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Subject[PATCH] x86: i8237: Register based on FADT legacy boot flag
Date
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® Core™ 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@linux.intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@intel.com>
---

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..5c7a0dea7e19 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 (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
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