linux-kernel - Re: [PATCH v3 22/51] PCI: Add alt

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Date:	Tue, 18 Aug 2015 22:28:47 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Wei Yang <weiyang@...ux.vnet.ibm.com>, TJ <linux@....tj>,
	Yijing Wang <wangyijing@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 22/51] PCI: Add alt_size allocation support

On Mon, Aug 17, 2015 at 5:03 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> On Mon, Jul 27, 2015 at 04:29:40PM -0700, Yinghai Lu wrote:
>> On system with several pcie switches, BIOS allocate very tight resources
>> to the bridge bar, and it is not aligned to min_align as kernel allocation
>> code.
>
> I can't parse this.

BIOS allocate resource in different way.

kernel is trying to find smallest align (min_align) and use it to get
aligned min_size.

>
>> For example:
>>       02:03.0---0c:00.0---0d:04.0---18:00.0
>> 18:00.0 need 0x10000000, and 0x00010000.
>> BIOS only allocate 0x10100000 to 0d:04.0 and above bridges.
>
> Do you mean the BIOS only allocated 0x10010000?

I can not find the exact bus layout on hand. only one similar ...

23 13:15:49  kernel: pci_bus 0000:10: scanning bus
Jun 23 13:15:49  kernel: pci 0000:10:00.0: [xxxx:xxxx] type 00 class 0x028000
Jun 23 13:15:49  kernel: pci 0000:10:00.0: reg 0x10: [mem
0xb0000000-0xbfffffff 64bit pref]
Jun 23 13:15:49  kernel: pci 0000:10:00.0: reg 0x18: [mem
0xc0000000-0xc000ffff 64bit pref]
Jun 23 13:15:49  kernel: pci_bus 0000:10: fixups for bus
Jun 23 13:15:49  kernel: pci 0000:05:04.0: PCI bridge to [bus 10-17]
Jun 23 13:15:49  kernel: pci 0000:05:04.0:   bridge window [mem
0xb0000000-0xc00fffff]
Jun 23 13:15:49  kernel: pci_bus 0000:10: bus scan returning with max=10

so device is using 0x10000000 and 0x00010000
and bridge is 0x10100000

As the bridge MMIO need to be aligned to 1M.

>
>> Later after using /sys/bus/pci/devices/0000:0c:00.0/remove to remove 0c:00.0,
>> rescan with /sys/bus/pci/rescan can not allocate 0x18000000 to 0c:00.0.
>>
>> another example:
>> 00:1c.0-[02-21]----00.0-[03-21]--+-01.0-[04-12]----00.0-[05-12]----19.0-[06-12]----00.0
>>                                  +-05.0-[13]--
>>                                  +-07.0-[14-20]----00.0-[15-20]--+-08.0-[16]--+-00.0
>>                                  |                               |            \-00.1
>>                                  |                               +-14.0-[17]----00.0
>>                                  |                               \-19.0-[18-20]----00.0
>>                                  \-09.0-[21]--
>> 06:00.0 need 0x4000000 and 0x800000.
>> BIOS only allocate 0x4800000 to 05:19.0 and 04:00.0.
>> when 05:19.0 get removed via /sys/bus/pci/devices/0000:05:19.0/remove,
>> rescan with /sys/bus/pci/rescan will fail.
>>  pci 0000:05:19.0: BAR 14: no space for [mem size 0x06000000]
>>  pci 0000:05:19.0: BAR 14: failed to assign [mem size 0x06000000]
>>  pci 0000:06:00.0: BAR 2: no space for [mem size 0x04000000 64bit]
>>  pci 0000:06:00.0: BAR 2: failed to assign [mem size 0x04000000 64bit]
>>  pci 0000:06:00.0: BAR 0: no space for [mem size 0x00800000]
>>  pci 0000:06:00.0: BAR 0: failed to assign [mem size 0x00800000]
>> current code try to use align 0x2000000 and size 0x6000000, but parent
>> bridge only have 0x4800000.
>
> I *think* you're saying:
>   - BIOS assigned space for device X
>   - We remove X via sysfs
>   - We rescan via sysfs and discover X
>   - We try to assign space for X
>   - We fail because we don't use the same algorithm as BIOS did
>
> If there is an optimal way to assign space for an arbitrary number of
> BARs, we could just adopt it.  I don't know what that is, and I don't
> know whether an optimal algorithm exists even in principle.
>
> If there is no single optimal algorithm, there will always be cases where
> we fail because we use a different algorithm than the firmware did.

That is what this patch try to do.

alt_size solution that is preferring smaller size and big alignment.

Use it together with min_align solution that is used in kernel.

>
>> Introduce alt_align/alt_size and store them in realloc list in addition
>> to addon info, and will try it after min_align/min_size allocation fails.
>
> What does "alt" mean?
>

alternative
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