Re: [PATCH v2 11/17] xen/arm: PCI host bridge discovery within XEN on ARM

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 24 Sep 2021, Stefano Stabellini wrote:
> On Fri, 24 Sep 2021, Rahul Singh wrote:
> > Hi Stefano,
> > 
> > > On 23 Sep 2021, at 8:12 pm, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
> > > wrote:
> > > 
> > > On Thu, 23 Sep 2021, Rahul Singh wrote:
> > >>>> +            goto err_exit;
> > >>>> +    }
> > >>> 
> > >>> This is unnecessary at the moment, right? Can we get rid of ops->init ?
> > >> 
> > >> No this is required for N1SDP board. Please check below patch.
> > >> https://gitlab.com/rahsingh/xen-integration/-/commit/6379ba5764df33d57547087cff4ffc078dc515d5
> > > 
> > > OK
> > > 
> > > 
> > >>>> +int pci_host_common_probe(struct dt_device_node *dev, const void 
> > >>>> *data)
> > >>>> +{
> > >>>> +    struct pci_host_bridge *bridge;
> > >>>> +    struct pci_config_window *cfg;
> > >>>> +    struct pci_ecam_ops *ops;
> > >>>> +    const struct dt_device_match *of_id;
> > >>>> +    int err;
> > >>>> +
> > >>>> +    if ( dt_device_for_passthrough(dev) )
> > >>>> +        return 0;
> > >>>> +
> > >>>> +    of_id = dt_match_node(dev->dev.of_match_table, dev->dev.of_node);
> > >>>> +    ops = (struct pci_ecam_ops *) of_id->data;
> > >>> 
> > >>> Do we really need dt_match_node and dev->dev.of_match_table to get
> > >>> dt_device_match.data?
> > >>> 
> > >> 
> > >>> data is passed as a parameter to pci_host_common_probe, isn't it enough
> > >>> to do:
> > >>> 
> > >>> ops = (struct pci_ecam_ops *) data;
> > >> 
> > >> As of now not required but in future we might need it if we implement 
> > >> other ecam supported bridge
> > >> 
> > >> static const struct dt_device_match gen_pci_dt_match[] = {               
> > >>        
> > >>    { .compatible = "pci-host-ecam-generic",                              
> > >>       
> > >>      .data =       &pci_generic_ecam_ops },
> > >> 
> > >>    { .compatible = "pci-host-cam-generic",
> > >>      .data = &gen_pci_cfg_cam_bus_ops },                                 
> > >> 
> > >>    { },                                                                  
> > >>       
> > >> };
> > > 
> > > Even if we add another ECAM-supported bridge, the following:
> > > 
> > > ops = (struct pci_ecam_ops *) data;
> > > 
> > > could still work, right? The probe function will directly receive as
> > > parameter the .data pointer. You shouldn't need the indirection via
> > > dt_match_node?
> > 
> > As per my understanding probe function will not get .data pointer.Probe 
> > data argument is NULL in most of the cases in XEN
> > Please have a look once dt_pci_init() -> device_init(..) call flow 
> > implementation.
> 
> You are right. Looking at the code, nobody is currently using
> dt_device_match.data and it is clear why: it is not passed to the
> device_desc.init function at all. As it is today, it is basically
> useless.
> 
> And there is only one case where device_init has a non-NULL data
> parameter and it is in xen/drivers/char/arm-uart.c. All the others are
> not even using the data parameter of device_init.
> 
> I think we need to change device_init so that dt_device_match.data can
> be useful. Sorry for the scope-creep but I think we should do the
> following:
> 
> - do not add of_match_table to struct device
> 
> - add one more parameter to device_desc.init:
>   int (*init)(struct dt_device_node *dev, struct device_desc *desc, const 
> void *data);
> 
> - change device_init to call desc->init with the right parameters:
>   desc->init(dev, desc, data);
> 
> This way pci_host_common_probe is just going to get a desc directly as
> parameter. I think it would make a lot more sense from an interface
> perspective. It does require a change in all the DT_DEVICE_START.init
> functions adding a struct device_desc *desc parameter, but it should be
> a mechanical change.
> 
> Alternatively we could just change device_init to pass
> device_desc.dt_match.data when the data parameter is NULL but it feels
> like a hack.
> 
> 
> What do you think?
 

Another idea that doesn't require a device_desc.init change and also
doesn't require a change to struct device is the following:


diff --git a/xen/arch/arm/pci/pci-host-common.c 
b/xen/arch/arm/pci/pci-host-common.c
index a88f20175e..1aa0ef4c1e 100644
--- a/xen/arch/arm/pci/pci-host-common.c
+++ b/xen/arch/arm/pci/pci-host-common.c
@@ -205,8 +205,7 @@ int pci_host_common_probe(struct dt_device_node *dev, const 
void *data)
     if ( dt_device_for_passthrough(dev) )
         return 0;
 
-    of_id = dt_match_node(dev->dev.of_match_table, dev->dev.of_node);
-    ops = (struct pci_ecam_ops *) of_id->data;
+    ops = (struct pci_ecam_ops *) data;
 
     bridge = pci_alloc_host_bridge();
     if ( !bridge )
diff --git a/xen/arch/arm/pci/pci-host-generic.c 
b/xen/arch/arm/pci/pci-host-generic.c
index 6b3288d6f3..66fb843f49 100644
--- a/xen/arch/arm/pci/pci-host-generic.c
+++ b/xen/arch/arm/pci/pci-host-generic.c
@@ -20,15 +20,19 @@
 #include <asm/pci.h>
 
 static const struct dt_device_match gen_pci_dt_match[] = {
-    { .compatible = "pci-host-ecam-generic",
-      .data =       &pci_generic_ecam_ops },
-
+    { .compatible = "pci-host-ecam-generic" },
     { },
 };
 
+static int pci_host_generic_probe(struct dt_device_node *dev,
+                                         const void *data)
+{
+    return pci_host_common_probe(dev, &pci_generic_ecam_ops);
+}
+
 DT_DEVICE_START(pci_gen, "PCI HOST GENERIC", DEVICE_PCI)
 .dt_match = gen_pci_dt_match,
-.init = pci_host_common_probe,
+.init = pci_host_generic_probe,
 DT_DEVICE_END
 
 /*