Re: [PATCH 1/1] hw/arm/sbsa-ref: use XHCI to replace EHCI

[Leif Lindholm]

On 2023-05-31 16:27, Peter Maydell wrote:
> On Wed, 31 May 2023 at 15:58, Graeme Gregory <graeme@xora.org.uk> wrote:
> > > The current sbsa-ref cannot use EHCI controller which is only
> > > able to do 32-bit DMA, since sbsa-ref doesn't have RAM above 4GB.
> > > Hence, this uses XHCI to provide a usb controller with 64-bit
> > > DMA capablity instead of EHCI.
> >
> > Should this be below 4G?
>
> It would be pretty disruptive to try to rearrange the memory
> map to put RAM below 4GB at this point, though in theory it's
> possible I guess. (I have a vague recollection that there was
> some reason the RAM was all put above 4GB, but can't find
> anything about that in my email archives. Perhaps Leif remembers?)

I think Graeme was just pointing out a typo in Marcin's email.

Yeah, we're not changing the DRAM base at this stage.
I think the reason we put no RAM below 4GB was simply that we had 
several real-world platforms where that was true, and given the intended 
use-case for the platform, we explicitly wanted to trigger issues those 
platforms might encounter.

> > Also has EHCI never worked, or has it worked in some modes and so this
> > change should be versioned?
>
> AIUI, EHCI has never worked and can never have worked, because
> this board's RAM is all above 4G and the QEMU EHCI controller
> implementation only allows DMA descriptors with 32-bit addresses.
>
> Looking back at the archives, it seems we discussed XHCI vs
> EHCI when the sbsa-ref board went in, and the conclusion was
> that XHCI would be better. But there wasn't a sysbus XHCI device
> at that point, so we ended up committing the sbsa-ref board
> with EHCI and a plan to switch to XHCI when the sysbus-xhci
> device was done, which we then forgot about:
> https://mail.gnu.org/archive/html/qemu-arm/2018-11/msg00638.html

Ah, thanks! That explains why we did the thing that made no sense :)

To skip the migration hazard, my prefernece is we just leave the EHCI 
device in for now, and add a separate XHCI on PCIe. We can drop the
EHCI device at some point in the future.

/
    Leif