linux-kernel - Re: [PATCH] dma-debug: add dumping facility via debugfs

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Message-ID: <2f43688f-9c08-a9c4-6175-caa3d2e40261@arm.com>
Date:   Wed, 16 Jan 2019 18:10:13 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Corentin Labbe <clabbe@...libre.com>, hch@....de,
        m.szyprowski@...sung.com
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-debug: add dumping facility via debugfs

On 16/01/2019 13:44, Corentin Labbe wrote:
> While debugging a DMA mapping leak, I needed to access
> debug_dma_dump_mappings() but easily from user space.
> 
> This patch adds a /sys/kernel/debug/dma-api/dump file which contain all
> current DMA mapping.
> 
> Signed-off-by: Corentin Labbe <clabbe@...libre.com>
> ---
>   kernel/dma/debug.c | 47 ++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 47 insertions(+)
> 
> diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
> index 23cf5361bcf1..9253382f5729 100644
> --- a/kernel/dma/debug.c
> +++ b/kernel/dma/debug.c
> @@ -144,6 +144,7 @@ static struct dentry *num_free_entries_dent __read_mostly;
>   static struct dentry *min_free_entries_dent __read_mostly;
>   static struct dentry *nr_total_entries_dent __read_mostly;
>   static struct dentry *filter_dent           __read_mostly;
> +static struct dentry *dump_dent             __read_mostly;
>   
>   /* per-driver filter related state */
>   
> @@ -840,6 +841,47 @@ static const struct file_operations filter_fops = {
>   	.llseek = default_llseek,
>   };
>   
> +static int dump_read(struct seq_file *seq, void *v)
> +{
> +	int idx;
> +
> +	for (idx = 0; idx < HASH_SIZE; idx++) {
> +		struct hash_bucket *bucket = &dma_entry_hash[idx];
> +		struct dma_debug_entry *entry;
> +		unsigned long flags;
> +
> +		spin_lock_irqsave(&bucket->lock, flags);
> +
> +		list_for_each_entry(entry, &bucket->list, list) {
> +			seq_printf(seq,
> +				   "%s %s %s idx %d P=%llx N=%lx D=%llx L=%llx %s %s\n",
> +				   dev_name(entry->dev),
> +				   dev_driver_string(entry->dev),
> +				   type2name[entry->type], idx,
> +				   phys_addr(entry), entry->pfn,
> +				   entry->dev_addr, entry->size,
> +				   dir2name[entry->direction],
> +				   maperr2str[entry->map_err_type]);
> +		}
> +
> +		spin_unlock_irqrestore(&bucket->lock, flags);
> +	}
> +	return 0;
> +}

It's a shame that this is pretty much a duplication of 
debug_dma_dump_mappings(), but there seems no straightforward way to 
define one in terms of the other :/

(although given that we'd rather not have that weird external interface 
anyway, maybe "now you can use debugfs instead" might be justification 
enough for cleaning it up...)

> +
> +static int dump_open(struct inode *inode, struct file *file)
> +{
> +	return single_open(file, dump_read, inode->i_private);
> +}
> +
> +static const struct file_operations dump_fops = {
> +	.owner = THIS_MODULE,
> +	.open = dump_open,
> +	.read = seq_read,
> +	.llseek = seq_lseek,
> +	.release = single_release,
> +};
> +

DEFINE_SHOW_ATTRIBUTE()?

Robin.

>   static int dma_debug_fs_init(void)
>   {
>   	dma_debug_dent = debugfs_create_dir("dma-api", NULL);
> @@ -894,6 +936,11 @@ static int dma_debug_fs_init(void)
>   	if (!filter_dent)
>   		goto out_err;
>   
> +	dump_dent = debugfs_create_file("dump", 0444,
> +					dma_debug_dent, NULL, &dump_fops);
> +	if (!dump_dent)
> +		goto out_err;
> +
>   	return 0;
>   
>   out_err:
>