Sorry for the late reply, I actually wrote this email over a week ago,
but never hit send. And the email was pushed back behind other
windows. :-/


On Fri,  3 Aug 2018 19:58:42 +0530
Sai Prakash Ranjan [off-list ref] wrote:

No need for the initialization, you could just have:

static struct rtb_state rtb;

And it will be initialized to all zeros. Or did you do that to document
that it is not enabled at boot?

+
+static int rtb_panic_notifier(struct notifier_block *this,
+					unsigned long event, void *ptr)
+{
+	rtb.enabled = 0;
+	return NOTIFY_DONE;
+}
+
+static struct notifier_block rtb_panic_blk = {
+	.notifier_call  = rtb_panic_notifier,
+	.priority = INT_MAX,
+};
+
+static void rtb_write_type(const char *log_type,
+			struct rtb_layout *start)
+{
+	start->log_type = log_type;
+}
+
+static void rtb_write_caller(u64 caller, struct rtb_layout *start)
+{
+	start->caller = caller;
+}
+
+static void rtb_write_data(u64 data, struct rtb_layout *start)
+{
+	start->data = data;
+}
+
+static void rtb_write_timestamp(struct rtb_layout *start)
+{
+	start->timestamp = sched_clock();
+}

Why have the above static functions? They are not very helpful, and
appear to be actually confusing. They are used once.

+
+static void uncached_logk_pc_idx(const char *log_type, u64 caller,
+				u64 data, int idx)
+{
+	struct rtb_layout *start;
+
+	start = &rtb.rtb[idx & (rtb.nentries - 1)];
+
+	rtb_write_type(log_type, start);
+	rtb_write_caller(caller, start);
+	rtb_write_data(data, start);
+	rtb_write_timestamp(start);

How is the above better than:

	start->log_type = log_type;
	start->caller = caller;
	start->data = data;
 	start->timestamp = sched_clock();

??

+	/* Make sure data is written */
+	mb();
+}
+
+static int rtb_get_idx(void)
+{
+	int i, offset;
+
+	i = atomic_inc_return(&rtb_idx);
+	i--;
+
+	/* Check if index has wrapped around */
+	offset = (i & (rtb.nentries - 1)) -
+		 ((i - 1) & (rtb.nentries - 1));
+	if (offset < 0) {
+		i = atomic_inc_return(&rtb_idx);
+		i--;
+	}
+
+	return i;
+}
+
+noinline void notrace uncached_logk(const char *log_type, void *data)

BTW, all files in this directory have their functions notrace by
default.

-- Steve

+{
+	int i;
+
+	if (!rtb.enabled)
+		return;
+
+	i = rtb_get_idx();
+	uncached_logk_pc_idx(log_type, (u64)(__builtin_return_address(0)),
+				(u64)(data), i);
+}
+EXPORT_SYMBOL(uncached_logk);
+
+int rtb_init(void)
+{
+	struct device_node *np;
+	u32 size;
+	int ret;
+
+	np = of_find_node_by_name(NULL, "ramoops");
+	if (!np)
+		return -ENODEV;
+
+	ret = of_property_read_u32(np, "rtb-size", &size);
+	if (ret) {
+		of_node_put(np);
+		return ret;
+	}
+
+	rtb.size = size;
+
+	/* Create a dummy platform device to use dma api */
+	rtb_dev = platform_device_register_simple("rtb", -1, NULL, 0);
+	if (IS_ERR(rtb_dev))
+		return PTR_ERR(rtb_dev);
+
+	/*
+	 * The device is a dummy, so arch_setup_dma_ops
+	 * is not called, thus leaving the device with dummy DMA ops
+	 * which returns null in case of arm64.
+	 */
+	of_dma_configure(&rtb_dev->dev, NULL, true);
+	rtb.rtb = dma_alloc_coherent(&rtb_dev->dev, rtb.size,
+					&rtb.phys, GFP_KERNEL);
+	if (!rtb.rtb)
+		return -ENOMEM;
+
+	rtb.nentries = rtb.size / sizeof(struct rtb_layout);
+	/* Round this down to a power of 2 */
+	rtb.nentries = __rounddown_pow_of_two(rtb.nentries);
+
+	memset(rtb.rtb, 0, rtb.size);
+	atomic_set(&rtb_idx, 0);
+
+	atomic_notifier_chain_register(&panic_notifier_list,
+						&rtb_panic_blk);
+	rtb.enabled = 1;
+	return 0;
+}
+
+void rtb_exit(void)
+{
+	dma_free_coherent(&rtb_dev->dev, rtb.size, rtb.rtb, rtb.phys);
+	platform_device_unregister(rtb_dev);
+}