Problem:

The card detect IRQ does not work with modern BIOS (that want
to use _DSD to provide the card detect GPIO to the driver).

Details:

The mmc core provides the mmc_gpiod_request_cd() API to let host drivers
request the gpio descriptor for the "card detect" pin.
This pin is specified in the ACPI for the SDHC device:

 * Either as a resource using _CRS. This is a method used by legacy BIOS.
   (The driver needs to tell which resource index).

 * Or as a named property ("cd-gpios"/"cd-gpio") in _DSD (which internally
   points to an entry in _CRS). This way, the driver can lookup using a
   string. This is what modern BIOS prefer to use.

This API finally results in a call to the following code:

struct gpio_desc *acpi_find_gpio(..., const char *con_id,...)
{
...
   /* Lookup gpio (using "<con_id>-gpio") in the _DSD */
...
   if (!acpi_can_fallback_to_crs(adev, con_id))
          return ERR_PTR(-ENOENT);
...
   /* Falling back to _CRS is allowed, Lookup gpio in the _CRS */
...
}

Note that this means that if the ACPI has _DSD properties, the kernel
will never use _CRS for the lookup (Because acpi_can_fallback_to_crs()
will always be false for any device hat has _DSD entries).

The SDHCI driver is thus currently broken on a modern BIOS, even if
BIOS provides both _CRS (for index based lookup) and _DSD entries (for
string based lookup). Ironically, none of these will be used for the
lookup currently because:

* Since the con_id is NULL, acpi_find_gpio() does not find a matching
  entry in DSDT. (The _DSDT entry has the property name = "cd-gpios")

* Because ACPI contains DSDT entries, thus acpi_can_fallback_to_crs()
  returns false (because device properties have been populated from
  _DSD), thus the _CRS is never used for the lookup.

Fix:

Try "cd" for lookup in the _DSD before falling back to using NULL so
as to try looking up in the _CRS.

I've tested this patch successfully with both Legacy BIOS (that
provide only _CRS method) as well as modern BIOS (that provide both
_CRS and _DSD). Also the use of "cd" appears to be fairly consistent
across other users of this API (other MMC host controller drivers).

Link: https://lkml.org/lkml/2018/9/25/1113


Signed-off-by: Rajat Jain <rajatja@google.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Fixes: f10e4bf6

 ("gpio: acpi: Even more tighten up ACPI GPIO lookups")
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>