[PATCH v2] x86: i8237: Register based on FADT legacy boot flag

[Rajneesh Bhardwaj]

>From Skylake onwards, the platform controller hub (Sunrisepoint PCH) does
not support legacy DMA operations to IO ports 81h-83h, 87h, 89h-8Bh, 8Fh.
Currently this driver registers as syscore ops and its resume function is
called on every resume from S3. On Skylake and Kabylake, this causes a
resume delay of around 100ms due to port IO operations, which is a problem.

This change allows to load the driver only when the platform bios
explicitly supports such devices or has a cut-off date earlier than 2017.

Please refer to chapter 21 of 6th Generation Intel Coreâ Processor
Platform Controller Hub Family: BIOS Specification.

https://www.intel.in/content/www/in/en/embedded/products/skylake/u-mobile/software-and-drivers.html

Cc: Alan Cox <alan@xxxxxxxxxxxxxxx>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Signed-off-by: Anshuman Gupta <anshuman.gupta@xxxxxxxxx>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@xxxxxxxxx>
---
Changes in v2:
 * changed to dma_inb()

This depends on recently introduced dmi_get_bios_year() helper.
https://patchwork.kernel.org/patch/10252151/
 arch/x86/kernel/i8237.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/arch/x86/kernel/i8237.c b/arch/x86/kernel/i8237.c
index 8eeaa81de066..bf5d8104e17b 100644
--- a/arch/x86/kernel/i8237.c
+++ b/arch/x86/kernel/i8237.c
@@ -9,6 +9,7 @@
  * your option) any later version.
  */
 
+#include <linux/dmi.h>
 #include <linux/init.h>
 #include <linux/syscore_ops.h>
 
@@ -49,6 +50,30 @@ static struct syscore_ops i8237_syscore_ops = {
 
 static int __init i8237A_init_ops(void)
 {
+	/*
+	 * From SKL PCH onwards, the port 0x61 bit 4 would stop toggle and
+	 * the legacy DMA device is removed in which the I/O ports (81h-83h,
+	 * 87h, 89h-8Bh, 8Fh)  related to it are removed as well. All
+	 * removed ports must return 0xff for a inb() request.
+	 *
+	 * Note: DMA_PAGE_2 (port 0x81) should not be checked for detecting
+	 * the presence of DMA device since it may be used by BIOS to decode
+	 * LPC traffic for POST codes. Original LPC only decodes one byte of
+	 * port 0x80 but some BIOS may choose to enhance PCH LPC port 0x8x
+	 * decoding.
+	 */
+	if (dma_inb(DMA_PAGE_0) == 0xFF)
+		return -ENODEV;
+
+	/*
+	 * It should be OK to not load this driver as newer SoC may not
+	 * support 8237 DMA or bus mastering from LPC. Platform firmware
+	 * must announce the support for such legacy devices via
+	 * ACPI_FADT_LEGACY_DEVICES field in FADT table.
+	 */
+	if (!x86_platform.legacy.devices.pnpbios && dmi_get_bios_year() >= 2017)
+		return -ENODEV;
+
 	register_syscore_ops(&i8237_syscore_ops);
 	return 0;
 }
-- 
2.7.4