linux-kernel - Re: [PATCH v2 4/5] PCI: keystone: Add quirk to mark AM654 RC BAR flag as IORESOURCE

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Date:   Fri, 4 Feb 2022 20:38:46 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
CC:     Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        <linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 4/5] PCI: keystone: Add quirk to mark AM654 RC BAR flag
 as IORESOURCE_UNSET

Hi Lorenzo,

On 11/01/22 11:53 am, Kishon Vijay Abraham I wrote:
> Hi Lorenzo,
> 
> On 04/01/22 9:27 pm, Lorenzo Pieralisi wrote:
>> On Fri, Nov 26, 2021 at 02:01:18PM +0530, Kishon Vijay Abraham I wrote:
>>> AM654 RootComplex has a hard coded 64 bit BAR of size 1MB and also has
>>> both MSI and MSI-X capability in it's config space. If PCIEPORTBUS is
>>> enabled, it tries to configure MSI-X and msix_mask_all() adds about 10
>>> Second boot up delay when it tries to write to undefined location.
>>>
>>> Add quirk to mark AM654 RC BAR flag as IORESOURCE_UNSET so that
>>> msix_map_region() returns NULL for Root Complex and avoid un-desirable
>>> writes to MSI-X table.
>>
>> I don't think this is the right fix (it is not even a fix, just a
>> plaster to workaround an issue).
>>
>> What do you mean by "writing to an undefined location" ?
>>
>> What does "a hard coded BAR" mean ?
>>
>> What happens if we _rightly_ write into it (ie to size it) ?
> 
> There are two parts w.r.t setting the BAR; one is during the configuration and
> the other is during the enumeration.
> i) During the configuration, the size of the BAR is configured and the inbound
> ATU is configured to map the BAR to a physical memory.
> ii) During the enumeration, the size of the BAR is obtained and an address is
> allocated and programmed in the BAR.
> 
> In the case of RC, for (i) above, the BAR size is configured as '0'
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/controller/dwc/pcie-designware-host.c#n556
> and the inbound ATU is not programmed at all.
> 
> However, in the case of AM654, the HW configures BAR0 for a fixed size of 1MB
> (irrespective of what SW programmed in [i]). While this was done more for a
> endpoint usecase, since the same IP is configured for both RC mode and EP mode,
> the fixed BAR size is seen with RC mode as well. AM654 also has MSI-X capability
> for RC mode (the IP should have been ideally configured to have MSI-X capability
> for EP mode). This results in PCIEPORTBUS doing some undesired access in
> msix_mask_all().
> 
> Here I configure IORESOURCE_UNSET so that memory is not allocated for RC BAR.

Do you need further clarifications on this?

Thank You,
Kishon

> 
> Thank You,
> Kishon
> 
> 
>>
>> Lorenzo
>>
>>> Signed-off-by: Kishon Vijay Abraham I <kishon@...com>
>>> ---
>>>  drivers/pci/controller/dwc/pci-keystone.c | 8 +++++++-
>>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/pci/controller/dwc/pci-keystone.c b/drivers/pci/controller/dwc/pci-keystone.c
>>> index 52d20fe17ee9..73e6626a0d8f 100644
>>> --- a/drivers/pci/controller/dwc/pci-keystone.c
>>> +++ b/drivers/pci/controller/dwc/pci-keystone.c
>>> @@ -557,8 +557,14 @@ static void ks_pcie_quirk(struct pci_dev *dev)
>>>  		{ 0, },
>>>  	};
>>>  
>>> -	if (pci_is_root_bus(bus))
>>> +	if (pci_is_root_bus(bus)) {
>>>  		bridge = dev;
>>> +		if (pci_match_id(am6_pci_devids, bridge)) {
>>> +			struct resource *r = &dev->resource[0];
>>> +
>>> +			r->flags |= IORESOURCE_UNSET;
>>> +		}
>>> +	}
>>>  
>>>  	/* look for the host bridge */
>>>  	while (!pci_is_root_bus(bus)) {
>>> -- 
>>> 2.17.1
>>>