[PATCH RFC 1/1] swiotlb: add debugfs to track swiotlb buffer usage

[Dongli Zhang]

The device driver will not be able to do dma operations once swiotlb buffer
is full, either because the driver is using so many IO TLB blocks inflight,
or because there is memory leak issue in device driver. To export the
swiotlb buffer usage via debugfs would help the user estimate the size of
swiotlb buffer to pre-allocate or analyze device driver memory leak issue.

As the swiotlb can be initialized at very early stage when debugfs cannot
register successfully, this patch creates the debugfs entry on demand.

Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
---
 kernel/dma/swiotlb.c | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 045930e..d3c8aa4 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -35,6 +35,9 @@
 #include <linux/scatterlist.h>
 #include <linux/mem_encrypt.h>
 #include <linux/set_memory.h>
+#ifdef CONFIG_DEBUG_FS
+#include <linux/debugfs.h>
+#endif
 
 #include <asm/io.h>
 #include <asm/dma.h>
@@ -73,6 +76,13 @@ static phys_addr_t io_tlb_start, io_tlb_end;
  */
 static unsigned long io_tlb_nslabs;
 
+#ifdef CONFIG_DEBUG_FS
+/*
+ * The number of used IO TLB block
+ */
+static unsigned long io_tlb_used;
+#endif
+
 /*
  * This is a free list describing the number of free entries available from
  * each index
@@ -100,6 +110,41 @@ static DEFINE_SPINLOCK(io_tlb_lock);
 
 static int late_alloc;
 
+#ifdef CONFIG_DEBUG_FS
+
+static struct dentry *d_swiotlb_usage;
+
+static int swiotlb_usage_show(struct seq_file *m, void *v)
+{
+	seq_printf(m, "%lu\n%lu\n", io_tlb_used, io_tlb_nslabs);
+	return 0;
+}
+
+static int swiotlb_usage_open(struct inode *inode, struct file *filp)
+{
+	return single_open(filp, swiotlb_usage_show, NULL);
+}
+
+static const struct file_operations swiotlb_usage_fops = {
+	.open           = swiotlb_usage_open,
+	.read           = seq_read,
+	.llseek         = seq_lseek,
+	.release        = single_release,
+};
+
+void swiotlb_create_debugfs(void)
+{
+	d_swiotlb_usage = debugfs_create_dir("swiotlb", NULL);
+
+	if (!d_swiotlb_usage)
+		return;
+
+	debugfs_create_file("usage", 0600, d_swiotlb_usage,
+			    NULL, &swiotlb_usage_fops);
+}
+
+#endif
+
 static int __init
 setup_io_tlb_npages(char *str)
 {
@@ -449,6 +494,11 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
 		pr_warn_once("%s is active and system is using DMA bounce buffers\n",
 			     sme_active() ? "SME" : "SEV");
 
+#ifdef CONFIG_DEBUG_FS
+	if (unlikely(!d_swiotlb_usage))
+		swiotlb_create_debugfs();
+#endif
+
 	mask = dma_get_seg_boundary(hwdev);
 
 	tbl_dma_addr &= mask;
@@ -528,6 +578,9 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
 		dev_warn(hwdev, "swiotlb buffer is full (sz: %zd bytes)\n", size);
 	return SWIOTLB_MAP_ERROR;
 found:
+#ifdef CONFIG_DEBUG_FS
+	io_tlb_used += nslots;
+#endif
 	spin_unlock_irqrestore(&io_tlb_lock, flags);
 
 	/*
@@ -588,6 +641,10 @@ void swiotlb_tbl_unmap_single(struct device *hwdev, phys_addr_t tlb_addr,
 		 */
 		for (i = index - 1; (OFFSET(i, IO_TLB_SEGSIZE) != IO_TLB_SEGSIZE -1) && io_tlb_list[i]; i--)
 			io_tlb_list[i] = ++count;
+
+#ifdef CONFIG_DEBUG_FS
+		io_tlb_used -= nslots;
+#endif
 	}
 	spin_unlock_irqrestore(&io_tlb_lock, flags);
 }
-- 
2.7.4