Re: [RFC PATCH 1/1] cxl/feature: Using Full Data Transfer only when offset is 0

[Davidlohr Bueso]

On Wed, 09 Apr 2025, Li Ming wrote:

> Per CXL r3.2 section 8.2.10.6.3 Set Feature(Opcode 0502h)
>
> "If the Feature data is transferred in its entirely, the caller makes
> one call to Set Feature with Action = Full Data Transfer. The Offset
> field is not used and shall be ignored."
>
> It implies if using Full Data Transfer, the received data should be
> updated from offset 0 on device side.
>
> Current driver implementation is if feature data size is less than mbox
> payload - set feature header, driver will use Full Data Transfer even if
> user provides an offset. Per above description, feature data will be
> written from offset 0 rather than the offset value user provided, the
> result will not be what user expects.
>
> The changes is checking if the offset caller provides is equal to 0, if
> yes, using Full Data Transfer, if not, using Initiate Data Transfer -
> Finish Data Transfer.
>
> After the changes, all Set Feature transfer scenarios are below:
>
> 1. data size + header is less than mbox payload and offset is 0
> 	Full Data Transfer
>
> 2. data size + header is less than mbox payload and offset is not 0
> 	Initiate Data Transfer(with all feature data) - Finish Data Transfer(with no feature data)
>
> 3. data size + header us greater than mbox payload with any value of offset
> 	Initiate Data Transfer - Continue Data Transfer - Finish Data Transfer
>
> Fixes: 14d502cc2718e ("cxl/mbox: Add SET_FEATURE mailbox command")
> Signed-off-by: Li Ming <ming.li@xxxxxxxxxxxx>

Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>