224 lines
8.0 KiB
Plaintext
224 lines
8.0 KiB
Plaintext
_DSD Device Properties Related to GPIO
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--------------------------------------
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With the release of ACPI 5.1, the _DSD configuration object finally
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allows names to be given to GPIOs (and other things as well) returned
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by _CRS. Previously, we were only able to use an integer index to find
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the corresponding GPIO, which is pretty error prone (it depends on
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the _CRS output ordering, for example).
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With _DSD we can now query GPIOs using a name instead of an integer
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index, like the ASL example below shows:
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// Bluetooth device with reset and shutdown GPIOs
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Device (BTH)
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{
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Name (_HID, ...)
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Name (_CRS, ResourceTemplate ()
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{
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GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionInputOnly,
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"\\_SB.GPO0", 0, ResourceConsumer) {15}
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GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionInputOnly,
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"\\_SB.GPO0", 0, ResourceConsumer) {27, 31}
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})
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Name (_DSD, Package ()
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{
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ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
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Package ()
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{
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Package () {"reset-gpios", Package() {^BTH, 1, 1, 0 }},
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Package () {"shutdown-gpios", Package() {^BTH, 0, 0, 0 }},
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}
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})
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}
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The format of the supported GPIO property is:
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Package () { "name", Package () { ref, index, pin, active_low }}
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ref - The device that has _CRS containing GpioIo()/GpioInt() resources,
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typically this is the device itself (BTH in our case).
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index - Index of the GpioIo()/GpioInt() resource in _CRS starting from zero.
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pin - Pin in the GpioIo()/GpioInt() resource. Typically this is zero.
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active_low - If 1 the GPIO is marked as active_low.
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Since ACPI GpioIo() resource does not have a field saying whether it is
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active low or high, the "active_low" argument can be used here. Setting
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it to 1 marks the GPIO as active low.
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In our Bluetooth example the "reset-gpios" refers to the second GpioIo()
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resource, second pin in that resource with the GPIO number of 31.
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It is possible to leave holes in the array of GPIOs. This is useful in
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cases like with SPI host controllers where some chip selects may be
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implemented as GPIOs and some as native signals. For example a SPI host
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controller can have chip selects 0 and 2 implemented as GPIOs and 1 as
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native:
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Package () {
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"cs-gpios",
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Package () {
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^GPIO, 19, 0, 0, // chip select 0: GPIO
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0, // chip select 1: native signal
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^GPIO, 20, 0, 0, // chip select 2: GPIO
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}
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}
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Other supported properties
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--------------------------
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Following Device Tree compatible device properties are also supported by
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_DSD device properties for GPIO controllers:
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- gpio-hog
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- output-high
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- output-low
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- input
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- line-name
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Example:
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Name (_DSD, Package () {
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// _DSD Hierarchical Properties Extension UUID
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ToUUID("dbb8e3e6-5886-4ba6-8795-1319f52a966b"),
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Package () {
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Package () {"hog-gpio8", "G8PU"}
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}
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})
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Name (G8PU, Package () {
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ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
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Package () {
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Package () {"gpio-hog", 1},
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Package () {"gpios", Package () {8, 0}},
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Package () {"output-high", 1},
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Package () {"line-name", "gpio8-pullup"},
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}
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})
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- gpio-line-names
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Example:
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Package () {
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"gpio-line-names",
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Package () {
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"SPI0_CS_N", "EXP2_INT", "MUX6_IO", "UART0_RXD", "MUX7_IO",
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"LVL_C_A1", "MUX0_IO", "SPI1_MISO"
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}
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}
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See Documentation/devicetree/bindings/gpio/gpio.txt for more information
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about these properties.
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ACPI GPIO Mappings Provided by Drivers
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--------------------------------------
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There are systems in which the ACPI tables do not contain _DSD but provide _CRS
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with GpioIo()/GpioInt() resources and device drivers still need to work with
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them.
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In those cases ACPI device identification objects, _HID, _CID, _CLS, _SUB, _HRV,
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available to the driver can be used to identify the device and that is supposed
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to be sufficient to determine the meaning and purpose of all of the GPIO lines
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listed by the GpioIo()/GpioInt() resources returned by _CRS. In other words,
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the driver is supposed to know what to use the GpioIo()/GpioInt() resources for
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once it has identified the device. Having done that, it can simply assign names
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to the GPIO lines it is going to use and provide the GPIO subsystem with a
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mapping between those names and the ACPI GPIO resources corresponding to them.
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To do that, the driver needs to define a mapping table as a NULL-terminated
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array of struct acpi_gpio_mapping objects that each contain a name, a pointer
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to an array of line data (struct acpi_gpio_params) objects and the size of that
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array. Each struct acpi_gpio_params object consists of three fields,
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crs_entry_index, line_index, active_low, representing the index of the target
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GpioIo()/GpioInt() resource in _CRS starting from zero, the index of the target
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line in that resource starting from zero, and the active-low flag for that line,
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respectively, in analogy with the _DSD GPIO property format specified above.
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For the example Bluetooth device discussed previously the data structures in
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question would look like this:
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static const struct acpi_gpio_params reset_gpio = { 1, 1, false };
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static const struct acpi_gpio_params shutdown_gpio = { 0, 0, false };
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static const struct acpi_gpio_mapping bluetooth_acpi_gpios[] = {
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{ "reset-gpios", &reset_gpio, 1 },
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{ "shutdown-gpios", &shutdown_gpio, 1 },
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{ },
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};
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Next, the mapping table needs to be passed as the second argument to
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acpi_dev_add_driver_gpios() that will register it with the ACPI device object
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pointed to by its first argument. That should be done in the driver's .probe()
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routine. On removal, the driver should unregister its GPIO mapping table by
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calling acpi_dev_remove_driver_gpios() on the ACPI device object where that
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table was previously registered.
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Using the _CRS fallback
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-----------------------
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If a device does not have _DSD or the driver does not create ACPI GPIO
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mapping, the Linux GPIO framework refuses to return any GPIOs. This is
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because the driver does not know what it actually gets. For example if we
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have a device like below:
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Device (BTH)
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{
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Name (_HID, ...)
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Name (_CRS, ResourceTemplate () {
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GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone,
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"\\_SB.GPO0", 0, ResourceConsumer) {15}
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GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone,
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"\\_SB.GPO0", 0, ResourceConsumer) {27}
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})
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}
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The driver might expect to get the right GPIO when it does:
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desc = gpiod_get(dev, "reset", GPIOD_OUT_LOW);
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but since there is no way to know the mapping between "reset" and
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the GpioIo() in _CRS desc will hold ERR_PTR(-ENOENT).
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The driver author can solve this by passing the mapping explictly
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(the recommended way and documented in the above chapter).
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The ACPI GPIO mapping tables should not contaminate drivers that are not
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knowing about which exact device they are servicing on. It implies that
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the ACPI GPIO mapping tables are hardly linked to ACPI ID and certain
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objects, as listed in the above chapter, of the device in question.
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Getting GPIO descriptor
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-----------------------
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There are two main approaches to get GPIO resource from ACPI:
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desc = gpiod_get(dev, connection_id, flags);
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desc = gpiod_get_index(dev, connection_id, index, flags);
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We may consider two different cases here, i.e. when connection ID is
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provided and otherwise.
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Case 1:
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desc = gpiod_get(dev, "non-null-connection-id", flags);
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desc = gpiod_get_index(dev, "non-null-connection-id", index, flags);
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Case 2:
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desc = gpiod_get(dev, NULL, flags);
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desc = gpiod_get_index(dev, NULL, index, flags);
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Case 1 assumes that corresponding ACPI device description must have
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defined device properties and will prevent to getting any GPIO resources
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otherwise.
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Case 2 explicitly tells GPIO core to look for resources in _CRS.
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Be aware that gpiod_get_index() in cases 1 and 2, assuming that there
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are two versions of ACPI device description provided and no mapping is
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present in the driver, will return different resources. That's why a
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certain driver has to handle them carefully as explained in previous
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chapter.
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