Hi Loic, On Wed, May 04, 2022 at 09:21:20AM +0200, Loic Poulain wrote: > Hi Mani, > > On Mon, 2 May 2022 at 12:42, Manivannan Sadhasivam > <manivannan.sadhasivam@xxxxxxxxxx> wrote: > > > > The endpoint device will only read the context wp when the host rings > > the doorbell. > > Are we sure about this statement? what if we update ctxt_wp while the > device is still processing the previous ring? is it going to continue > processing the new ctxt_wp or wait for a new doorbell interrupt? what > about burst mode in which we don't ring at all (ring_db is no-op)? > Good point. I think my statement was misleading. But still this scenario won't happen as per my undestanding. Please see below. > > And moreover the doorbell write is using writel(). This > > guarantess that the prior writes will be completed before ringing > > doorbell. > > Yes but the barrier is to ensure that descriptor/ring content is > updated before we actually pass it to device ownership, it's not about > ordering with the doorbell write, but the memory coherent ones. > I see a clear data dependency between writing the ring element and updating the context pointer. For instance, ``` struct mhi_ring_element *mhi_tre; mhi_tre = ring->wp; /* Populate mhi_tre */ ... /* Increment wp */ ring->wp += el_size; /* Update ctx wp */ ring->ctx_wp = ring->iommu_base + (ring->wp - ring->base); ``` This is analogous to: ``` Read PTR A; Update PTR A; Increment PTR A; Write PTR A to PTR B; ``` Here, because of the data dependency due to "ring->wp", the CPU or compiler won't be ordering the instructions. I think that's one of the reason we never hit any issue due to this. Thanks, Mani > > > > So there is no need of an additional dma_wmb() to order the coherent > > memory writes w.r.t each other. Even if the writes gets reordered, it > > won't affect the endpoint device. > > > > Cc: Loic Poulain <loic.poulain@xxxxxxxxxx> > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxx> > > --- > > drivers/bus/mhi/host/main.c | 5 ----- > > 1 file changed, 5 deletions(-) > > > > diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c > > index 966ffc2458b9..6706a82d3aa8 100644 > > --- a/drivers/bus/mhi/host/main.c > > +++ b/drivers/bus/mhi/host/main.c > > @@ -138,11 +138,6 @@ void mhi_ring_chan_db(struct mhi_controller *mhi_cntrl, > > > > db = ring->iommu_base + (ring->wp - ring->base); > > > > - /* > > - * Writes to the new ring element must be visible to the hardware > > - * before letting h/w know there is new element to fetch. > > - */ > > - dma_wmb(); > > *ring->ctxt_wp = cpu_to_le64(db); > > > > mhi_chan->db_cfg.ring_db(mhi_cntrl, &mhi_chan->db_cfg, > > -- > > 2.25.1 > > > > Regards, > Loic
