Re: [PATCH] bus: mhi: ep: Fix chained transfer handling in read path

[Akhil Vinod]

On 7/16/2025 12:10 PM, Manivannan Sadhasivam wrote:
> On Wed, Jul 09, 2025 at 04:03:17PM GMT, Sumit Kumar wrote:
> > From: Sumit Kumar <sumk@xxxxxxxxxxxxxxxx>
> >
> > The current implementation of mhi_ep_read_channel, in case of chained
> > transactions, assumes the End of Transfer(EOT) bit is received with the
> > doorbell. As a result, it may incorrectly advance mhi_chan->rd_offset
> > beyond wr_offset during host-to-device transfers when EOT has not yet
> > arrived. This can lead to access of unmapped host memory, causing
> > IOMMU faults and processing of stale TREs.
> >
> > This change modifies the loop condition to ensure rd_offset remains behind
> > wr_offset, allowing the function to process only valid TREs up to the
> > current write pointer. This prevents premature reads and ensures safe
> > traversal of chained TREs.
> >
> > Fixes: 5301258899773 ("bus: mhi: ep: Add support for reading from the host")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Co-developed-by: Akhil Vinod <akhvin@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Akhil Vinod <akhvin@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Sumit Kumar <sumk@xxxxxxxxxxxxxxxx>
> > ---
> >   drivers/bus/mhi/ep/main.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/bus/mhi/ep/main.c b/drivers/bus/mhi/ep/main.c
> > index b3eafcf2a2c50d95e3efd3afb27038ecf55552a5..2e134f44952d1070c62c24aeca9effc7fd325860 100644
> > --- a/drivers/bus/mhi/ep/main.c
> > +++ b/drivers/bus/mhi/ep/main.c
> > @@ -468,7 +468,7 @@ static int mhi_ep_read_channel(struct mhi_ep_cntrl *mhi_cntrl,
> >   
> >   			mhi_chan->rd_offset = (mhi_chan->rd_offset + 1) % ring->ring_size;
> >   		}
> > -	} while (buf_left && !tr_done);
> > +	} while (buf_left && !tr_done && mhi_chan->rd_offset != ring->wr_offset);
> You should use mhi_ep_queue_is_empty() for checking the available elements to
> process. And with this check in place, the existing check in
> mhi_ep_process_ch_ring() becomes redundant.
>
> - Mani

Yes, agreed that the check can be replaced with the mhi_ep_queue_is_empty, but the existing
check in mhi_ep_process_ch_ring() is still necessary because there can be a case where
there are multiple chained transactions in the ring.

Example: The ring at the time mhi_ep_read_channel is executing may look like:
chained | chained |  EOT#1 | chained | chained | EOT#2
 
If we remove the check from mhi_ep_process_ch_ring, we bail out of the first transaction itself
and the remaining packets won't be processed. mhi_ep_read_channel in its current form is designed
for a single MHI packet only.

- Akhil