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Subject[PATCH 3/3] firmware: google: Probe for a GSMI handler in firmware
Date
From: Arthur Heymans <arthur@aheymans.xyz>

Currently this driver is loaded if the DMI string matches coreboot
and has a proper smi_command in the ACPI FADT table, but a GSMI handler in
SMM is an optional feature in coreboot.

So probe for a SMM GSMI handler before initializing the driver.
If the smihandler leaves the calling argument in %eax in the SMM save state
untouched that generally means the is no handler for GSMI.

Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
---
drivers/firmware/google/gsmi.c | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)

diff --git a/drivers/firmware/google/gsmi.c b/drivers/firmware/google/gsmi.c
index 974c769b75cf..a3eaa9f41125 100644
--- a/drivers/firmware/google/gsmi.c
+++ b/drivers/firmware/google/gsmi.c
@@ -746,6 +746,7 @@ MODULE_DEVICE_TABLE(dmi, gsmi_dmi_table);
static __init int gsmi_system_valid(void)
{
u32 hash;
+ u16 cmd, result;

if (!dmi_check_system(gsmi_dmi_table))
return -ENODEV;
@@ -780,6 +781,23 @@ static __init int gsmi_system_valid(void)
return -ENODEV;
}

+ /* Test the smihandler with a bogus command. If it leaves the
+ * calling argument in %ax untouched, there is no handler for
+ * GSMI commands.
+ */
+ cmd = GSMI_CALLBACK | 0xff << 8;
+ asm volatile (
+ "outb %%al, %%dx\n\t"
+ : "=a" (result)
+ : "0" (cmd),
+ "d" (acpi_gbl_FADT.smi_command)
+ : "memory", "cc"
+ );
+ if (cmd == result) {
+ pr_info("gsmi: no gsmi handler in firmware\n");
+ return -ENODEV;
+ }
+
/* Found */
return 0;
}
--
2.21.0
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