Re: [Xen-devel] [PATCH 21/57] ARM: GICv2: extend LR read/write functions to cover EOI and source

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 08/03/18 16:25, Andre Przywara wrote:
> On 06/03/18 16:06, Julien Grall wrote:
> > On 05/03/18 16:03, Andre Przywara wrote:
> > > diff --git a/xen/include/asm-arm/gic.h b/xen/include/asm-arm/gic.h
> > > index 8fab458d7f..89a07ae6b4 100644
> > > --- a/xen/include/asm-arm/gic.h
> > > +++ b/xen/include/asm-arm/gic.h
> > > @@ -223,9 +223,11 @@ struct gic_lr {
> > >       /* Virtual IRQ */
> > >       uint32_t virq;
> > >       uint8_t priority;
> > > -   uint8_t state;
> > > -   uint8_t hw_status;
> > > -   uint8_t grp;
> > > +   uint8_t source;
> > > +   uint8_t state:2;
> > > +   uint8_t hw_status:1;
> > > +   uint8_t grp:1;
> > > +   uint8_t eoi:1;
> > I would much prefer to introduce an union with specific information for
> > the physical interrupt (i.e GICH_LR.HW = 1) in one side and purely
> > virtual in the other side.
> Feel free to send a patch ;-)
Feel free to find someone else acking your patch ;).

> For my part I just wanted to add the two bits I need, without
> introducing any fragile assumptions and code changes about which fields
> are used exclusively with others and which not.
> I figured that I can just be a *user* of this existing interface,
> extending it as needed, but not tinkering too much. Which was what
> Stefano asked for.
> To that extent one could argue to just reuse the existing GICv3 LR
> format directly, which should be a superset of the GICv2 format. Not
> sure that this is useful, though.
While I agree that interface should not be changed too much, this new 
change just does not make sense. It makes more complicate for a reader 
(even after having read the spec...) to understand why some fields are 
only set in certain circumstance. So from my side, this is a Nack.
> > That would also help to understand the purpose of each field without
> > looking at the write_lr callback details.
> > Also, please mention that source is GICv2 only.
> I am not sure the purpose of the Xen code is to educate people about how
> the GIC works. Since you seem like to spec references, I can add a
> comment to that regard, if you are OK with that.
Well, it does not mean you need to have dry code... If you write GICv2 
only the user will have some tips where to look in the spec. I don't 
even ask a specific section.
Cheers,

--
Julien Grall

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