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From: Peter Maydell <peter.maydell@linaro.org>
Date: Tue, 21 Feb 2017 10:59:46 +0000
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Subject: Re: [Qemu-devel] [PATCH 0/2] add bcm2835 sdhost controller to
 bcm2835 platform
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To: Clement Deschamps <clement.deschamps@antfield.fr>
Cc: QEMU Developers <qemu-devel@nongnu.org>, qemu-arm <qemu-arm@nongnu.org>, Andrew Baumann <andrew.baumann@microsoft.com>, =?UTF-8?Q?Gr=C3=A9gory_ESTRADE?= <gregory.estrade@gmail.com>

On 20 February 2017 at 17:34, Clement Deschamps
<clement.deschamps@antfield.fr> wrote:
> This patches add the Arasan SD controller to the BCM2835/36 platforms.

Cool!

> On the real hardware, both SD controllers are wired to the same SD card.
> Selection is done via multi-function pins through the GPIO controller.  I'm not
> sure what would be the good solution to emulate this in QEMU.  For now on we
> pass two sd cards on the command line which is not ideal.

That's unfortunate. I think we definitely need to fix this before we
can take the controller support into QEMU, because of the user-visible
effects on the required QEMU command line syntax.

I wonder if we can model this by effectively ejecting the card from
one SDBus and inserting it into the other when the GPIO controller
setting is configured. I think the way we want to do this is to
remove the SDState object from one SDBus qbus and insert it into
the other -- the functions in hw/sd/core.c all already cope with
"there's no card on this bus".

This might be a bit awkward because we effectively want to do a
hotplug/hot-unplug from our code but we don't want to expose to
the user that the device is hotpluggable. It may just be a matter
of doing an object_unparent() and then qdev_set_parent_bus() to move
the card object from one bus to the other... or it could get more
complicated.

thanks
-- PMM