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

Alexandra Winter
2025-01-20T11:35:44Z

On 20.01.25 11:07, Julian Ruess wrote:
> 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.
>

An API should always be extendable

>>
>> 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);
>>>> --
>