Re: [Xen-devel] [GSOC-2019] Problem with initializing crossbar on bb-x15 in dom0

[Julien Grall <julien.grall@xxxxxxx>]

Hi Denis,

On 7/6/19 5:50 PM, Denis Obrezkov wrote:
> > > Ok, I changed it and was able to boot to the previous error. I think a
> > > logical next step would be to set up the uart somehow?
> > Do you mean the UART used by Xen?
> Yes, to be able to switch between xen and dom0, to download a produced
> dt for dom0 for example.
Yes it would be sensible to try to implement a crossbar driver in Xen 
and test with the UART. Looking at the driver in Linux, this should not 
be too difficult.
> > > > But this feels weird to map then unmap the mmio. Instead, you should
> > > > blacklist the crossbar device. Have a look at the field blacklist_dev in
> > > > platform_desc.
> > > Hm, I can see that in the device tree the crossbar has a phandle
> > > property <0x00000008> and the main node has an interrupt-parent property
> > > 0x00000008. So, all the interrupts seems to be mapped to the crossbar.
> > > Wouldn't be that a problem if we blacklist the device?
> >
> > The Device is owned by Xen, so technically Dom0 does not see the
> > hardware one. Instead it sees a virtual and therefore the node should be
> > created to reflect it.
> >
> > The purpose of recreating the node is you can alter it to match what we
> > actually exposed to the domain (property values may differ). It may
> > happen that a lot of information are exactly the same as the hardware
> > and can just be copied.
> >
> > This is, for instance, what we do for the GIC and timer.
> I mean if we expose only GIC to the dom0 then we need to change the
> interrupt-parent property to make all nodes have GIC's phandle as their
> interrupt-parent. And when dom0 tries to modify irq connections then xen
> should modify the crossbar. It seems to be a bit error prone approach.
I don't think I ever suggested to not expose the crossbar to Dom0. 
Instead, I suggested to virtualize for Dom0, so it can be used by Xen as 
well.
> > > Also, the tegra
> > > implementation blacklist only a uart.
> > I don't understand this.
> In here [1] you can find that only uart is blacklisted (in
> tegra_blacklist_dev[]). So, in tegra they didn't blacklist their version
> of the crossbar.
This series has not been merged. In other word, the code is not yet 
matching the expectations of the maintainers.
I pointed you to this series, because I think some of the idea could be 
re-used for implementing the crossbar.
In this particular case, it has been suggested to use blacklist_dev 
rather than unmapping (see [2]).
Cheers,

> [1]
> https://lists.xenproject.org/archives/html/xen-devel/2017-04/msg00993.html
[2] 
https://lists.xenproject.org/archives/html/xen-devel/2017-04/msg01979.html
>
--
Julien Grall

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