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e179f69141
The legacy PIC may or may not be available and we need a mechanism to detect the existence of the legacy PIC that is applicable for all hardware (both physical as well as virtual) currently supported by Linux. On Hyper-V, when our legacy firmware presented to the guests, emulates the legacy PIC while when our EFI based firmware is presented we do not emulate the PIC. To support Hyper-V EFI firmware, we had to set the legacy_pic to the null_legacy_pic since we had to bypass PIC based calibration in the early boot code. While, on the EFI firmware, we know we don't emulate the legacy PIC, we need a generic mechanism to detect the presence of the legacy PIC that is not based on boot time state - this became apparent when we tried to get kexec to work on Hyper-V EFI firmware. This patch implements the proposal put forth by H. Peter Anvin <hpa@linux.intel.com>: Write a known value to the PIC data port and read it back. If the value read is the value written, we do have the PIC, if not there is no PIC and we can safely set the legacy_pic to null_legacy_pic. Since the read from an unconnected I/O port returns 0xff, we will use ~(1 << PIC_CASCADE_IR) (0xfb: mask all lines except the cascade line) to probe for the existence of the PIC. In version V1 of the patch, I had cleaned up the code based on comments from Peter. In version V2 of the patch, I have addressed additional comments from Peter. In version V3 of the patch, I have addressed Jan's comments (JBeulich@suse.com). In version V4 of the patch, I have addressed additional comments from Peter. Signed-off-by: K. Y. Srinivasan <kys@microsoft.com> Link: http://lkml.kernel.org/r/1397501029-29286-1-git-send-email-kys@microsoft.com Cc: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: H. Peter Anvin <hpa@linux.intel.com> |
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