bit in the PCI_EXP_LNKSTA register.
> >>>>
> >>>> Fix the existing workaround, to ensure that the Physical Layer training
> >>>> has also completed, in addition to the Data Link initialization.
> >>>>
> >>>> Fixes: 4740b969aaf5 ("PCI: cadence: Retrain Link to work around Gen2 training defect")
> >>>> Signed-off-by: Siddharth Vadapalli <s-vadapalli@xxxxxx>
> >>>> Reviewed-by: Vignesh Raghavendra <vigneshr@xxxxxx>
> >>>> ---
> >>>> Changes from v1:
> >>>> 1. Collect Reviewed-by tag from Vignesh Raghavendra.
> >>>> 2. Rebase on next-20230315.
> >>>>
> >>>> v1:
> >>>> https://lore.kernel.org/r/20230102075656.260333-1-s-vadapalli@xxxxxx
> >>>>
> >>>>  .../controller/cadence/pcie-cadence-host.c    | 27 +++++++++++++++++++
> >>>>  1 file changed, 27 insertions(+)
> >>>>
> >>>> diff --git a/drivers/pci/controller/cadence/pcie-cadence-host.c b/drivers/pci/controller/cadence/pcie-cadence-host.c
> >>>> index 940c7dd701d6..5b14f7ee3c79 100644
> >>>> --- a/drivers/pci/controller/cadence/pcie-cadence-host.c
> >>>> +++ b/drivers/pci/controller/cadence/pcie-cadence-host.c
> >>>> @@ -12,6 +12,8 @@
> >>>>  
> >>>>  #include "pcie-cadence.h"
> >>>>  
> >>>> +#define LINK_RETRAIN_TIMEOUT HZ
> >>>> +
> >>>>  static u64 bar_max_size[] = {
> >>>>  	[RP_BAR0] = _ULL(128 * SZ_2G),
> >>>>  	[RP_BAR1] = SZ_2G,
> >>>> @@ -77,6 +79,27 @@ static struct pci_ops cdns_pcie_host_ops = {
> >>>>  	.write		= pci_generic_config_write,
> >>>>  };
> >>>>  
> >>>> +static int cdns_pcie_host_training_complete(struct cdns_pcie *pcie)
> >>>
> >>> This is kind of weird because it's named like a predicate, i.e., "this
> >>> function tells me whether link training is complete", but it returns
> >>> *zero* for success.
> >>>
> >>> This is the opposite of j721e_pcie_link_up(), which returns "true"
> >>> when the link is up, so code like this reads naturally:
> >>>
> >>>   if (pcie->ops->link_up(pcie))
> >>>     /* do something if the link is up */
> >>
> >> I agree. The function name can be changed to indicate that it is
> >> waiting for completion rather than indicating completion. If this is
> >> the only change, I will post a patch to fix it. On the other hand,
> >> based on your comments in the next section, I am thinking of an
> >> alternative approach of merging the current
> >> "cdns_pcie_host_training_complete()" function's operation as well
> >> into the "cdns_pcie_host_wait_for_link()" function. If this is
> >> acceptable, I will post a different patch and the name change patch
> >> won't be necessary.
> > 
> > Yeah, sorry, I meant to delete this part of my response after I wrote
> > the one below.
> > 
> >>>> @@ -118,6 +141,10 @@ static int cdns_pcie_retrain(struct cdns_pcie *pcie)
> >>>>  		cdns_pcie_rp_writew(pcie, pcie_cap_off + PCI_EXP_LNKCTL,
> >>>>  				    lnk_ctl);
> >>>>  
> >>>> +		ret = cdns_pcie_host_training_complete(pcie);
> >>>> +		if (ret)
> >>>> +			return ret;
> >>>> +
> >>>>  		ret = cdns_pcie_host_wait_for_link(pcie);
> >>>
> >>> It seems a little clumsy that we wait for two things in succession:
> >>>
> >>>   - cdns_pcie_host_training_complete() waits up to 1s for
> >>>     PCI_EXP_LNKSTA_LT to be cleared
> >>>
> >>>   - cdns_pcie_host_wait_for_link() waits between .9s and 1s for
> >>>     LINK_UP_DL_COMPLETED on j721e (and not at all for other platforms)
> >>
> >> Is it acceptable to merge "cdns_pcie_host_training_complete()" into
> >> "cdns_pcie_host_wait_for_link()"?
> > 
> > That's what I'm proposing.  Maybe someone who is more familiar with
> > Cadence would have an argument against it, but I think making it
> > structurally the same as dw_pcie_wait_for_link() would be a good
> > thing.
> 
> Thank you for the confirmation. I will work on it and post a patch.

Ping, do you still plan to do this?  Lorenzo currently has the
v2 patch on his pci/controller/cadence branch [1], but I haven't
merged it into -next yet on the assumption that a new version is
coming.

Bjorn

[1] https://git.kernel.org/pub/scm/linux/kernel/git/pci/pci.git/commit/?h=controller/cadence&id=0e12f8302369