Re: [PATCH v3 12/25] bus: mhi: ep: Add support for ring management

[Alex Elder]

On 2/18/22 2:07 AM, Manivannan Sadhasivam wrote:
> On Tue, Feb 15, 2022 at 02:03:13PM -0600, Alex Elder wrote:
> > On 2/12/22 12:21 PM, Manivannan Sadhasivam wrote:
> > > Add support for managing the MHI ring. The MHI ring is a circular queue
> > > of data structures used to pass the information between host and the
> > > endpoint.
> > >
> > > MHI support 3 types of rings:
> > >
> > > 1. Transfer ring
> > > 2. Event ring
> > > 3. Command ring
> > >
> > > All rings reside inside the host memory and the MHI EP device maps it to
> > > the device memory using blocks like PCIe iATU. The mapping is handled in
> > > the MHI EP controller driver itself.
> > >
> > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxx>
> >
> > Great explanation.  One more thing to add, is that the command
> > and transfer rings are directed from the host to the MHI EP device,
> > while the event rings are directed from the EP device toward the
> > host.
>
> That's correct, will add.
>
> > I notice that you've improved a few things I had notes about,
> > and I don't recall suggesting them.  I'm very happy about that.
> >
> > I have a few more comments here, some worth thinking about
> > at least.
> >
> > 					-Alex
> >
> > > ---
> > >    drivers/bus/mhi/ep/Makefile   |   2 +-
> > >    drivers/bus/mhi/ep/internal.h |  33 +++++
> > >    drivers/bus/mhi/ep/main.c     |  59 +++++++-
> > >    drivers/bus/mhi/ep/ring.c     | 267 ++++++++++++++++++++++++++++++++++
> > >    include/linux/mhi_ep.h        |  11 ++
> > >    5 files changed, 370 insertions(+), 2 deletions(-)
> > >    create mode 100644 drivers/bus/mhi/ep/ring.c
> > >
> > > diff --git a/drivers/bus/mhi/ep/Makefile b/drivers/bus/mhi/ep/Makefile
> > > index a1555ae287ad..7ba0e04801eb 100644
> > > --- a/drivers/bus/mhi/ep/Makefile
> > > +++ b/drivers/bus/mhi/ep/Makefile

. . .

> > > diff --git a/drivers/bus/mhi/ep/main.c b/drivers/bus/mhi/ep/main.c
> > > index 950b5bcabe18..2c8045766292 100644
> > > --- a/drivers/bus/mhi/ep/main.c
> > > +++ b/drivers/bus/mhi/ep/main.c
> > > @@ -18,6 +18,48 @@
> > >    static DEFINE_IDA(mhi_ep_cntrl_ida);
> >
> > The following function handles command or channel interrupt work.
>
> Both

What I meant was to suggest a comment that stated that it
is used for both of those.  Not really a bit deal though.

> > > +static void mhi_ep_ring_worker(struct work_struct *work)
> > > +{
> > > +	struct mhi_ep_cntrl *mhi_cntrl = container_of(work,
> > > +				struct mhi_ep_cntrl, ring_work);
> > > +	struct device *dev = &mhi_cntrl->mhi_dev->dev;
> > > +	struct mhi_ep_ring_item *itr, *tmp;
> > > +	struct mhi_ep_ring *ring;
> > > +	struct mhi_ep_chan *chan;
> > > +	unsigned long flags;
> > > +	LIST_HEAD(head);
> > > +	int ret;
> > > +
> > > +	/* Process the command ring first */
> > > +	ret = mhi_ep_process_ring(&mhi_cntrl->mhi_cmd->ring);
> > > +	if (ret) {
> >
> > At the moment I'm not sure where this work gets scheduled.
> > But what if there is no command to process?  It looks
> > like you go update the cached pointer no matter what
> > to see if there's anything new.  But it seems like you
> > ought to be able to do this when interrupted for a
> > command rather than all the time.
>
> No, ring cache is not getting updated all the time. If you look into
> process_ring(), first the write pointer is read from MMIO and there is a
> check to see if there are elements in the ring or not. Only if that
> check passes, the ring cache will get updated.
>
> Since the same work item is used for both cmd and transfer rings, this
> check is necessary. The other option would be to use different work items
> for command and transfer rings. This is something I want to try once
> this initial version gets merged.

OK.  I accept your explanation (even though I confess I did not
go back and look at the code again...).

Thanks Mani.

					-Alex

> Thanks,
> Mani