Re: [PATCH v2 07/12] iio: buffer-dma: Use DMABUFs instead of custom solution

[Paul Cercueil <paul@xxxxxxxxxxxxxxx>]

Hi Jonathan,

Le lun., mars 28 2022 at 18:54:25 +0100, Jonathan Cameron 
<jic23@xxxxxxxxxx> a écrit :
> On Mon,  7 Feb 2022 12:59:28 +0000
> Paul Cercueil <paul@xxxxxxxxxxxxxxx> wrote:
>
> >  Enhance the current fileio code by using DMABUF objects instead of
> >  custom buffers.
> >
> >  This adds more code than it removes, but:
> >  - a lot of the complexity can be dropped, e.g. custom kref and
> >    iio_buffer_block_put_atomic() are not needed anymore;
> >  - it will be much easier to introduce an API to export these DMABUF
> >    objects to userspace in a following patch.
> >
> >  Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx>
> Hi Paul,
>
> I'm a bit rusty on dma mappings, but you seem to have
> a mixture of streaming and coherent mappings going on in here.

That's OK, so am I. What do you call "streaming mappings"?

> Is it the case that the current code is using the coherent mappings
> and a potential 'other user' of the dma buffer might need
> streaming mappings?

Something like that. There are two different things; on both cases, 
userspace needs to create a DMABUF with IIO_BUFFER_DMABUF_ALLOC_IOCTL, 
and the backing memory is allocated with dma_alloc_coherent().

- For the userspace interface, you then have a "cpu access" IOCTL 
(DMA_BUF_IOCTL_SYNC), that allows userspace to inform when it will 
start/finish to process the buffer in user-space (which will 
sync/invalidate the data cache if needed). A buffer can then be 
enqueued for DMA processing (TX or RX) with the new 
IIO_BUFFER_DMABUF_ENQUEUE_IOCTL.

- When the DMABUF created via the IIO core is sent to another driver 
through the driver's custom DMABUF import function, this driver will 
call dma_buf_attach(), which will call iio_buffer_dma_buf_map(). Since 
it has to return a "struct sg_table *", this function then simply 
creates a sgtable with one entry that points to the backing memory.

Note that I added the iio_buffer_dma_buf_map() / _unmap() functions 
because the dma-buf core would WARN() if these were not provided. But 
since this code doesn't yet support importing/exporting DMABUFs to 
other drivers, these are never called, and I should probably just make 
them return a ERR_PTR() unconditionally.

Cheers,
-Paul

> Jonathan
>
> >  ---
> >   drivers/iio/buffer/industrialio-buffer-dma.c | 192 
> > ++++++++++++-------
> >   include/linux/iio/buffer-dma.h               |   8 +-
> >   2 files changed, 122 insertions(+), 78 deletions(-)
> >
> >  diff --git a/drivers/iio/buffer/industrialio-buffer-dma.c 
> > b/drivers/iio/buffer/industrialio-buffer-dma.c
> >  index 15ea7bc3ac08..54e6000cd2ee 100644
> >  --- a/drivers/iio/buffer/industrialio-buffer-dma.c
> >  +++ b/drivers/iio/buffer/industrialio-buffer-dma.c
> >  @@ -14,6 +14,7 @@
> >   #include <linux/poll.h>
> >   #include <linux/iio/buffer_impl.h>
> >   #include <linux/iio/buffer-dma.h>
> >  +#include <linux/dma-buf.h>
> >   #include <linux/dma-mapping.h>
> >   #include <linux/sizes.h>
> >
> >  @@ -90,103 +91,145 @@
> >    * callback is called from within the custom callback.
> >    */
> >
> >  -static void iio_buffer_block_release(struct kref *kref)
> >  -{
> >  -	struct iio_dma_buffer_block *block = container_of(kref,
> >  -		struct iio_dma_buffer_block, kref);
> >  -
> >  -	WARN_ON(block->state != IIO_BLOCK_STATE_DEAD);
> >  -
> >  -	dma_free_coherent(block->queue->dev, PAGE_ALIGN(block->size),
> >  -					block->vaddr, block->phys_addr);
> >  -
> >  -	iio_buffer_put(&block->queue->buffer);
> >  -	kfree(block);
> >  -}
> >  -
> >  -static void iio_buffer_block_get(struct iio_dma_buffer_block 
> > *block)
> >  -{
> >  -	kref_get(&block->kref);
> >  -}
> >  -
> >  -static void iio_buffer_block_put(struct iio_dma_buffer_block 
> > *block)
> >  -{
> >  -	kref_put(&block->kref, iio_buffer_block_release);
> >  -}
> >  -
> >  -/*
> >  - * dma_free_coherent can sleep, hence we need to take some special 
> > care to be
> >  - * able to drop a reference from an atomic context.
> >  - */
> >  -static LIST_HEAD(iio_dma_buffer_dead_blocks);
> >  -static DEFINE_SPINLOCK(iio_dma_buffer_dead_blocks_lock);
> >  -
> >  -static void iio_dma_buffer_cleanup_worker(struct work_struct *work)
> >  -{
> >  -	struct iio_dma_buffer_block *block, *_block;
> >  -	LIST_HEAD(block_list);
> >  -
> >  -	spin_lock_irq(&iio_dma_buffer_dead_blocks_lock);
> >  -	list_splice_tail_init(&iio_dma_buffer_dead_blocks, &block_list);
> >  -	spin_unlock_irq(&iio_dma_buffer_dead_blocks_lock);
> >  -
> >  -	list_for_each_entry_safe(block, _block, &block_list, head)
> >  -		iio_buffer_block_release(&block->kref);
> >  -}
> >  -static DECLARE_WORK(iio_dma_buffer_cleanup_work, 
> > iio_dma_buffer_cleanup_worker);
> >  -
> >  -static void iio_buffer_block_release_atomic(struct kref *kref)
> >  -{
> >  +struct iio_buffer_dma_buf_attachment {
> >  +	struct scatterlist sgl;
> >  +	struct sg_table sg_table;
> >   	struct iio_dma_buffer_block *block;
> >  -	unsigned long flags;
> >  -
> >  -	block = container_of(kref, struct iio_dma_buffer_block, kref);
> >  -
> >  -	spin_lock_irqsave(&iio_dma_buffer_dead_blocks_lock, flags);
> >  -	list_add_tail(&block->head, &iio_dma_buffer_dead_blocks);
> >  -	spin_unlock_irqrestore(&iio_dma_buffer_dead_blocks_lock, flags);
> >  -
> >  -	schedule_work(&iio_dma_buffer_cleanup_work);
> >  -}
> >  -
> >  -/*
> >  - * Version of iio_buffer_block_put() that can be called from 
> > atomic context
> >  - */
> >  -static void iio_buffer_block_put_atomic(struct 
> > iio_dma_buffer_block *block)
> >  -{
> >  -	kref_put(&block->kref, iio_buffer_block_release_atomic);
> >  -}
> >  +};
> >
> >   static struct iio_dma_buffer_queue *iio_buffer_to_queue(struct 
> > iio_buffer *buf)
> >   {
> >   	return container_of(buf, struct iio_dma_buffer_queue, buffer);
> >   }
> >
> >  +static struct iio_buffer_dma_buf_attachment *
> >  +to_iio_buffer_dma_buf_attachment(struct sg_table *table)
> >  +{
> >  +	return container_of(table, struct iio_buffer_dma_buf_attachment, 
> > sg_table);
> >  +}
> >  +
> >  +static void iio_buffer_block_get(struct iio_dma_buffer_block 
> > *block)
> >  +{
> >  +	get_dma_buf(block->dmabuf);
> >  +}
> >  +
> >  +static void iio_buffer_block_put(struct iio_dma_buffer_block 
> > *block)
> >  +{
> >  +	dma_buf_put(block->dmabuf);
> >  +}
> >  +
> >  +static int iio_buffer_dma_buf_attach(struct dma_buf *dbuf,
> >  +				     struct dma_buf_attachment *at)
> >  +{
> >  +	at->priv = dbuf->priv;
> >  +
> >  +	return 0;
> >  +}
> >  +
> >  +static struct sg_table *iio_buffer_dma_buf_map(struct 
> > dma_buf_attachment *at,
> >  +					       enum dma_data_direction dma_dir)
> >  +{
> >  +	struct iio_dma_buffer_block *block = at->priv;
> >  +	struct iio_buffer_dma_buf_attachment *dba;
> >  +	int ret;
> >  +
> >  +	dba = kzalloc(sizeof(*dba), GFP_KERNEL);
> >  +	if (!dba)
> >  +		return ERR_PTR(-ENOMEM);
> >  +
> >  +	sg_init_one(&dba->sgl, block->vaddr, PAGE_ALIGN(block->size));
> >  +	dba->sg_table.sgl = &dba->sgl;
> >  +	dba->sg_table.nents = 1;
> >  +	dba->block = block;
> >  +
> >  +	ret = dma_map_sgtable(at->dev, &dba->sg_table, dma_dir, 0);
> >  +	if (ret) {
> >  +		kfree(dba);
> >  +		return ERR_PTR(ret);
> >  +	}
> >  +
> >  +	return &dba->sg_table;
> >  +}
> >  +
> >  +static void iio_buffer_dma_buf_unmap(struct dma_buf_attachment *at,
> >  +				     struct sg_table *sg_table,
> >  +				     enum dma_data_direction dma_dir)
> >  +{
> >  +	struct iio_buffer_dma_buf_attachment *dba =
> >  +		to_iio_buffer_dma_buf_attachment(sg_table);
> >  +
> >  +	dma_unmap_sgtable(at->dev, &dba->sg_table, dma_dir, 0);
> >  +	kfree(dba);
> >  +}
> >  +
> >  +static void iio_buffer_dma_buf_release(struct dma_buf *dbuf)
> >  +{
> >  +	struct iio_dma_buffer_block *block = dbuf->priv;
> >  +	struct iio_dma_buffer_queue *queue = block->queue;
> >  +
> >  +	WARN_ON(block->state != IIO_BLOCK_STATE_DEAD);
> >  +
> >  +	mutex_lock(&queue->lock);
> >  +
> >  +	dma_free_coherent(queue->dev, PAGE_ALIGN(block->size),
> >  +			  block->vaddr, block->phys_addr);
> >  +	kfree(block);
> >  +
> >  +	mutex_unlock(&queue->lock);
> >  +	iio_buffer_put(&queue->buffer);
> >  +}
> >  +
> >  +static const struct dma_buf_ops iio_dma_buffer_dmabuf_ops = {
> >  +	.attach			= iio_buffer_dma_buf_attach,
> >  +	.map_dma_buf		= iio_buffer_dma_buf_map,
> >  +	.unmap_dma_buf		= iio_buffer_dma_buf_unmap,
> >  +	.release		= iio_buffer_dma_buf_release,
> >  +};
> >  +
> >   static struct iio_dma_buffer_block *iio_dma_buffer_alloc_block(
> >   	struct iio_dma_buffer_queue *queue, size_t size)
> >   {
> >   	struct iio_dma_buffer_block *block;
> >  +	DEFINE_DMA_BUF_EXPORT_INFO(einfo);
> >  +	struct dma_buf *dmabuf;
> >  +	int err = -ENOMEM;
> >
> >   	block = kzalloc(sizeof(*block), GFP_KERNEL);
> >   	if (!block)
> >  -		return NULL;
> >  +		return ERR_PTR(err);
> >
> >   	block->vaddr = dma_alloc_coherent(queue->dev, PAGE_ALIGN(size),
> >   		&block->phys_addr, GFP_KERNEL);
> >  -	if (!block->vaddr) {
> >  -		kfree(block);
> >  -		return NULL;
> >  +	if (!block->vaddr)
> >  +		goto err_free_block;
> >  +
> >  +	einfo.ops = &iio_dma_buffer_dmabuf_ops;
> >  +	einfo.size = PAGE_ALIGN(size);
> >  +	einfo.priv = block;
> >  +	einfo.flags = O_RDWR;
> >  +
> >  +	dmabuf = dma_buf_export(&einfo);
> >  +	if (IS_ERR(dmabuf)) {
> >  +		err = PTR_ERR(dmabuf);
> >  +		goto err_free_dma;
> >   	}
> >
> >  +	block->dmabuf = dmabuf;
> >   	block->size = size;
> >   	block->state = IIO_BLOCK_STATE_DONE;
> >   	block->queue = queue;
> >   	INIT_LIST_HEAD(&block->head);
> >  -	kref_init(&block->kref);
> >
> >   	iio_buffer_get(&queue->buffer);
> >
> >   	return block;
> >  +
> >  +err_free_dma:
> >  +	dma_free_coherent(queue->dev, PAGE_ALIGN(size),
> >  +			  block->vaddr, block->phys_addr);
> >  +err_free_block:
> >  +	kfree(block);
> >  +	return ERR_PTR(err);
> >   }
> >
> >   static void _iio_dma_buffer_block_done(struct iio_dma_buffer_block 
> > *block)
> >  @@ -223,7 +266,7 @@ void iio_dma_buffer_block_done(struct 
> > iio_dma_buffer_block *block)
> >   	_iio_dma_buffer_block_done(block);
> >   	spin_unlock_irqrestore(&queue->list_lock, flags);
> >
> >  -	iio_buffer_block_put_atomic(block);
> >  +	iio_buffer_block_put(block);
> >   	iio_dma_buffer_queue_wake(queue);
> >   }
> >   EXPORT_SYMBOL_GPL(iio_dma_buffer_block_done);
> >  @@ -249,7 +292,8 @@ void iio_dma_buffer_block_list_abort(struct 
> > iio_dma_buffer_queue *queue,
> >   		list_del(&block->head);
> >   		block->bytes_used = 0;
> >   		_iio_dma_buffer_block_done(block);
> >  -		iio_buffer_block_put_atomic(block);
> >  +
> >  +		iio_buffer_block_put(block);
> >   	}
> >   	spin_unlock_irqrestore(&queue->list_lock, flags);
> >
> >  @@ -340,8 +384,8 @@ int iio_dma_buffer_request_update(struct 
> > iio_buffer *buffer)
> >
> >   		if (!block) {
> >   			block = iio_dma_buffer_alloc_block(queue, size);
> >  -			if (!block) {
> >  -				ret = -ENOMEM;
> >  +			if (IS_ERR(block)) {
> >  +				ret = PTR_ERR(block);
> >   				goto out_unlock;
> >   			}
> >   			queue->fileio.blocks[i] = block;
> >  diff --git a/include/linux/iio/buffer-dma.h 
> > b/include/linux/iio/buffer-dma.h
> >  index 490b93f76fa8..6b3fa7d2124b 100644
> >  --- a/include/linux/iio/buffer-dma.h
> >  +++ b/include/linux/iio/buffer-dma.h
> >  @@ -8,7 +8,6 @@
> >   #define __INDUSTRIALIO_DMA_BUFFER_H__
> >
> >   #include <linux/list.h>
> >  -#include <linux/kref.h>
> >   #include <linux/spinlock.h>
> >   #include <linux/mutex.h>
> >   #include <linux/iio/buffer_impl.h>
> >  @@ -16,6 +15,7 @@
> >   struct iio_dma_buffer_queue;
> >   struct iio_dma_buffer_ops;
> >   struct device;
> >  +struct dma_buf;
> >
> >   /**
> >    * enum iio_block_state - State of a struct iio_dma_buffer_block
> >  @@ -39,8 +39,8 @@ enum iio_block_state {
> >    * @vaddr: Virutal address of the blocks memory
> >    * @phys_addr: Physical address of the blocks memory
> >    * @queue: Parent DMA buffer queue
> >  - * @kref: kref used to manage the lifetime of block
> >    * @state: Current state of the block
> >  + * @dmabuf: Underlying DMABUF object
> >    */
> >   struct iio_dma_buffer_block {
> >   	/* May only be accessed by the owner of the block */
> >  @@ -56,13 +56,13 @@ struct iio_dma_buffer_block {
> >   	size_t size;
> >   	struct iio_dma_buffer_queue *queue;
> >
> >  -	/* Must not be accessed outside the core. */
> >  -	struct kref kref;
> >   	/*
> >   	 * Must not be accessed outside the core. Access needs to hold
> >   	 * queue->list_lock if the block is not owned by the core.
> >   	 */
> >   	enum iio_block_state state;
> >  +
> >  +	struct dma_buf *dmabuf;
> >   };
> >
> >   /**