Re: [PATCH v3 01/17] platform-msi: allow creation of MSI domain without interrupt number

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hi Marc,

Marc Zyngier <marc.zyngier@arm.com> wrote on Fri, 29 Jun 2018 15:38:31
+0100:

> On Fri, 29 Jun 2018 08:40:35 +0100,
> Miquel Raynal <miquel.raynal@bootlin.com> wrote:
> > 
> > Hi Marc,
> > 
> > Marc Zyngier <marc.zyngier@arm.com> wrote on Thu, 28 Jun 2018 12:12:04
> > +0100:
> >   
> > > On 22/06/18 16:14, Miquel Raynal wrote:  
> > > > platform_msi_alloc_priv_data() checks that a number of interrupts is
> > > > always given. This extra-check has no real impact and just prevents
> > > > uselessly the user to create an MSI tree domain: remove it.
> > > > 
> > > > Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
> > > > ---
> > > >  drivers/base/platform-msi.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/base/platform-msi.c b/drivers/base/platform-msi.c
> > > > index 60d6cc618f1c..9f001f4ccc0f 100644
> > > > --- a/drivers/base/platform-msi.c
> > > > +++ b/drivers/base/platform-msi.c
> > > > @@ -203,7 +203,7 @@ platform_msi_alloc_priv_data(struct device *dev, unsigned int nvec,
> > > >  	 * accordingly (which would impact the max number of MSI
> > > >  	 * capable devices).
> > > >  	 */
> > > > -	if (!dev->msi_domain || !write_msi_msg || !nvec || nvec > MAX_DEV_MSIS)
> > > > +	if (!dev->msi_domain || !write_msi_msg || nvec > MAX_DEV_MSIS)
> > > >  		return ERR_PTR(-EINVAL);
> > > >  
> > > >  	if (dev->msi_domain->bus_token != DOMAIN_BUS_PLATFORM_MSI) {
> > > >     
> > > 
> > > Huh... It's not that simple.
> > > 
> > > Yes, it allows you to get a tree via platform_msi_create_device_domain
> > > (assuming that's why you're changing it -- your commit message doesn't
> > > say much)  
> > 
> > Indeed. That was exactly my intention.
> >   
> > > but it also has some impact on the way msi_domain_prepare_irqs
> > > works (see how it is called from platform_msi_create_device_domain).
> > > 
> > > Importantly, the msi_prepare callback takes nvec as a parameter, and
> > > that ends up trickling down to the irqchip, or whatever will setup the
> > > MSI domain. Things like the GICv3 ITS do rely on that to carve out the
> > > LPI space that subsequently gets used to populate the domain.
> > > 
> > > So no, you can't do it like that. If you really want a tree, add a
> > > helper that does so.  
> > 
> > So if I understand correctly, what should be done is writting a new
> > helper that would do something similar to
> > platform_msi_create_device_domain(), but creating instead a tree domain
> > and still giving msi_domain_prepare_irqs() a meaningful number (as
> > nvec) that would be the maximum number of MSIs that could be allocated.
> > Am I right here?  
> 
> What I have in mind is the following (untested) patch. Let me know if
> that works for you.

Ok, that's really close to what I wrote this morning, cool. I was not
sure that was the only thing to modify.

Thanks,
Miquèl

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