Hi Peter,

On 18-07-18, 13:06, Peter Ujfalusi wrote:

quoted

quoted

+struct dma_async_tx_descriptor;
+
+struct dma_descriptor_metadata_ops {
+	int (*attach)(struct dma_async_tx_descriptor *desc, void *data,
+		      size_t len);

How does one detach?

I have not thought about detach, but clients can just attach NULL I guess.

So what are the implication of attach and detach here, should the data
be deref by dmaengine driver and drop the ref.

Should anyone do refcounting?

quoted

When should the client free up the memory, IOW when
does dma driver drop ref to data.

The metadata is for the descriptor so the DMA driver might want to
access to it while the descriptor is valid.

Typically clients can free up their metadata storage after the dma
completion callback. On DEV_TO_MEM the metadata is going to be placed in
the provided buffer when the transfer is completed.

That sounds okay to me

quoted

quoted

+	void *(*get_ptr)(struct dma_async_tx_descriptor *desc,
+			 size_t *payload_len, size_t *max_len);

so what is this supposed to do..?

My issue with the attach in general is that it will need additional
memcpy to move the metadata from/to the client buffer to it's place.

With get_ptr the client can get the pointer to the actual place where
the metadata resides and modify/read it in place w/o memcpy.

I know, I know... We need to trust the clients, but with high throughput
peripherals the memcpy is taxing.

Okay I am not sure I have understood fully, so with attach you set
a pointer (containing metdata?) so why do you need additional one..

quoted

quoted

+	int (*set_len)(struct dma_async_tx_descriptor *desc,
+		       size_t payload_len);

attach already has length, so how does this help?

So, DMA drivers can implement either or both:
1. attach()
2. get_ptr() / set_len()

Ah okay, what are the reasons for providing two methods and not a single
one

Clients must not mix the two way of handling the metadata.
The set_len() is intended to tell the DMA driver the client provided
metadata size (in MEM_TO_DEV case mostly).

MEM_TO_DEV flow on client side:
get_ptr()
fill in the metadata to the pointer (not exceeding max_len)
set_len() to tell the DMA driver the amount of valid bytes written

DEV_TO_MEM flow on client side:
In the completion callback, get_ptr()
the metadata is payload_len bytes and can be accessed in the return pointer.

I would think to unify this..

BTW: The driver which is going to need this is now accessible in public:
https://git.ti.com/ti-linux-kernel/ti-linux-kernel/trees/ti-linux-4.14.y/drivers/dma/ti

or in my wip tree:
https://github.com/omap-audio/linux-audio/tree/peter/ti-linux-4.14.y/wip/drivers/dma/ti

prefixed with k3-*

-- 
~Vinod