linux-imx

// SPDX-License-Identifier: GPL-2.0

#include <linux/mm.h>

#include <linux/gfp.h>

#include <linux/kernel.h>

​

#include <asm/mce.h>

​

#include "debugfs.h"

​

/*

 * RAS Correctable Errors Collector

 *

 * This is a simple gadget which collects correctable errors and counts their

 * occurrence per physical page address.

 *

 * We've opted for possibly the simplest data structure to collect those - an

 * array of the size of a memory page. It stores 512 u64's with the following

 * structure:

 *

 * [63 ... PFN ... 12 | 11 ... generation ... 10 | 9 ... count ... 0]

 *

 * The generation in the two highest order bits is two bits which are set to 11b

 * on every insertion. During the course of each entry's existence, the

 * generation field gets decremented during spring cleaning to 10b, then 01b and

 * then 00b.

 *

 * This way we're employing the natural numeric ordering to make sure that newly

 * inserted/touched elements have higher 12-bit counts (which we've manufactured)

 * and thus iterating over the array initially won't kick out those elements

 * which were inserted last.

 *

 * Spring cleaning is what we do when we reach a certain number CLEAN_ELEMS of

 * elements entered into the array, during which, we're decaying all elements.

 * If, after decay, an element gets inserted again, its generation is set to 11b

 * to make sure it has higher numerical count than other, older elements and

 * thus emulate an an LRU-like behavior when deleting elements to free up space

 * in the page.

 *

 * When an element reaches it's max count of count_threshold, we try to poison

 * it by assuming that errors triggered count_threshold times in a single page

 * are excessive and that page shouldn't be used anymore. count_threshold is

 * initialized to COUNT_MASK which is the maximum.

 *

 * That error event entry causes cec_add_elem() to return !0 value and thus

 * signal to its callers to log the error.

 *

 * To the question why we've chosen a page and moving elements around with

 * memmove(), it is because it is a very simple structure to handle and max data

 * movement is 4K which on highly optimized modern CPUs is almost unnoticeable.

 * We wanted to avoid the pointer traversal of more complex structures like a

 * linked list or some sort of a balancing search tree.

 *

 * Deleting an element takes O(n) but since it is only a single page, it should

 * be fast enough and it shouldn't happen all too often depending on error

 * patterns.

 */

​

#undef pr_fmt

#define pr_fmt(fmt) "RAS: " fmt

​

/*

 * We use DECAY_BITS bits of PAGE_SHIFT bits for counting decay, i.e., how long

 * elements have stayed in the array without having been accessed again.