Subject: Re: [RFC net-next 4/7] net/ism: Add kernel-doc comments for ism functions

From: "Julian Ruess" <[email-masked]>
Date: Mon, 20 Jan 2025 11:07:54 +0100

On Mon Jan 20, 2025 at 10:56 AM CET, Alexandra Winter wrote:
>
>
> On 20.01.25 07:32, Dust Li wrote:
> >> +	/**
> >> +	 * move_data() - write into a remote dmb
> >> +	 * @dev: Local sending ism device
> >> +	 * @dmb_tok: Token of the remote dmb
> >> +	 * @idx: signalling index
> >> +	 * @sf: signalling flag;
> >> +	 *      if true, idx will be turned on at target ism interrupt mask
> >> +	 *      and target device will be signalled, if required.
> >> +	 * @offset: offset within target dmb
> >> +	 * @data: pointer to data to be sent
> >> +	 * @size: length of data to be sent
> >> +	 *
> >> +	 * Use dev to write data of size at offset into a remote dmb
> >> +	 * identified by dmb_tok. Data is moved synchronously, *data can
> >> +	 * be freed when this function returns.
> > When considering the API, I found this comment may be incorrect.
> > 
> > IIUC, in copy mode for PCI ISM devices, the CPU only tells the
> > device to perform a DMA copy. As a result, when this function returns,
> > the device may not have completed the DMA copy.
> > 
>
> No, it is actually one of the properties of ISM vPCI that the data is
> moved synchronously inside the move_data() function. (on PCI layer the
> data is moved inside the __zpci_store_block() command).
> Obviously for loopback move_data() is also synchornous.

That is true for the IBM ISM vPCI device but maybe we
should design the API also for future PCI devices
that do not move data synchronously.

>
> SMC-D does not make use of it, instead they re-use the same
> conn->sndbuf_desc for the lifetime of a connection.
>
>
> > In zero-copy mode for loopback, the source and destination share the
> > same buffer. If the source rewrites the buffer, the destination may
> > encounter corrupted data. The source should only reuse the data after
> > the destination has finished reading it.
> > 
>
> That is true independent of the question, whether the move is
> synchronous or not.
> It is the clients' responsibility to make sure a sender does not
> overwrite unread data. SMC uses the write-pointers and read-pointer for
> that.
>
>
> > Best regards,
> > Dust
> > 
> >> +	 *
> >> +	 * If signalling flag (sf) is true, bit number idx bit will be
> >> +	 * turned on in the ism signalling mask, that belongs to the
> >> +	 * target dmb, and handle_irq() of the ism client that owns this
> >> +	 * dmb will be called, if required. The target device may chose to
> >> +	 * coalesce multiple signalling triggers.
> >> +	 */
> >> 	int (*move_data)(struct ism_dev *dev, u64 dmb_tok, unsigned int idx,
> >> 			 bool sf, unsigned int offset, void *data,
> >> 			 unsigned int size);
> >> --