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Re: [Qemu-ppc] [Qemu-devel] [PULL 075/118] macio: handle non-block ATAPI

From: Mark Cave-Ayland
Subject: Re: [Qemu-ppc] [Qemu-devel] [PULL 075/118] macio: handle non-block ATAPI DMA transfers
Date: Mon, 23 Jun 2014 23:41:42 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Icedove/24.5.0

On 23/06/14 20:26, BALATON Zoltan wrote:

(add Kevin to CC)

I'm afraid as you're the only person that can boot MorphOS this far
then we need you to diagnose and suggest a suitable alternative by
comparing the before and after output. Since MacOS is already a
supported client then if no solution can be found then it is likely
that this patch will be reverted :(

So should I revert the patch for now? We're already in soft freeze.

Well let's see if Zoltan can make any headway with debugging over the next few days; if there's no progress by the weekend then sadly my recommendation would be to revert in time for -rc0 as this definitely causes intermittent boot failures in Darwin for me.

It would be nicer if it could be fixed instead of reverting. You could
help detangling the macio.c code for a start.

Just to clarify here: the macio/DBDMA code is quite complicated, but this is because this device has to work around to the fact that currently the DMA I/O routines currently need sector alignment whereas macio requires byte-level alignment. There has been quite a lot of work at the lower levels to support byte-level alignment (see Kevin's series at http://lists.gnu.org/archive/html/qemu-devel/2014-01/msg02163.html) but until we can specify transfers to byte granularity in the dma_bdrv_*() APIs then there isn't much we can do to clean up the macio.c code.

Kevin, are there any plans to bubble the byte-granularity block layer changes up to the dma_bdrv_*() APIs in the near future?

Please bear in mind that QEMU supports a large number of OSs, and there is already an enthusiastic group of people using Alex's OS X work (see emaculation for many examples) so introducing an intermittent fault on a supported OS is not an option here.

I should also re-emphasise that Alex/Andreas work on many different parts of QEMU, and my work is currently unsponsored so while we are all keen to improve QEMU to the point where it can emulate new OSs such as MorphOS, it's not the case that we can simply drop what we are doing at the time to focus on an issue that affects a single OS which is new and currently unsupported.

Now I think it's fair to say that I've spent quite a few hours helping you and coming up with the original version of this patch, and I'm glad that you are now seeing success with this. But what is important to us right now heading towards a release is that patches don't cause any regressions.

All I can say is that debugging this stuff isn't easy, particularly with MorphOS which has some rather unusual behaviours. But what we really need from you now over the next few days is for you to compare the debug output between the working and non-working cases and figure out if we can fix this in time for the 2.1 release. You have everything you need (including my acceptance test of booting both MorphOS and Darwin ISOs), so time to take a deep breath and begin what should be a challenging yet ultimately rewarding debugging process :)



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