Re: [PATCH v8 01/17] Xen: ACPI: Hide UART used by Xen

[Stefano Stabellini]

On Fri, 25 Mar 2016, Rafael J. Wysocki wrote:
> On Friday, March 25, 2016 04:05:49 PM Shannon Zhao wrote:
> > From: Shannon Zhao <shannon.zhao@xxxxxxxxxx>
> > 
> > ACPI 6.0 introduces a new table STAO to list the devices which are used
> > by Xen and can't be used by Dom0. On Xen virtual platforms, the physical
> > UART is used by Xen. So here it hides UART from Dom0.
> > 
> > CC: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx> (supporter:ACPI)
> > CC: Len Brown <lenb@xxxxxxxxxx> (supporter:ACPI)
> > CC: linux-acpi@xxxxxxxxxxxxxxx (open list:ACPI)
> > Signed-off-by: Shannon Zhao <shannon.zhao@xxxxxxxxxx>
> 
> So I said it looked good, but now that I think about it, I have a question. ->
> 
> > ---
> >  drivers/acpi/scan.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 68 insertions(+)
> > 
> > diff --git a/drivers/acpi/scan.c b/drivers/acpi/scan.c
> > index 5f28cf7..5420cc5 100644
> > --- a/drivers/acpi/scan.c
> > +++ b/drivers/acpi/scan.c
> > @@ -45,6 +45,7 @@ static LIST_HEAD(acpi_scan_handlers_list);
> >  DEFINE_MUTEX(acpi_device_lock);
> >  LIST_HEAD(acpi_wakeup_device_list);
> >  static DEFINE_MUTEX(acpi_hp_context_lock);
> > +static u64 spcr_uart_addr;
> >  
> >  struct acpi_dep_data {
> >  	struct list_head node;
> > @@ -1453,6 +1454,41 @@ static int acpi_add_single_object(struct acpi_device **child,
> >  	return 0;
> >  }
> >  
> > +static acpi_status acpi_get_resource_memory(struct acpi_resource *ares,
> > +					    void *context)
> > +{
> > +	struct resource *res = context;
> > +
> > +	if (acpi_dev_resource_memory(ares, res))
> > +		return AE_CTRL_TERMINATE;
> 
> -> It looks like this will terminate on the first memory resource found,
> but what if there are more of them?
> 
> Or is it guaranteed that there will be only one for the device objects in
> question?

Given that spcr_uart_addr is actually the only Base Address specified by
the SPCR table, I think it is assumed that this kind of devices only
have one memory region.


> If not, then it would better to check res.start == spcr_uart_addr here too
> and only terminate if there's a match.
>
> > +
> > +	return AE_OK;
> > +}
> > +
> > +static bool acpi_device_should_be_hidden(acpi_handle handle)
> > +{
> > +	acpi_status status;
> > +	struct resource res;
> > +
> > +	/* Check if it should ignore the UART device */
> > +	if (spcr_uart_addr != 0) {
> > +		if (!acpi_has_method(handle, METHOD_NAME__CRS))
> > +			return false;
> > +
> > +		status = acpi_walk_resources(handle, METHOD_NAME__CRS,
> > +					     acpi_get_resource_memory, &res);
> > +		if (ACPI_FAILURE(status))
> > +			return false;
> > +
> > +		if (res.start == spcr_uart_addr) {
> > +			printk(KERN_INFO PREFIX "The UART device in SPCR table will be hidden\n");
> > +			return true;
> > +		}
> > +	}
> > +
> > +	return false;
> > +}
> > +
> >  static int acpi_bus_type_and_status(acpi_handle handle, int *type,
> >  				    unsigned long long *sta)
> >  {
> > @@ -1466,6 +1502,9 @@ static int acpi_bus_type_and_status(acpi_handle handle, int *type,
> >  	switch (acpi_type) {
> >  	case ACPI_TYPE_ANY:		/* for ACPI_ROOT_OBJECT */
> >  	case ACPI_TYPE_DEVICE:
> > +		if (acpi_device_should_be_hidden(handle))
> > +			return -ENODEV;
> > +
> >  		*type = ACPI_BUS_TYPE_DEVICE;
> >  		status = acpi_bus_get_status_handle(handle, sta);
> >  		if (ACPI_FAILURE(status))
> > @@ -1916,9 +1955,24 @@ static int acpi_bus_scan_fixed(void)
> >  	return result < 0 ? result : 0;
> >  }
> >  
> > +static void __init acpi_get_spcr_uart_addr(void)
> > +{
> > +	acpi_status status;
> > +	struct acpi_table_spcr *spcr_ptr;
> > +
> > +	status = acpi_get_table(ACPI_SIG_SPCR, 0,
> > +				(struct acpi_table_header **)&spcr_ptr);
> > +	if (ACPI_SUCCESS(status))
> > +		spcr_uart_addr = spcr_ptr->serial_port.address;
> > +	else
> > +		printk(KERN_WARNING PREFIX "STAO table present, but SPCR is missing\n");
> > +}
> > +
> >  int __init acpi_scan_init(void)
> >  {
> >  	int result;
> > +	acpi_status status;
> > +	struct acpi_table_stao *stao_ptr;
> >  
> >  	acpi_pci_root_init();
> >  	acpi_pci_link_init();
> > @@ -1934,6 +1988,20 @@ int __init acpi_scan_init(void)
> >  
> >  	acpi_scan_add_handler(&generic_device_handler);
> >  
> > +	/*
> > +	 * If there is STAO table, check whether it needs to ignore the UART
> > +	 * device in SPCR table.
> > +	 */
> > +	status = acpi_get_table(ACPI_SIG_STAO, 0,
> > +				(struct acpi_table_header **)&stao_ptr);
> > +	if (ACPI_SUCCESS(status)) {
> > +		if (stao_ptr->header.length > sizeof(struct acpi_table_stao))
> > +			printk(KERN_INFO PREFIX "STAO Name List not yet supported.");
> > +
> > +		if (stao_ptr->ignore_uart)
> > +			acpi_get_spcr_uart_addr();
> > +	}
> > +
> >  	mutex_lock(&acpi_scan_lock);
> >  	/*
> >  	 * Enumerate devices in the ACPI namespace.
> > 
>