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Re: [PATCH v6 3/9] qdev: add clock input&output support to devices.

From: Philippe Mathieu-Daudé
Subject: Re: [PATCH v6 3/9] qdev: add clock input&output support to devices.
Date: Wed, 4 Dec 2019 10:53:41 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2

On 12/4/19 10:05 AM, Damien Hedde wrote:
On 12/2/19 3:34 PM, Peter Maydell wrote:
On Wed, 4 Sep 2019 at 13:56, Damien Hedde <address@hidden> wrote:

+ * qdev_pass_clock:
+ * @dev: the device to forward the clock to
+ * @name: the name of the clock to be added (can't be NULL)
+ * @container: the device which already has the clock
+ * @cont_name: the name of the clock in the container device
+ *
+ * Add a clock @name to @dev which forward to the clock @cont_name in 
+ */

'container' seems odd terminology here, because I would expect
the usual use of this function to be when a 'container' object
like an SoC wants to forward a clock to one of its components;
in that case the 'container' SoC would be @dev, wouldn't it?

Yes. I agree it is confusing.
This function just allow a a device 'A' to exhibit a clock from another
device 'B' (typically a composing sub-device, inside a soc like you
said). The device A is not supposed to interact with the clock itself.
The original sub-device is the true
owner/controller of the clock and is the only one which should use the
clock API: setting a callback on it (input clock); or updating the clock
frequency (output clock).
Basically, it just add the clock into the device clock namespace without
interfering with it.

We should get this to be the same way round as qdev_pass_gpios(),
which takes "DeviceState *dev, DeviceState *container", and
passes the gpios that exist on 'dev' over to 'container' so that
'container' now has gpios which it did not before.

Ok, I'll invert the arguments.

Also, your use of 'forward to' is inconsistent: in the 'dev'
documentation you say we're forwarding the clock to 'dev',
but in the body of the documentation you say we're forwarding
the clock to the clock in 'container'.

I'll try to clarify this and make the documentation more consistent with
the comments here.

I think the way to resolve this is to stick to the terminology
in the function name itself:
  @dev: the device which has the clock
  @name: the name of the clock on @dev
  @container: the name of the device which the clock should
   be passed to
  @cont_name: the name to use for the clock on @container

I think container is confusing because depending on how we reason (clock
in a device; device in another device), we end up thinking the opposite.

Maybe we can use "exhibit" instead of "container" in the 2nd pair of
parameters, or prefix use "origin" or "owner" as a prefix for the first
pair of parameters.

@sink vs @source?

Q: if you pass a clock to another device with this function,
does it still exist to be used directly on the original
device? For qdev_pass_gpios it does not (I think), but
this is more accident of implementation than anything else.

It depends what we mean by "used by".
Original device can:
+ set the callback in case it is an input
+ update the frequency in case it is an output

Hmm here you use @input vs @output...

But since an input clock can only be connected once,
I think the logic here is that any connection should now go through the
new device. But this is not checked and using one or the other is
exactly the same.

Maybe I misunderstood the meaning of qdev_pass_gpios().

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