Re: [PATCH v3 15/25] bus: mhi: ep: Add support for processing MHI endpoint interrupts

[Alex Elder]

On 2/22/22 2:18 AM, Manivannan Sadhasivam wrote:
> On Tue, Feb 15, 2022 at 04:39:30PM -0600, Alex Elder wrote:
> > On 2/12/22 12:21 PM, Manivannan Sadhasivam wrote:
> > > Add support for processing MHI endpoint interrupts such as control
> > > interrupt, command interrupt and channel interrupt from the host.
> > >
> > > The interrupts will be generated in the endpoint device whenever host
> > > writes to the corresponding doorbell registers. The doorbell logic
> > > is handled inside the hardware internally.
> > >
> > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxx>
> >
> > Unless I'm mistaken, you have some bugs here.
> >
> > Beyond that, I question whether you should be using workqueues
> > for handling all interrupts.  For now, it's fine, but there
> > might be room for improvement after this is accepted upstream
> > (using threaded interrupt handlers, for example).
>
> Only reason I didn't use bottom halves is that the memory for TRE buffers need
> to be allocated each time, so essentially the caller should not sleep.

Threaded interrupt handlers can sleep.  If scheduled, they run
immediately after hard interrupt handlers.  For receive buffers,
yes, replacing a receive buffer just consumed would require an
allocation, but for transmit I think it might be possible to
avoid the need to do a memory allocation.  (Things to think
about at some future date.)

> This is currently a limitation of iATU where there are only 8 windows for
> mapping the host memory and each memory region size is also limited.

Those are hard limitations, and probably what constrains you the most.

					-Alex
> > 					-Alex
> >
> > > ---
> > >    drivers/bus/mhi/ep/main.c | 113 +++++++++++++++++++++++++++++++++++++-
> > >    include/linux/mhi_ep.h    |   2 +
> > >    2 files changed, 113 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/bus/mhi/ep/main.c b/drivers/bus/mhi/ep/main.c
> > > index ccb3c2795041..072b872e735b 100644
> > > --- a/drivers/bus/mhi/ep/main.c
> > > +++ b/drivers/bus/mhi/ep/main.c
> > > @@ -185,6 +185,56 @@ static void mhi_ep_ring_worker(struct work_struct *work)
> > >    	}
> > >    }
> > > +static void mhi_ep_queue_channel_db(struct mhi_ep_cntrl *mhi_cntrl,
> > > +				    unsigned long ch_int, u32 ch_idx)
> > > +{
> > > +	struct device *dev = &mhi_cntrl->mhi_dev->dev;
> > > +	struct mhi_ep_ring_item *item;
> > > +	struct mhi_ep_ring *ring;
> > > +	unsigned int i;
> >
> > Why not u32 i?  And why is the ch_int argument unsigned long?  The value
> > passed in is a u32.
>
> for_each_set_bit() expects the 2nd argument to be of type "unsigned long".
>
> Thanks,
> Mani