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Re: [Qemu-devel] [PATCH 2/3] ioh3420: Provide a unique bus name and an i
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH 2/3] ioh3420: Provide a unique bus name and an interrupt mapping function |
Date: |
Wed, 20 Aug 2014 14:33:57 +0200 |
On Wed, Aug 20, 2014 at 02:06:38PM +0200, Markus Armbruster wrote:
> Knut Omang <address@hidden> writes:
>
> > On Wed, 2014-08-20 at 10:52 +0200, Paolo Bonzini wrote:
> >> Il 20/08/2014 08:53, Knut Omang ha scritto:
> >> > A unique bus name is necessary to be able to refer to each instance
> >> > from the command line and monitors.
> >>
> >> Is it needed? Can't you just add id= to the -device option?
> >
> > Yes, as far as I understand the problem is that the id= would work on
> > the ioh3420 device itself, while what is needed here is to name the
> > secondary bus of the ioh3420, which I haven't found a way to name from
> > the command line.
>
> Bus names in qdev are a mess. Here are the rules:
>
> 1. If code provides a name, that's the name.
>
> 2. Else, if the device has an ID, the name is ID.N, where N counts the
> device's buses from zero.
>
> 3. Else, the name is BUS-TYPE-NAME.N, where N counts the buses of this
> type from zero.
>
> This results in a usable bus name unless device IDs collide with bus
> type names, or the code provides names that collide.
>
> The user needs to take care to use IDs that don't collide with bus
> names. Adding new bus names may screw some users.
>
> The user needs to take further care to use IDs whenever the code
> provides a bus name that collides. Adding code that provides bus names
> may screw some users.
>
> Broken by design.
>
> The problem here is "code provides names that collide": device
> q35-pcihost provides the name "pcie.0". Bound to collide with the first
> PCIE bus named under rule 3. For instance, if you next add an ioh3420
> without ID, its bus is also named "pcie.0".
>
> Rule 1 should be taken out and shot. Unfortunately, that'll break ABI
> left & right. Instead, we can try to reduce its use. The appended does
> exactly that for q35-pcihost. With it applied, the bus provided by
> q35-pcihost still gets the same name "pcie.0", but under rule 3 instead
> of rule 1. Rule 3 then names further PCIE buses "pcie.1", "pcie.2", ...
> instead of "pcie.0", "pcie.1", ... Better, but it's still an ABI break.
>
> > Maybe an even better solution would be to have default names for
> > everything, if not specified, from a user friendliness perspective?
>
> Buses *have* a default name! You're confusing this with device IDs,
> which exist only when the user sets one.
>
> Changes in this area are difficult, because the names are all ABI.
> Names that cannot be used are fair game, of course.
>
> > I suppose this is a more general issue of sensible default values
> > though, but the fact that it is easy to create devices which cannot be
> > referred has caused me some confusion from time to time.
>
> Picking default qdev IDs risks collisions with the user's IDs. We
> shouldn't do that. We do it anyway in a few places, for historical
> reasons.
>
> QOM paths might be a sane way to let users refer to devices without IDs.
>
>
> While writing the above, I stumbled another rule 1 screwup: pci_bridge.c
> attempts to "improve" the boring standard bus names chosen via rule 2 or
> 3.
>
> pci_bridge_initfn() provides a bus name of its own (commit 8a3d80f
> pci_bridge: user-friendly default bus name):
> a. If pci_bridge_map_irq() set a bus name, that's the name.
>
> b. Else, if the device has an ID, that's the name. Thus, ID.N is
> "improved" to just ID, at the cost of a special case: now users have to
> avoid not just IDs of the form BUS-TYPE-NAME.N, but also plain
> BUS-TYPE-NAME.
>
> Callers of pci_bridge_map_irq() generally provide a name. Some names
> contain spaces, thus can't collide (but would be bloody inconvenient on
> the command line or in the monitor). Others don't, but thankfully the
> ones I checked are in dead code. Craptastic.
>
>
> diff --git a/hw/pci-host/q35.c b/hw/pci-host/q35.c
> index 37f228e..469aafd 100644
> --- a/hw/pci-host/q35.c
> +++ b/hw/pci-host/q35.c
> @@ -47,7 +47,7 @@ static void q35_host_realize(DeviceState *dev, Error **errp)
> sysbus_add_io(sbd, MCH_HOST_BRIDGE_CONFIG_DATA, &pci->data_mem);
> sysbus_init_ioports(sbd, MCH_HOST_BRIDGE_CONFIG_DATA, 4);
>
> - pci->bus = pci_bus_new(DEVICE(s), "pcie.0",
> + pci->bus = pci_bus_new(DEVICE(s), NULL,
> s->mch.pci_address_space, s->mch.address_space_io,
> 0, TYPE_PCIE_BUS);
> qdev_set_parent_bus(DEVICE(&s->mch), BUS(pci->bus));
This is for the root bus, I think it won't help Knut who's trying to
add devices behind root ports.
- [Qemu-devel] [PATCH 2/3] ioh3420: Provide a unique bus name and an interrupt mapping function, Knut Omang, 2014/08/20
- Re: [Qemu-devel] [PATCH 2/3] ioh3420: Provide a unique bus name and an interrupt mapping function, Paolo Bonzini, 2014/08/20
- Re: [Qemu-devel] [PATCH 2/3] ioh3420: Provide a unique bus name and an interrupt mapping function, Markus Armbruster, 2014/08/20
- Re: [Qemu-devel] [PATCH 2/3] ioh3420: Provide a unique bus name and an interrupt mapping function,
Michael S. Tsirkin <=
- Re: [Qemu-devel] [PATCH 2/3] ioh3420: Provide a unique bus name and an interrupt mapping function, Markus Armbruster, 2014/08/20
- Re: [Qemu-devel] [PATCH 2/3] ioh3420: Provide a unique bus name and an interrupt mapping function, Paolo Bonzini, 2014/08/20
- Re: [Qemu-devel] [PATCH 2/3] ioh3420: Provide a unique bus name and an interrupt mapping function, Markus Armbruster, 2014/08/20
Re: [Qemu-devel] [PATCH 2/3] ioh3420: Provide a unique bus name and an interrupt mapping function, Andreas Färber, 2014/08/20