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Date:   Thu, 9 Jun 2022 12:19:33 +0200
From:   Pali =?utf-8?B?Um9ow6Fy?= <pali@kernel.org>
To:     Tyrel Datwyler <tyreld@linux.ibm.com>
Cc:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Paul Mackerras <paulus@samba.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH] powerpc/pci: Add config option for using OF 'reg' for
 PCI domain
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On Tuesday 24 May 2022 11:17:56 Pali Rohár wrote:
> On Friday 06 May 2022 00:33:02 Pali Rohár wrote:
> > On Thursday 05 May 2022 15:10:01 Tyrel Datwyler wrote:
> > > On 5/5/22 02:31, Pali Rohár wrote:
> > > > Hello!
> > > > 
> > > > On Thursday 05 May 2022 07:16:40 Christophe Leroy wrote:
> > > >> Le 04/05/2022 à 19:57, Pali Rohár a écrit :
> > > >>> Since commit 63a72284b159 ("powerpc/pci: Assign fixed PHB number based on
> > > >>> device-tree properties"), powerpc kernel always fallback to PCI domain
> > > >>> assignment from OF / Device Tree 'reg' property of the PCI controller.
> > > >>>
> > > >>> PCI code for other Linux architectures use increasing assignment of the PCI
> > > >>> domain for individual controllers (assign the first free number), like it
> > > >>> was also for powerpc prior mentioned commit.
> > > >>>
> > > >>> Upgrading powerpc kernels from LTS 4.4 version (which does not contain
> > > >>> mentioned commit) to new LTS versions brings a regression in domain
> > > >>> assignment.
> > > >>
> > > >> Can you elaborate why it is a regression ?
> > > >>63a72284b159
> > > >> That commit says 'no functionnal changes', I'm having hard time 
> > > >> understanding how a nochange can be a regression.
> > > > 
> > > > It is not 'no functional change'. That commit completely changed PCI
> > > > domain assignment in a way that is incompatible with other architectures
> > > > and also incompatible with the way how it was done prior that commit.
> > > 
> > > I agree that the "no functional change" statement is incorrect. However, for
> > > most powerpc platforms it ended up being simply a cosmetic behavior change. As
> > > far as I can tell there is nothing requiring domain ids to increase montonically
> > > from zero or that each architecture is required to use the same domain numbering
> > > scheme.
> > 
> > That is truth. But it looks really suspicious why domains are not
> > assigned monotonically. Some scripts / applications are using PCI
> > location (domain:bus:dev:func) for remembering PCI device and domain
> > change can cause issue for config files. And some (older) applications
> > expects existence of domain zero. In systems without hot plug support
> > with small number of domains (e.g. 3) it means that there are always
> > domains 0, 1 and 2.
> > 
> > > Its hard to call this a true regression unless it actually broke
> > > something. The commit in question has been in the kernel since 4.8 which was
> > > released over 5 1/2 years ago.
> > 
> > I agree, it really depends on how you look at it.
> > 
> > The important is that lot of people are using LTS versions and are doing
> > upgrades when LTS support is dropped. Which for 4.4 now happened. So not
> > all smaller or "cosmetic" changes could be detected until longer LTS
> > period pass.
> > 
> > > With all that said looking closer at the code in question I think it is fair to
> > > assume that the author only intended this change for powernv and pseries
> > > platforms and not every powerpc platform. That change was done to make
> > > persistent naming easier to manage in userspace.
> > 
> > I agree that this behavior change may be useful in some situations and I
> > do not object this need.
> > 
> > > Your change defaults back to
> > > the old behavior which will now break both powernv and pseries platforms with
> > > regard to hotplugging and persistent naming.
> > 
> > I was aware of it, that change could cause issues. And that is why I
> > added config option for choosing behavior. So users would be able to
> > choose what they need.
> > 
> > > We could properly limit it to powernv and pseries by using ibm,fw-phb-id instead
> > > of reg property in the look up that follows a failed ibm,opal-phbid lookup. I
> > > think this is acceptable as long as no other powerpc platforms have started
> > > using this behavior for persistent naming.
> > 
> > And what about setting that new config option to enabled by default for
> > those series?
> > 
> > Or is there issue with introduction of the new config option?
> 
> PING? Any opinion?

PING?

> > One of the point is that it is really a good idea to have similar/same
> > behavior for all linux platforms. And if it cannot be enabled by default
> > (for backward compatibility) add at least some option, so new platforms
> > can start using it or users can decide to switch behavior.
> > 
> > > -Tyrel
> > > 
> > > > For example, prior that commit on P2020 RDB board were PCI domains 0, 1 and 2.
> > > > 
> > > > $ lspci
> > > > 0000:00:00.0 PCI bridge: Freescale Semiconductor Inc P2020E (rev 21)
> > > > 0000:01:00.0 USB controller: Texas Instruments TUSB73x0 SuperSpeed USB 3.0 xHCI Host Controller (rev 02)
> > > > 0001:02:00.0 PCI bridge: Freescale Semiconductor Inc P2020E (rev 21)
> > > > 0001:03:00.0 Network controller: Qualcomm Atheros AR93xx Wireless Network Adapter (rev 01)
> > > > 0002:04:00.0 PCI bridge: Freescale Semiconductor Inc P2020E (rev 21)
> > > > 0002:05:00.0 Network controller: Qualcomm Atheros QCA986x/988x 802.11ac Wireless Network Adapter
> > > > 
> > > > After that commit on P2020 RDB board are PCI domains 0x8000, 0x9000 and 0xa000.
> > > > 
> > > > $ lspci
> > > > 8000:00:00.0 PCI bridge: Freescale Semiconductor Inc P2020E (rev 21)
> > > > 8000:01:00.0 USB controller: Texas Instruments TUSB73x0 SuperSpeed USB 3.0 xHCI Host Controller (rev 02)
> > > > 9000:02:00.0 PCI bridge: Freescale Semiconductor Inc P2020E (rev 21)
> > > > 9000:03:00.0 Network controller: Qualcomm Atheros AR93xx Wireless Network Adapter (rev 01)
> > > > a000:04:00.0 PCI bridge: Freescale Semiconductor Inc P2020E (rev 21)
> > > > a000:05:00.0 Network controller: Qualcomm Atheros QCA986x/988x 802.11ac Wireless Network Adapter
> > > > 
> > > > It is somehow strange that PCI domains are not indexed one by one and
> > > > also that there is no domain 0
> > > > 
> > > > With my patch when CONFIG_PPC_PCI_DOMAIN_FROM_OF_REG is not set, then
> > > > previous behavior used and PCI domains are again 0, 1 and 2.
> > > > 
> > > >> Usually we don't commit regressions to mainline ...
> > > >>
> > > >>
> > > >>>
> > > >>> Fix this issue by introducing a new option CONFIG_PPC_PCI_DOMAIN_FROM_OF_REG
> > > >>> When this options is disabled then powerpc kernel would assign PCI domains
> > > >>> in the similar way like it is doing kernel for other architectures and also
> > > >>> how it was done prior that commit.
> > > >>
> > > >> You don't define CONFIG_PPC_PCI_DOMAIN_FROM_OF_REG on by default, it 
> > > >> means this commit will change the behaviour. Is that expected ?
> > > >>
> > > >> Is that really worth a user selectable option ? Is the user able to 
> > > >> decide what he needs ?
> > > > 
> > > > Well, I hope that maintainers of that code answer to these questions.
> > > > 
> > > > In any case, I think that it could be a user selectable option as in
> > > > that commit is explained that in some situation is makes sense to do
> > > > PCI domain numbering based on DT reg.
> > > > 
> > > > But as I pointed above, upgrading from 4.4 TLS kernel to some new TLS
> > > > kernel brings above regression, so I think that there should be a way to
> > > > disable this behavior.
> > > > 
> > > > In my opinion, for people who are upgrading from 4.4 TLS kernel, this
> > > > option should be turned off by default (= do not change behavior). For
> > > > people who want same behaviour on powerpc as on other platforms, also it
> > > > should be turned off by default.
> > > > 
> > > >>>
> > > >>> Fixes: 63a72284b159 ("powerpc/pci: Assign fixed PHB number based on device-tree properties")
> > > >>
> > > >> Is that really a fix ? What is the problem really ?
> > > > 
> > > > Problem is that PCI domains were changed in a way that is not compatible
> > > > neither with version prior that commit and neither with how other linux
> > > > platforms assign PCI domains for controllers.
> > > > 
> > > >>> Signed-off-by: Pali Rohár <pali@kernel.org>
> > > >>> ---
> > > >>>   arch/powerpc/Kconfig             | 10 ++++++++++
> > > >>>   arch/powerpc/kernel/pci-common.c |  4 ++--
> > > >>>   2 files changed, 12 insertions(+), 2 deletions(-)
> > > >>>
> > > >>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> > > >>> index 174edabb74fa..4dd3e3acddda 100644
> > > >>> --- a/arch/powerpc/Kconfig
> > > >>> +++ b/arch/powerpc/Kconfig
> > > >>> @@ -375,6 +375,16 @@ config PPC_OF_PLATFORM_PCI
> > > >>>   	depends on PCI
> > > >>>   	depends on PPC64 # not supported on 32 bits yet
> > > >>>   
> > > >>> +config PPC_PCI_DOMAIN_FROM_OF_REG
> > > >>> +	bool "Use OF reg property for PCI domain"
> > > >>> +	depends on PCI
> > > >>
> > > >> Should it depend on PPC_OF_PLATFORM_PCI instead ?
> > > > 
> > > > No, PPC_OF_PLATFORM_PCI has line "depends on PPC64 # not supported on 32
> > > > bits yet". But it is already used also for e.g. P2020 which is 32-bit
> > > > platform.
> > > > 
> > > >>> +	help
> > > >>> +	  By default PCI domain for host bridge during its registration is
> > > >>> +	  chosen as the lowest unused PCI domain number.
> > > >>> +
> > > >>> +	  When this option is enabled then PCI domain is determined from
> > > >>> +	  the OF / Device Tree 'reg' property.
> > > >>> +
> > > >>>   config ARCH_SUPPORTS_UPROBES
> > > >>>   	def_bool y
> > > >>>   
> > > >>> diff --git a/arch/powerpc/kernel/pci-common.c b/arch/powerpc/kernel/pci-common.c
> > > >>> index 8bc9cf62cd93..8cb6fc5302ae 100644
> > > >>> --- a/arch/powerpc/kernel/pci-common.c
> > > >>> +++ b/arch/powerpc/kernel/pci-common.c
> > > >>> @@ -74,7 +74,6 @@ void __init set_pci_dma_ops(const struct dma_map_ops *dma_ops)
> > > >>>   static int get_phb_number(struct device_node *dn)
> > > >>>   {
> > > >>>   	int ret, phb_id = -1;
> > > >>> -	u32 prop_32;
> > > >>>   	u64 prop;
> > > >>>   
> > > >>>   	/*
> > > >>> @@ -83,7 +82,8 @@ static int get_phb_number(struct device_node *dn)
> > > >>>   	 * reading "ibm,opal-phbid", only present in OPAL environment.
> > > >>>   	 */
> > > >>>   	ret = of_property_read_u64(dn, "ibm,opal-phbid", &prop);
> > > >>
> > > >> This looks like very specific, it is not reflected in the commit log.
> > > > 
> > > > I have not changed nor touched this "ibm,opal-phbid" setting. And it was
> > > > not also touched in that mentioned patch. I see that no DTS file in
> > > > kernel use this option (so probably only DTS files supplied by
> > > > bootloader use it). So I thought that there is not reason to mention in
> > > > commit message.
> > > > 
> > > > But if you think so, I can add some info to commit message about it.
> > > > 
> > > >>> -	if (ret) {
> > > >>> +	if (ret && IS_ENABLED(CONFIG_PPC_PCI_DOMAIN_FROM_OF_REG)) {
> > > >>> +		u32 prop_32;
> > > >>>   		ret = of_property_read_u32_index(dn, "reg", 1, &prop_32);
> > > >>>   		prop = prop_32;
> > > >>>   	}
> > >