Re: [PATCH] i3c: mipi-i3c-hci: fix IBI payload length calculation for final status

[Adrian Hunter]

On 02/04/2026 11:07, Billy Tsai wrote:
> On 31/03/2026 13:27, Billy Tsai wrote:
>>> In DMA mode, the IBI status descriptor encodes the payload using
>>> CHUNKS (number of chunks) and DATA_LENGTH (valid bytes in the last
>>> chunk). All preceding chunks are implicitly full-sized.
>>>
>>> The current code accumulates full chunk sizes for non-final status
>>> descriptors, but for the final status descriptor it only adds
>>> DATA_LENGTH. This ignores the contribution of the preceding full
>>> chunks described by the same final status entry.
>>>
>>> As a result, the computed IBI payload length is truncated whenever
>>> the final status spans multiple chunks. For example, with a chunk
>>> size of 4 bytes, CHUNKS=2 and DATA_LENGTH=1 should result in a total
>>> payload size of 5 bytes, but the current code reports only 1 byte.
>>>
>>> Fix the calculation by adding the size of (CHUNKS - 1) full chunks
>>> plus DATA_LENGTH for the last chunk.
>>>
>>> Fixes: 9ad9a52cce28 ("i3c/master: introduce the mipi-i3c-hci driver")
>>> Signed-off-by: Billy Tsai <billy_tsai@xxxxxxxxxxxxxx>
>>> ---
>>>  drivers/i3c/master/mipi-i3c-hci/dma.c | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/drivers/i3c/master/mipi-i3c-hci/dma.c b/drivers/i3c/master/mipi-i3c-hci/dma.c
>>> index b903a2da1fd1..f4c76f168276 100644
>>> --- a/drivers/i3c/master/mipi-i3c-hci/dma.c
>>> +++ b/drivers/i3c/master/mipi-i3c-hci/dma.c
>>> @@ -721,6 +721,7 @@ static void hci_dma_process_ibi(struct i3c_hci *hci, struct hci_rh_data *rh)
>>>               if (!(ibi_status & IBI_LAST_STATUS)) {
>>>                       ibi_size += chunks * rh->ibi_chunk_sz;
>>>               } else {
>>> +                     ibi_size += (chunks - 1) * rh->ibi_chunk_sz;
> 
>> That assumes chunks is not 0.  It would be better to
>> defend against that possibility i.e.
>>
>>                         if (chunks) {
>>                                 ibi_size += (chunks - 1) * rh->ibi_chunk_sz;
>>                                 ibi_size += FIELD_GET(IBI_DATA_LENGTH, ibi_status);
>>                         }
>>
> 
> As expected, the value should never be 0, as this is guaranteed by the hardware.

Not sure the spec. actually says that anywhere.

> 
> If we add a check for 0, it may be appropriate to include a WARN_ON message to
> indicate unexpected hardware behavior.

Please no.  If the target driver does not get the amount of data
it is expecting, then it can complain.