Re: [Xen-devel] [PATCH v2 05/10] arm: make it possible to enable/disable UART drivers

[Julien Grall <julien.grall@xxxxxxx>]

On 22/05/18 10:37, Jan Beulich wrote:
> > > > On 22.05.18 at 11:25, <julien.grall@xxxxxxx> wrote:
> > On 05/22/2018 10:22 AM, Jan Beulich wrote:
> > > > > > On 22.05.18 at 10:51, <julien.grall@xxxxxxx> wrote:
> > > > On 05/22/2018 07:34 AM, Jan Beulich wrote:
> > > > > > > > Stefano Stabellini <sstabellini@xxxxxxxxxx> 05/22/18 2:53 AM >>>
> > > > > > @@ -53,7 +53,8 @@ config HAS_SCIF
> > > > > > or Renesas R-Car Gen 2/3 based board say Y.
> > > > >     >
> > > > > > config HAS_EHCI
> > > > > > -       bool
> > > > > > +       bool "EHCI UART driver"
> > > > > ... here iirc Julien had already pointed out that the drivers should not become
> > > > > optional on x86, i.e. in the former case you want to attach a conditional to the
> > > > > prompt while in the latter case I don't see why you add the prompt in the first
> > > > > place if you mean to make it x86-specific. Which by itself is questionable
> > > > > though: Why would this driver be x86-specific, when so far it (consciously)
> > > > > hasn't been? If anything I could see it depend on HAS_PCI.
> > > > I don't expect this piece of hardware to come up on Arm.
> > > I can read two meanings into this: You expect the driver to not work on
> > > ARM, or you're confident the EHCI protocol of accessing USB host
> > > controller hardware not used anywhere in devices used on ARM. Which
> > > of the two you mean determines what the correct approach here is.
> > I am quite confident the EHCI protocol is not used on Arm.
> And why would that be? I thought PCI was universal enough that I could
> put basically any card into a system independent of CPU architecture,
> perhaps with the exception of such requiring option ROMs but not offering
> an EFI Byte Code flavor of that code.
Well, ARM ecosystem has already plenty of UART implemented directly in 
the SoC. The most common one are ns16550 and pl011.
On server side, the SBSA (Standard Base Server Architecture) mandates 
platform to implement the Arm Generic UART (a subset of PL011).
So I see very limited reason to have an EHCI PCI card on ARM platform.

Let's imagine someone really want to use an EHCI PCI card. That driver, 
aside enabling PCI, will require some bits for Arm (such as calling the 
initial setup) and likely going to need some testing. That effort is not 
worth given that Xen will always be able to use another UART (thanks to 
our 7 different driver).
The user should use either the pl011 or ns16550 on Arm for Xen. If 
someone wants something different, then he needs to come up with an 
explanation why it is needed and the patch/testing for it.
Meanwhile, that driver should not be selectable on Arm.

Cheers,

--
Julien Grall

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