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Re: [PATCH v7 1/8] vt82c686: Implement control of serial port io ranges

From: Mark Cave-Ayland
Subject: Re: [PATCH v7 1/8] vt82c686: Implement control of serial port io ranges via config regs
Date: Tue, 23 Mar 2021 21:58:44 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0

On 23/03/2021 12:54, BALATON Zoltan wrote:

On Wed, 10 Mar 2021, BALATON Zoltan wrote:
In VIA super south bridge the io ranges of superio components
(parallel and serial ports and FDC) can be controlled by superio
config registers to set their base address and enable/disable them.
This is not easy to implement in QEMU because ISA emulation is only
designed to set io base address once on creating the device and io
ranges are registered at creation and cannot easily be disabled or
moved later.

In this patch we hack around that but only for serial ports because
those have a single io range at port base that's relatively easy to
handle and it's what guests actually use and set address different
than the default.

We do not attempt to handle controlling the parallel and FDC regions
because those have multiple io ranges so handling them would be messy
and guests either don't change their deafult or don't care. We could
even get away with disabling and not emulating them, but since they
are already there, this patch leaves them mapped at their default
address just in case this could be useful for a guest in the future.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>

Could this patch be reviewed now please? I've dropped it from later versions to avoid this holding back the series but now that it won't be in 6.0 I'd like to go back to this. This is implementing the behaviour of the real hardware better than the unsettable default value we have as a replacement. That approach also works for the guests I've tried (MorphOS and Linux) but if we can do better than why not do it?

Do bear in mind that several people have already looked at this patch and haven't felt comfortable enough to review it, and I've also said in a previous email that this isn't the right approach. Given that the ISA bus partly uses the ioport address to reference the device, manually moving the memory regions around for devices on the bus without the ISA bus seeing those changes is going to cause issues.

IIRC the ability to dynamically change the standard ISA addresses was present in several motherboards from that era, and given that this functionality hasn't been implemented in QEMU this already tells us that no firmware is currently is using it.

I don't understand why using the hard-coded addresses in the v10 is a problem here? The addresses you added in the comments representing the programmed values are the standard ISA device addresses, so if those are what the firmware is programming then there will be no change. You also reported that it works fine with both your MorphOS and Linux test images, indicating that neither of these guest OSs require the feature.

If you find a guest OS that needs the functionality then certainly we can talk about trying to come up with a solution, but for me the extra complexity of this approach and the fact that you're overriding the management of the device by the ISA bus is why I haven't given it a R-B tag (I should add that my R-B for v10 using the hard-coded ISA addresses still stands).



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