pstore ramoops读写分析

pstore读写接口分别是ramoops_pstore_read()和ramoops_pstore_write_buf()，参考代码kernel4.9:

static struct ramoops_context oops_cxt = {
	.pstore = {
		.owner	= THIS_MODULE,
		.name	= "ramoops",
		.open	= ramoops_pstore_open,
		.read	= ramoops_pstore_read,
		.write_buf	= ramoops_pstore_write_buf,
		.write_buf_user	= ramoops_pstore_write_buf_user, 
		.erase	= ramoops_pstore_erase,
	},
};

我们先看write，pstore是写了后才有读出。

ramoops_pstore_write_buf会call persistent_ram_write:

static int notrace ramoops_pstore_write_buf(enum pstore_type_id type,
					    enum kmsg_dump_reason reason,
					    u64 *id, unsigned int part,
					    const char *buf,
					    bool compressed, size_t size,
					    struct pstore_info *psi)
{
	struct ramoops_context *cxt = psi->data;
	struct persistent_ram_zone *prz;
	size_t hlen;

	if (type == PSTORE_TYPE_CONSOLE) {
		if (!cxt->cprz)
			return -ENOMEM;
		persistent_ram_write(cxt->cprz, buf, size);
		return 0;

backend ramoops write:

int notrace persistent_ram_write(struct persistent_ram_zone *prz,
	const void *s, unsigned int count)
{
	int rem;
	int c = count;
	size_t start;

	if (unlikely(c > prz->buffer_size)) {
		s += c - prz->buffer_size;
		c = prz->buffer_size;
	}

	buffer_size_add(prz, c);

	start = buffer_start_add(prz, c);

	rem = prz->buffer_size - start;
	if (unlikely(rem < c)) {
		persistent_ram_update(prz, s, start, rem);
		s += rem;
		c -= rem;
		start = 0;
	}
	persistent_ram_update(prz, s, start, c);

	persistent_ram_update_header_ecc(prz);

	return count;
}

看样子是在确定buffer大小和地址后call persistent_ram_update来写buffer，我们先看下buffer_size_add:

/* increase the size counter until it hits the max size */
static void buffer_size_add(struct persistent_ram_zone *prz, size_t a)
{
	size_t old;
	size_t new;
	unsigned long flags = 0;

	if (!(prz->flags & PRZ_FLAG_NO_LOCK))
		raw_spin_lock_irqsave(&prz->buffer_lock, flags);

	old = atomic_read(&prz->buffer->size);
	if (old == prz->buffer_size)
		goto exit;

	new = old + a;
	if (new > prz->buffer_size)
		new = prz->buffer_size;
	atomic_set(&prz->buffer->size, new); //设置prz->buffer->size

再看下buffer_start_add:

/* increase and wrap the start pointer, returning the old value */
static size_t buffer_start_add(struct persistent_ram_zone *prz, size_t a)
{
	int old;
	int new;
	unsigned long flags = 0;

	if (!(prz->flags & PRZ_FLAG_NO_LOCK))
		raw_spin_lock_irqsave(&prz->buffer_lock, flags);

	old = atomic_read(&prz->buffer->start);
	new = old + a;
	while (unlikely(new >= prz->buffer_size))
		new -= prz->buffer_size;
	atomic_set(&prz->buffer->start, new); //设置prz->buffer->start
	...
	return old;

这里就有几个疑问了:

1. prz->buffer的->size和->start初始值如何定义的？
2. What is prz->buffer->start, prz->buffer->start, prz->buffer_size？

问题1：prz init时会call persistent_ram_zap初始化为0:

void persistent_ram_zap(struct persistent_ram_zone *prz)
{
	atomic_set(&prz->buffer->start, 0);
	atomic_set(&prz->buffer->size, 0);

问题2：ram在分配时会设置prz->buffer_size:

static int persistent_ram_buffer_map(phys_addr_t start, phys_addr_t size,
		struct persistent_ram_zone *prz, int memtype)
{
	prz->paddr = start;
	prz->size = size;

	if (pfn_valid(start >> PAGE_SHIFT))
		prz->vaddr = persistent_ram_vmap(start, size, memtype);
	else
		prz->vaddr = persistent_ram_iomap(start, size, memtype);

	if (!prz->vaddr) {
		pr_err("%s: Failed to map 0x%llx pages at 0x%llx\n", __func__,
			(unsigned long long)size, (unsigned long long)start);
		return -ENOMEM;
	}

	prz->buffer = prz->vaddr + offset_in_page(start);
	prz->buffer_size = size - sizeof(struct persistent_ram_buffer); //tj: 这里设置

这里size就是每个zone配置的大小:

err = ramoops_init_prz(dev, cxt, &cxt->cprz, &paddr,
		       cxt->console_size, 0);

->buffer偏移了start，所以应该是zone的开头保留了一个persistent_ram_buffer大小：

struct persistent_ram_buffer {
	uint32_t    sig;
	atomic_t    start;
	atomic_t    size;
	uint8_t     data[0];
};

这个data[0]后面才是如console message等。回过头来再看persistent_ram_write就通了。

再来看read，read是在pstore文件系统准备好就开始：

ramoops_probe -> pstore_register -> pstore_get_records (if pstore is mounted) -> (psi->read)

static ssize_t ramoops_pstore_read(u64 *id, enum pstore_type_id *type,
				   int *count, struct timespec *time,
				   char **buf, bool *compressed,
				   ssize_t *ecc_notice_size,
				   struct pstore_info *psi)
{
	...
	/* Find the next valid persistent_ram_zone for DMESG */
	while (cxt->dump_read_cnt < cxt->max_dump_cnt && !prz) {
		prz = ramoops_get_next_prz(cxt->przs, &cxt->dump_read_cnt,
					   cxt->max_dump_cnt, id, type,
					   PSTORE_TYPE_DMESG, 1);
		if (!prz_ok(prz))
			continue;
		header_length = ramoops_read_kmsg_hdr(persistent_ram_old(prz),
						      time, compressed);
		/* Clear and skip this DMESG record if it has no valid header */
		if (!header_length) {
			persistent_ram_free_old(prz);
			persistent_ram_zap(prz);
			prz = NULL;
		}
	}

	if (!prz_ok(prz))
		prz = ramoops_get_next_prz(&cxt->cprz, &cxt->console_read_cnt,
					   1, id, type, PSTORE_TYPE_CONSOLE, 0);
	if (!prz_ok(prz))
		prz = ramoops_get_next_prz(&cxt->fprz, &cxt->ftrace_read_cnt,
					   1, id, type, PSTORE_TYPE_FTRACE, 0);
	if (!prz_ok(prz))
		prz = ramoops_get_next_prz(&cxt->mprz, &cxt->pmsg_read_cnt,
					   1, id, type, PSTORE_TYPE_PMSG, 0);
	if (!prz_ok(prz))
		return 0; //tj: 一个也没有找到

扫描整个pstore ram区域，通过call ramoops_get_next_prz来找到一个有效的persist ram zone。

看下ramoops_get_next_pr:

static struct persistent_ram_zone *
ramoops_get_next_prz(struct persistent_ram_zone *przs[], uint *c, uint max,
		     u64 *id,
		     enum pstore_type_id *typep, enum pstore_type_id type,
		     bool update)
{
	struct persistent_ram_zone *prz;
	int i = (*c)++;

	if (i >= max)
		return NULL;

	prz = przs[i];
	if (!prz)
		return NULL;

	/* Update old/shadowed buffer. */
	if (update)
		persistent_ram_save_old(prz);

	if (!persistent_ram_old_size(prz))
		return NULL;

	*typep = type;
	*id = i;

	return prz;
}

console没有update，看下persistent_ram_old_size:

size_t persistent_ram_old_size(struct persistent_ram_zone *prz)
{
	return prz->old_log_size;
}

old_log_size要非0才能找到，在persistent_ram_save_old会保存：

void persistent_ram_save_old(struct persistent_ram_zone *prz)
{
	struct persistent_ram_buffer *buffer = prz->buffer;
	size_t size = buffer_size(prz);
	size_t start = buffer_start(prz);

	if (!size)
		return; //tj: 只有写过这里才会pass

	if (!prz->old_log) {
		persistent_ram_ecc_old(prz);
		prz->old_log = kmalloc(size, GFP_KERNEL);
	}
	if (!prz->old_log) {
		pr_err("failed to allocate buffer\n");
		return;
	}

	prz->old_log_size = size; //tj: 这里保存
	memcpy_fromio(prz->old_log, &buffer->data[start], size - start);
	memcpy_fromio(prz->old_log + size - start, &buffer->data[0], start);
}

可见，这里old_log_size就是由buffer_size而来。console在下面保留的：

static int persistent_ram_post_init(struct persistent_ram_zone *prz, u32 sig,
				    struct persistent_ram_ecc_info *ecc_info)
{
	int ret;

	ret = persistent_ram_init_ecc(prz, ecc_info);
	if (ret)
		return ret;

	sig ^= PERSISTENT_RAM_SIG;

	if (prz->buffer->sig == sig) {
		if (buffer_size(prz) > prz->buffer_size ||
		    buffer_start(prz) > buffer_size(prz))
			pr_info("found existing invalid buffer, size %zu, start %zu\n",
				buffer_size(prz), buffer_start(prz));
		else {
			pr_debug("found existing buffer, size %zu, start %zu\n",
				 buffer_size(prz), buffer_start(prz));
			persistent_ram_save_old(prz); //tj: 找到就保存
			return 0;

有些时候，我们需要在kernel起来查看bootloader log，这里提供linux修改部分(基于4.9)参考，我本地验证ok：