Re: [PATCH v6 08/12] dma-buf: heaps: Add debugfs support

[Thierry Reding <thierry.reding@kernel.org>]

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On Wed, Sep 09, 2026 at 01:04:03PM +0200, Christian König wrote:
> Since AMDs mail servers blocked me from sending this out, here once more from my gmail account.
> 
> Regards,
> Christian.
> 
> On 9/4/26 15:00, Christian König wrote:
> > On 9/4/26 12:44, Thierry Reding wrote:
> >> From: Thierry Reding <treding@nvidia.com>
> >>
> >> Add a callback to struct dma_heap_ops that heap providers can implement
> >> to show information about the state of the heap in debugfs. A top-level
> >> directory named "dma_heap" is created in debugfs and individual files
> >> will be named after the heaps.
> >>
> >> Reviewed-by: Maxime Ripard <mripard@kernel.org>
> >> Signed-off-by: Thierry Reding <treding@nvidia.com>
> > 
> > One minimal nit pick below, with that taken care of Reviewed-by: Christian König <christian.koenig@amd.com>.
> > 
> >> ---
> >> Changes in v5:
> >> - fix failure handling during debugfs root directory creation
> >> - add more cleanup to the newly introduced dma_heap_exit()
> >> ---
> >>  drivers/dma-buf/dma-heap.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++
> >>  include/linux/dma-heap.h   |  2 ++
> >>  2 files changed, 54 insertions(+)
> >>
> >> diff --git a/drivers/dma-buf/dma-heap.c b/drivers/dma-buf/dma-heap.c
> >> index 8a6c2b6e0cc6..65c80dac6500 100644
> >> --- a/drivers/dma-buf/dma-heap.c
> >> +++ b/drivers/dma-buf/dma-heap.c
> >> @@ -7,6 +7,7 @@
> >>   */
> >>  
> >>  #include <linux/cdev.h>
> >> +#include <linux/debugfs.h>
> >>  #include <linux/device.h>
> >>  #include <linux/dma-buf.h>
> >>  #include <linux/dma-heap.h>
> >> @@ -225,6 +226,40 @@ const char *dma_heap_get_name(struct dma_heap *heap)
> >>  }
> >>  EXPORT_SYMBOL_NS_GPL(dma_heap_get_name, "DMA_BUF_HEAP");
> >>  
> >> +#ifdef CONFIG_DEBUG_FS
> >> +static int dma_heap_debug_show(struct seq_file *s, void *unused)
> >> +{
> >> +	struct dma_heap *heap = s->private;
> >> +	int err = 0;
> >> +
> >> +	if (heap->ops && heap->ops->show)
> >> +		err = heap->ops->show(s, heap);
> >> +
> >> +	return err;
> >> +}
> >> +DEFINE_SHOW_ATTRIBUTE(dma_heap_debug);
> >> +
> >> +static struct dentry *dma_heap_debugfs_dir;
> >> +
> >> +static void dma_heap_init_debugfs(void)
> >> +{
> >> +	dma_heap_debugfs_dir = debugfs_create_dir("dma_heap", NULL);
> >> +}
> >> +
> >> +static void dma_heap_exit_debugfs(void)
> >> +{
> >> +	debugfs_remove_recursive(dma_heap_debugfs_dir);
> >> +}
> >> +#else
> >> +static void dma_heap_init_debugfs(void)
> >> +{
> >> +}
> >> +
> >> +static void dma_heap_exit_debugfs(void)
> >> +{
> >> +}
> >> +#endif
> >> +
> >>  /**
> >>   * dma_heap_add - adds a heap to dmabuf heaps
> >>   * @exp_info: information needed to register this heap
> >> @@ -299,6 +334,13 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
> >>  
> >>  	/* Add heap to the list */
> >>  	list_add(&heap->list, &heap_list);
> >> +
> >> +#ifdef CONFIG_DEBUG_FS
> >> +	if (heap->ops && heap->ops->show)
> >> +		debugfs_create_file(heap->name, 0444, dma_heap_debugfs_dir,
> >> +				    heap, &dma_heap_debug_fops);
> >> +#endif
> >> +
> >>  	mutex_unlock(&heap_list_lock);
> >>  
> >>  	return heap;
> >> @@ -335,6 +377,16 @@ static int dma_heap_init(void)
> >>  	}
> >>  	dma_heap_class->devnode = dma_heap_devnode;
> >>  
> >> +	dma_heap_init_debugfs();
> >> +
> >>  	return 0;
> >>  }
> >>  subsys_initcall(dma_heap_init);
> >> +
> >> +static void __exit dma_heap_exit(void)
> >> +{
> >> +	dma_heap_exit_debugfs();
> >> +	class_destroy(dma_heap_class);
> >> +	unregister_chrdev_region(dma_heap_devt, NUM_HEAP_MINORS);
> >> +}
> >> +__exitcall(dma_heap_exit);
> >> diff --git a/include/linux/dma-heap.h b/include/linux/dma-heap.h
> >> index 648328a64b27..1c9bed1f4dde 100644
> >> --- a/include/linux/dma-heap.h
> >> +++ b/include/linux/dma-heap.h
> >> @@ -12,6 +12,7 @@
> >>  #include <linux/types.h>
> >>  
> >>  struct dma_heap;
> >> +struct seq_file;
> >>  
> >>  /**
> >>   * struct dma_heap_ops - ops to operate on a given heap
> >> @@ -24,6 +25,7 @@ struct dma_heap_ops {
> >>  				    unsigned long len,
> >>  				    u32 fd_flags,
> >>  				    u64 heap_flags);
> >> +	int (*show)(struct seq_file *s, struct dma_heap *heap);
> > 
> > I found it good coding practice for objects ops pointer tables to have the object the function works with as first parameter.
> > 
> > Absolutely not a must have, but just looks a little bit more consistent.

Agreed, I generally prefer that too. However, in this case I thought it
more important to model this after how pretty much every debugfs file
operation works, which is to take the seq_file as first parameter. It's
the same order as for the main dma_heap_debug_show() entry point that
this is called from, so it's consistent that way.

I guess an alternative way to think about it is that in this case the
object that we're really working with is the seq_file and the dma_heap
is what gets written to that seq_file.

Thierry

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