Re: [RFC PATCH 1/2] PCI: Add pci=hpreserve= to reserve memory windows for a hotplug bridge

[Maciej Grochowski <maciej.grochowski@sony.com>]

On Mon, 5 Oct 2026, Ilpo Järvinen wrote:
> > Add "pci=hpreserve=<reservation>@<pci_dev>[; ...]" to request a minimum
>
> Why is this a new parameter instead of adding the per device functionality
> into hpmmio{,pref}size=x ?

Hi Ilpo,

Mainly to avoid giving one option two meanings.  As I read the
allocator, on the hotplug path (pci_assign_unassigned_bridge_resources())
hpmmio{,pref}size is only an optional size: it is passed as add_size, so
it lands on the realloc list, the required-only retry can drop it, and
pci_bridge_distribute_available_resources() then sets each empty
hotplug port's window to an equal share of what remains of the parent
window.  It also has no alignment of its own.  This case needs a required, aligned size
on one specific port, so I kept it separate rather than have
"hpmmioprefsize=1032M" mean "best effort" globally and "must fit" per
device.  I also wanted the bus number reservation in the same entry.

That said, the separation isn't essential, and I'd be happy to fold it
into the existing options for v2, e.g.:

  pci=hpmmioprefsize=1032M@0000:80:01.1/00.0/03.0,hpbussize=11@0000:80:01.1/00.0/03.0

where a bare value keeps today's global, optional meaning and a value
with a device specification applies only to the matching hotplug
bridges.  "[; ...]" would also cover the cover letter's grouping
question (same size for several paths), at the cost of repeating the
path once per option.  A couple of questions before I respin:

1. Should a per-device size be required, as in this RFC, or optional
   like the global one?  My understanding is that optional is not enough
   here, since the retry can drop it and distribution hands each empty
   port only an equal share of the parent window.  I haven't isolated
   this in a test with valid bus numbers; the run I did with the global
   "hpmmioprefsize=1032M,hpbussize=11" failed earlier on bus numbers
   (hpbussize is a per-bridge minimum, so the first ports drained the
   pool).  If you'd rather keep the semantics uniform, I could make it
   optional and teach distribution to honour it, but I expect that to be
   a bigger change.

2. How far should the @<pci_dev> form reach?  This case needs it only
   on hpmmioprefsize and hpbussize; I'd add hpmmiosize too.  hpiosize and
   hpmemsize are a few more parser branches, but nothing here exercises
   them.  Add them anyway, or only the knobs with a real need?

> > Tested-by: Maciej Grochowski <maciej.grochowski@sony.com>
>
> The usual assumption the patch submitter has actually tested the patch so
> normal custom is to not include self tested-bys.

Understood, I'll drop it.

Thanks,
Maciej