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Date: Fri, 10 May 2019 23:20:43 +0530 From: Vidya Sagar <vidyas@...dia.com> To: Bjorn Helgaas <helgaas@...nel.org> CC: Thierry Reding <thierry.reding@...il.com>, "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>, "robh+dt@...nel.org" <robh+dt@...nel.org>, "mark.rutland@....com" <mark.rutland@....com>, Jonathan Hunter <jonathanh@...dia.com>, "kishon@...com" <kishon@...com>, "catalin.marinas@....com" <catalin.marinas@....com>, "will.deacon@....com" <will.deacon@....com>, "jingoohan1@...il.com" <jingoohan1@...il.com>, "gustavo.pimentel@...opsys.com" <gustavo.pimentel@...opsys.com>, Mikko Perttunen <mperttunen@...dia.com>, "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>, "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>, "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, Krishna Thota <kthota@...dia.com>, Manikanta Maddireddy <mmaddireddy@...dia.com>, "sagar.tv@...il.com" <sagar.tv@...il.com> Subject: Re: [PATCH V5 03/16] PCI: Export pcie_bus_config symbol On 5/10/2019 10:16 PM, Bjorn Helgaas wrote: > Hi Vidya, > > On Fri, May 10, 2019 at 11:51:24AM +0530, Vidya Sagar wrote: >>> -----Original Message----- >>> From: linux-pci-owner@...r.kernel.org <linux-pci-owner@...r.kernel.org> On >>> Behalf Of Thierry Reding >>> Sent: Friday, May 3, 2019 4:38 PM >>> To: Vidya Sagar <vidyas@...dia.com> >>> On Wed, Apr 24, 2019 at 10:49:51AM +0530, Vidya Sagar wrote: >>>> Export pcie_bus_config to enable host controller drivers setting it to >>>> a specific configuration be able to build as loadable modules >>>> >>>> Signed-off-by: Vidya Sagar <vidyas@...dia.com> > >>> It doesn't look to me like this is something that host controller drivers are >>> supposed to change. This is set via the pci kernel command- line parameter, >>> meaning it's a way of tuning the system configuration. >>> Drivers should not be allowed to override this after the fact. >>> >>> Why do we need to set this? >> Here is the reason I'm doing it. >> First things first, Tegra194 supports MPS up to 256 bytes. >> Assume there are two endpoints with MPS supported up to >> a) 128 bytes (Ex:- Realtek NIC with 8168 controller) >> b) 256 bytes (Ex:- Kingston NVMe drive) >> Now, leaving "pcie_bus_config" untouched in the driver sets it to >> PCIE_BUS_DEFAULT by default. With this setting, for both (a) and (b), >> MPS is set to 128, which means, even though Tegra194 supports 256 MPS, it is not >> set to 256 even in case of (b) thereby not using RP's 256 MPS feature. >> If I explicitly set pcie_bus_config=PCIE_BUS_PERFORMACE in the code, then 256 MPS is set when >> (b) is connected, but when (a) is connected, for root port MPS 256 is set and for >> endpoint MPS 128 is set, because of which root port tries to send packets with 256 >> payload that breaks functionality of Realtek NIC card. >> The best option I've found out is that when I set 256 in PCI_EXP_DEVCTL of root port >> explicitly before link up and use pcie_bus_config=PCIE_BUS_SAFE, then, I get the best of both >> PCIE_BUS_DEFAULT and PCIE_BUS_PERFORMANCE i.e. with (a) connected, MPS is set to 128 in both RP >> and EP and with (b) connected, MPS is set to 256 in both RP and EP. >> >> So, is it like, pcie_bus_config shouldn't be set to anything explicitly in the driver and depending on the >> platform and what is connected to root port, kernel parameter can be passed with appropriate setting? > > Host controller drivers shouldn't change this unless there's some host > controller defect that means the generic code can't do the right > thing. Even then, I'd prefer that the host controller driver merely > set a quirk bit that describes the defect, e.g., "mps_*_broken". Then > the generic code could pay attention to that and we wouldn't have to > make "pcie_bus_config" a part of the ABI. > > From your description, it sounds like there's nothing actually wrong > with the Tegra194 hardware, but the generic code isn't as smart about > setting MPS as it possibly could be. My solution to that would be to > make the generic code smarter so everybody can benefit. > > Bjorn Thanks Bjorn for your take on this. I'll drop this patch from the current series and make a note to optimize PCIE_BUS_DEFAULT to do a better job of setting MPS in the best possible way. > >>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c index >>>> f5ff01dc4b13..731f78508601 100644 >>>> --- a/drivers/pci/pci.c >>>> +++ b/drivers/pci/pci.c >>>> @@ -94,6 +94,7 @@ unsigned long pci_hotplug_mem_size = >>>> DEFAULT_HOTPLUG_MEM_SIZE; unsigned long pci_hotplug_bus_size = >>>> DEFAULT_HOTPLUG_BUS_SIZE; >>>> >>>> enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_DEFAULT; >>>> +EXPORT_SYMBOL_GPL(pcie_bus_config); >>>> >>>> /* >>>> * The default CLS is used if arch didn't set CLS explicitly and not >>>> -- >>>> 2.17.1 >>>>
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