Re: [Xen-devel] Xen ARM small task (WAS: Re: [Xen Question])

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 05/12/16 20:25, Stefano Stabellini wrote:
> On Mon, 5 Dec 2016, Julien Grall wrote:
[...]

> > > > But in the case of baremetal application, we want to be able to do logging
> > > > only (i.e not reading). They will have a driver for the host UART. It
> > > > would be
> > > > pointless and potentially difficult to emulate a full UART here. This is
> > > > where
> > > > vuart come in place (see the use case mentioned by Edgar).
> > > I was suggesting to kill two birds with one stone and just do PL011 for
> > > DomUs (disabled by default, of course). Instead, you would keep the two
> > > use cases separate? PL011 for VM spec guests and vuart for baremetal
> > > apps?
> > We have to keep those use cases separated. We already went in lengthly
> > discussion and you agreed on it. Anyway, I will summarize here.
> Ops, sorry, after reading the rest of your email I realize that there
> was a misunderstanding. Let me explain. I agree that keeping full PL011
> emulation and small vuart emulation separate can be a good thing. I also
> agree that those are two different use cases.
>
> But I didn't realize that the vuart feature and the expose memory layout
> feature were tightly connected for you. I would keep them independent.
> In fact, vuart or PL011, I would make a new virtual uart option
> available to any guests including the ones without host memory layout,
> using an iomem style option. The new uart could be advertised as PL011
> on device tree, while in fact only emulating part of it via vuart.c (of
> course the fact that doesn't completely emulate a PL011 should be
> clearly documented).
>
> On the other hand, you would basically embedded the vuart option into the
> host memory layout option and expose it as the same uart as the one on
> the host, which of course changes from platform to platform.
>
> Although I like the approach I suggested because I believe it gives
> users more flexibility and is less unexpected, because the type of the
> uart is well defined, I don't think this is an important decision. The
> host memory layout feature is the important one to have and guests that
> need a virtual uart will still be able to get one with full PL011
> emulation one day. The end results are very similar. So I would leave it
> to the contributor or, if you feel strongly about it, I'll leave it up
> to you.
After reading what you suggest, I am fine with that. I thought you 
wanted to only support PL011, hence why I was arguing back.
Cheers,


--
Julien Grall

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