Re: [edk2-devel] [PATCH v2 0/4] Add extra pci roots support for Arm

["Ni, Ray" <ray.ni@intel.com>]

> -----Original Message-----
> From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of Jiahui Cen via groups.io
> Sent: Friday, December 4, 2020 2:49 PM
> To: devel@edk2.groups.io; lersek@redhat.com
> Cc: Justen, Jordan L <jordan.l.justen@intel.com>; ard.biesheuvel@arm.com; leif@nuviainc.com; xieyingtai@huawei.com;
> miaoyubo@huawei.com; xuxiaoyang2@huawei.com
> Subject: Re: [edk2-devel] [PATCH v2 0/4] Add extra pci roots support for Arm
> 
> Hi Laszlo,
> 
> During the modification of this patch series, I got a confusing problem
> that the EDK2's Alignment seems not fit with Linux on ARM.
> 
> EDK2, in CalculateResourceAperture, aligns the offset of child nodes
> in descending order and uses the *Bridge's alignment* to align the
> total length of Bridge.
> 
> However, Linux, in pbus_size_mem, would align the size of child nodes
> and use *the half of max alignment of child nodes* to align the total
> length of Bridge.
> 
> eg. A Root Bridge with Bus [d2]
>     -+-[0000:d2]---01.0-[d3]----01.0
>     where [d2:01.00] is a pcie-pci-bridge with BAR0 (mem, 64-bit, non-pref) [size=256]
>           [d3:01.00] is a PCI Device with BAR0 (mem, 64-bit, pref) [size=128K]
>                                           BAR4 (mem, 64-bit, pref) [size=64M]
> 
>     In EDK2, the Resource Map would be:
>         PciBus: Resource Map for Root Bridge PciRoot(0xD2)
>         Type =  Mem64; Base = 0x8004000000;     Length = 0x4200000;     Alignment = 0x3FFFFFF
>            Base = 0x8004000000; Length = 0x4100000;     Alignment = 0x3FFFFFF;  Owner = PPB [D2|01|00:**]; Type = PMem64
>            Base = 0x8008100000; Length = 0x100; Alignment = 0xFFF;      Owner = PPB [D2|01|00:10]
> 
>         PciBus: Resource Map for Bridge [D2|01|00]
>         Type = PMem64; Base = 0x8004000000;     Length = 0x4100000;     Alignment = 0x3FFFFFF
>            Base = 0x8004000000; Length = 0x4000000;     Alignment = 0x3FFFFFF;  Owner = PCI [D3|01|00:20]
>            Base = 0x8008000000; Length = 0x20000;       Alignment = 0x1FFFF;    Owner = PCI [D3|01|00:10]
>         Type =  Mem64; Base = 0x8008100000;     Length = 0x100; Alignment = 0xFFF
> 
>     It shows that with EDK2's alignment [d2:01.00] only requires
>     a PMem64 resource range of 0x4100000.
> 
>     However in Linux, [d2:01.00]'s BAR14 (Prefetchable Memory
>     Resource behind the Bridge) requires a PMem64 resource range
>     of 0x06000000, which comes from (0x4000000 + 0x20000) with
>     alignment=0x1FFFFFF the half of the max alignment 0x3FFFFFF.

How is 0x3FFFFFF calculated?
0x3FFFFFF = 0x8000000 / 2 - 1, and 0x8000000 is the minimum value that's
power of 2 and bigger than 0x6000000. Is my understanding correct?

I don't understand why this calculation method is chosen.
It seems a kernel bug to me.
I thought Linux doesn't handle the pci resource assignment but just use what was
assigned by firmware.

> 
> Furthermore, Linux Kernel will treat pcie-root-port as a hotplugable
> bridge and try to require more resource. But EDK2 seems not support
> hotplug padding for ARM?

Hotplug padding is supported through HotPlugInit protocol in edk2.