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

[Siddharth Vadapalli <s-vadapalli@ti.com>]

Hello Bjorn,

On 06/06/23 21:50, Bjorn Helgaas wrote:
> 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@ti.com>
>>>>>> Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>
>>>>>> ---
>>>>>> 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@ti.com
>>>>>>
>>>>>>  .../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.

Sorry for the delay. I have posted the V3 patch at:
https://lore.kernel.org/r/20230607091427.852473-1-s-vadapalli@ti.com/

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

-- 
Regards,
Siddharth.