Re: [PATCH] bus: mhi: ep: Update read pointer only after buffer is written

[Manivannan Sadhasivam]

On Wed, Apr 09, 2025 at 04:17:43PM +0530, Sumit Kumar wrote:
> Inside mhi_ep_ring_add_element, the read pointer (rd_offset) is updated
> before the buffer is written, potentially causing race conditions where
> the host sees an updated read pointer before the buffer is actually
> written. Updating rd_offset prematurely can lead to the host accessing
> an uninitialized or incomplete element, resulting in data corruption.
> 
> Invoke the buffer write before updating rd_offset to ensure the element
> is fully written before signaling its availability.
> 

Hmm. I was under assumption that the host wouldn't access the rd_offset before
raising the interrupt.

But anyway, the fix looks legitimate irrespective of that.

> Fixes: bbdcba57a1a2 ("bus: mhi: ep: Add support for ring management")
> cc: stable@xxxxxxxxxxxxxxx
> Co-developed-by: Youssef Samir <quic_yabdulra@xxxxxxxxxxx>
> Signed-off-by: Youssef Samir <quic_yabdulra@xxxxxxxxxxx>
> Signed-off-by: Sumit Kumar <quic_sumk@xxxxxxxxxxx>

Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxx>

- Mani

> ---
> ---
>  drivers/bus/mhi/ep/ring.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/bus/mhi/ep/ring.c b/drivers/bus/mhi/ep/ring.c
> index aeb53b2c34a8cd859393529d0c8860462bc687ed..26357ee68dee984d70ae5bf39f8f09f2cbcafe30 100644
> --- a/drivers/bus/mhi/ep/ring.c
> +++ b/drivers/bus/mhi/ep/ring.c
> @@ -131,19 +131,23 @@ int mhi_ep_ring_add_element(struct mhi_ep_ring *ring, struct mhi_ring_element *e
>  	}
>  
>  	old_offset = ring->rd_offset;
> -	mhi_ep_ring_inc_index(ring);
>  
>  	dev_dbg(dev, "Adding an element to ring at offset (%zu)\n", ring->rd_offset);
> +	buf_info.host_addr = ring->rbase + (old_offset * sizeof(*el));
> +	buf_info.dev_addr = el;
> +	buf_info.size = sizeof(*el);
> +
> +	ret = mhi_cntrl->write_sync(mhi_cntrl, &buf_info);
> +	if (ret)
> +		return ret;
> +
> +	mhi_ep_ring_inc_index(ring);
>  
>  	/* Update rp in ring context */
>  	rp = cpu_to_le64(ring->rd_offset * sizeof(*el) + ring->rbase);
>  	memcpy_toio((void __iomem *) &ring->ring_ctx->generic.rp, &rp, sizeof(u64));
>  
> -	buf_info.host_addr = ring->rbase + (old_offset * sizeof(*el));
> -	buf_info.dev_addr = el;
> -	buf_info.size = sizeof(*el);
> -
> -	return mhi_cntrl->write_sync(mhi_cntrl, &buf_info);
> +	return ret;
>  }
>  
>  void mhi_ep_ring_init(struct mhi_ep_ring *ring, enum mhi_ep_ring_type type, u32 id)
> 
> ---
> base-commit: 1e26c5e28ca5821a824e90dd359556f5e9e7b89f
> change-id: 20250328-rp_fix-d7ebc18bc3be
> 
> Best regards,
> -- 
> Sumit Kumar <quic_sumk@xxxxxxxxxxx>
> 

-- 
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