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

[Manivannan Sadhasivam <mani@kernel.org>]

On Thu, Jul 17, 2025 at 10:18:54PM GMT, Akhil Vinod wrote:
> 
> 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@qti.qualcomm.com>
> > > 
> > > 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@vger.kernel.org
> > > Co-developed-by: Akhil Vinod <akhvin@qti.qualcomm.com>
> > > Signed-off-by: Akhil Vinod <akhvin@qti.qualcomm.com>
> > > Signed-off-by: Sumit Kumar <sumk@qti.qualcomm.com>
> > > ---
> > >   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.
> 

Then you should ignore the EOT flag by removing '!tr_done' check and just check
for buf_left and mhi_ep_process_ch_ring(). Having the same check in caller and
callee doesn't make sense.

- Mani

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