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Re: [Qemu-devel] [PATCH 4/4] i8254: convert to qdev

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 4/4] i8254: convert to qdev
Date: Sun, 06 Mar 2011 11:47:58 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20101027 Lightning/1.0b1 Thunderbird/3.0.10

On 03/06/2011 10:47 AM, Blue Swirl wrote:
The PIT is not an ISA device.  Modelling it as such is worse than leaving it
No. These days, PIT is part of Super I/O chip, which is accessed via
LPC bus. LPC is from software point of view equal to ISA. Therefore,
in absence of LPC, ISA is correct.

In the system we model, the PIT is part of the PIIX3. The right way to model it is as a DeviceState that's no_user=1 and created as part of the initialized of PIIX3 (for the PC at least).

LPC is still an expansion bus and it's primarily used for discrete components like a TPM. For components that are all part of a Super I/O chip, there really just isn't a bus in the middle.

-PITState *pit_init(int base, qemu_irq irq);
-void pit_set_gate(PITState *pit, int channel, int val);
-int pit_get_gate(PITState *pit, int channel);
-int pit_get_initial_count(PITState *pit, int channel);
-int pit_get_mode(PITState *pit, int channel);
-int pit_get_out(PITState *pit, int channel, int64_t current_time);
+void pit_set_gate(ISADevice *dev, int channel, int val);
+int pit_get_gate(ISADevice *dev, int channel);
+int pit_get_initial_count(ISADevice *dev, int channel);
+int pit_get_mode(ISADevice *dev, int channel);
+int pit_get_out(ISADevice *dev, int channel, int64_t current_time);

Making these functions take an ISADevice hurts type safety.  They should
take a PITState.
These functions should be removed anyway. For example, pcspk probably
should be merged with i825, or a signal like interface could be used.

It's tough to do that without doing a real simulation. The PC speaker uses one of the PIT channel outputs to drive its tone output. But we don't want to do something like use a qemu_irq with a timer programmed to generate the right frequency wave to drive the tone.

So having the PC speaker hold a reference to the PIT and providing a high level method interface makes sense to me.


Anthony Liguori

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