On Mon, 19 Feb 2024, Mark Cave-Ayland wrote:
On 19/02/2024 12:00, BALATON Zoltan wrote:
On Mon, 19 Feb 2024, Philippe Mathieu-Daudé wrote:
On 19/2/24 12:27, BALATON Zoltan wrote:
On Mon, 19 Feb 2024, Philippe Mathieu-Daudé wrote:
On 16/2/24 20:54, Philippe Mathieu-Daudé wrote:
On 16/2/24 18:14, BALATON Zoltan wrote:
On Fri, 16 Feb 2024, Philippe Mathieu-Daudé wrote:
We want to set another qdev property (a link) for the pl110
and pl111 devices, we can not use sysbus_create_simple() which
only passes sysbus base address and IRQs as arguments. Inline
it so we can set the link property in the next commit.
Signed-off-by: Philippe Mathieu-Daudé <philmd@xxxxxxxxxx>
---
hw/arm/realview.c | 5 ++++-
hw/arm/versatilepb.c | 6 +++++-
hw/arm/vexpress.c | 10 ++++++++--
3 files changed, 17 insertions(+), 4 deletions(-)
diff --git a/hw/arm/realview.c b/hw/arm/realview.c
index 9058f5b414..77300e92e5 100644
--- a/hw/arm/realview.c
+++ b/hw/arm/realview.c
@@ -238,7 +238,10 @@ static void realview_init(MachineState *machine,
sysbus_create_simple("pl061", 0x10014000, pic[7]);
gpio2 = sysbus_create_simple("pl061", 0x10015000, pic[8]);
- sysbus_create_simple("pl111", 0x10020000, pic[23]);
+ dev = qdev_new("pl111");
+ sysbus_realize_and_unref(SYS_BUS_DEVICE(dev), &error_fatal);
+ sysbus_mmio_map(SYS_BUS_DEVICE(dev), 0, 0x10020000);
+ sysbus_connect_irq(SYS_BUS_DEVICE(dev), 0, pic[23]);
Not directly related to this patch but this blows up 1 line into 4
just to allow setting a property. Maybe just to keep some simplicity
we'd rather need either a sysbus_realize_simple function that takes a
sysbus device instead of the name and does not create the device
itself or some way to pass properties to sysbus create simple (but the
latter may not be easy to do in a generic way so not sure about that).
What do you think?
Unfortunately sysbus doesn't scale in heterogeneous setup.
Regarding the HW modelling API complexity you are pointing at, we'd
like to move from the current imperative programming paradigm to a
declarative one, likely DSL driven. Meanwhile it is being investigated
(as part of "Dynamic Machine"), I'm trying to get the HW APIs right
I'm aware of that activity but we're currently still using board code to
construct machines and probably will continue to do so for a while. Also
because likely not all current machines will be converted to new
declarative way so having a convenient API for that is still useful.
(As for the language to describe the devices of a machine and their
connections declaratively the device tree does just that but dts is not a
very user friendly descrtiption language so I haven't brought that up as
a possibility. But you may still could get some clues by looking at the
problems it had to solve to at least get a requirements for the machine
description language.)
for heterogeneous emulation. Current price to pay is a verbose
imperative QDev API, hoping we'll get later a trivial declarative one
(like this single sysbus_create_simple call), where we shouldn't worry
about the order of low level calls, whether to use link or not, etc.
Having a detailed low level API does not prevent a more convenient for
current use higher level API on top so keeping that around for current
machines would allow you to chnage the low level API without having to
change all the board codes because you's only need to update the simple
high level API.
So what is your suggestion here, add a new complex helper to keep
a one-line style?
DeviceState *sysbus_create_simple_dma_link(const char *typename,
hwaddr baseaddr,
const char *linkname,
Object *linkobj,
qemu_irq irq);
I think just having sysbus_realize_simple that does the same as
sysbus_create_simple minus creating the device would be enough because then
the cases where you need to set properties could still use it after
qdev_new or init and property_set but hide the realize and connecting the
device behind this single call.
I can't say I'm a fan of sysbus_create_simple() because its use of varargs to
populate qdev properties is based upon the assumptions that the properties
defined with device_class_set_props() are stored in a list. I can see there
could be potential in future to store properties in other structures such as
a hash, and keeping this API would prevent this change. FWIW my personal
preference would be to remove this API completely.
I wonder why this is that important since you never modified
any of the files changed by this series:
For new people trying to contribute to QEMU QDev is overwhelming so having
some way to need less of it to do simple things would help them to get
started.
It depends what how you define "simple": for QEMU developers most people
search for similar examples in the codebase and copy/paste them. I'd much
rather have a slightly longer, but consistent API for setting properties
rather than coming up with many special case wrappers that need to be
maintained just to keep the line count down for "simplicity".
It's not just about keeping the line count down, although that helps with
readablility, it's simpler to see what the code does if one has to go
through less QDev and QOM details, and new people are unfamiliar with
those so when they see the five lines creating the single device they
won't get what it does while a sysbus_create_simple call is very self
explaining. Maybe sysbus_create_simple is not the best API and not one we
can keep but by point is that as long as we have board code and it's the
main way to create machines that developers have to work with then we
should have some simple API to do that and don't leave them with only low
level QOM and QDev calls that are not high level enough to creare a
machine conveniently. If the direction is to eventually don't need any
code to create a machine then don't spend much time on designing that API
but at least keep what we have as long as it's possible. Removing the
device creation from sysbus_create_simple is not a big change but allows
board code to keep using it for now instead of ending up an unreadable low
level calls that makes it harder to see at a glance what a board consists
of.
I think that Phil's approach here is the best one for now, particularly given
that it allows us to take another step towards heterogeneous machines. As the
work in this area matures it might be that we can consider other approaches,
but that's not a decision that can be made right now and so shouldn't be a
reason to block this change.
I did not say this patch should not be accepred or anything like that.
Just if there's a way with not too much work to make this simpler (as in
more readable and understandable for people not familiar with low levels
of QEMU) then I think that's worth trying and keeping at least most of the
functions of sysbus_create_simple as sysbus_realize_simple is not much
work to do but avoids blowing up the board code with a lot of low level
QOM stuff that I'd rather keep out of there unless it could be made less
overwhelming and verbose. Also keeping a higher level API for board code
would help this refactoring because if the low level calls are not all
over the board code then they would need to change less as the changes
could be done within the higher level API implementation.
But at the end this is just my opinion and Philippe is free to do what he
wants. I ust shared this view point in case he can take it into account
but if not then it's not the end of the world.
Regards,
BALATON Zoltan
