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Date:   Fri, 18 Feb 2022 13:37:04 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
To:     Alex Elder <elder@linaro.org>
Cc:     mhi@lists.linux.dev, quic_hemantk@quicinc.com,
        quic_bbhatt@quicinc.com, quic_jhugo@quicinc.com,
        vinod.koul@linaro.org, bjorn.andersson@linaro.org,
        dmitry.baryshkov@linaro.org, quic_vbadigan@quicinc.com,
        quic_cang@quicinc.com, quic_skananth@quicinc.com,
        linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 12/25] bus: mhi: ep: Add support for ring management
Message-ID: <20220218080704.GD11964@workstation>
References: <20220212182117.49438-1-manivannan.sadhasivam@linaro.org>
 <20220212182117.49438-13-manivannan.sadhasivam@linaro.org>
 <20504c14-6df5-83d1-53e9-85c682a5a3c7@linaro.org>
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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@linaro.org>
> 
> 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
> > @@ -1,2 +1,2 @@
> >   obj-$(CONFIG_MHI_BUS_EP) += mhi_ep.o
> > -mhi_ep-y := main.o mmio.o
> > +mhi_ep-y := main.o mmio.o ring.o
> > diff --git a/drivers/bus/mhi/ep/internal.h b/drivers/bus/mhi/ep/internal.h
> > index 2c756a90774c..48d6e9667d55 100644
> > --- a/drivers/bus/mhi/ep/internal.h
> > +++ b/drivers/bus/mhi/ep/internal.h
> > @@ -112,6 +112,18 @@ enum mhi_ep_execenv {
> >   	MHI_EP_UNRESERVED
> >   };
> > +/* Transfer Ring Element macros */
> > +#define MHI_EP_TRE_PTR(ptr) (ptr)
> > +#define MHI_EP_TRE_DWORD0(len) (len & MHI_MAX_MTU)
> 
> The above looks funny.  This assumes MHI_MAX_MTU is
> a mask value (likely one less than a power-of-2).
> That doesn't seem obvious to me; use modulo if you
> must, but better, just ensure len is in range rather
> than silently truncating it if it's not.
> 
> > +#define MHI_EP_TRE_DWORD1(bei, ieot, ieob, chain) ((2 << 16) | (bei << 10) \
> > +	| (ieot << 9) | (ieob << 8) | chain)
> 
> You should probably use FIELD_PREP() to compute the value
> here, since you're using FIELD_GET() to extract the field
> values below.
> 
> > +#define MHI_EP_TRE_GET_PTR(tre) ((tre)->ptr)
> > +#define MHI_EP_TRE_GET_LEN(tre) ((tre)->dword[0] & 0xffff)
> > +#define MHI_EP_TRE_GET_CHAIN(tre) FIELD_GET(BIT(0), (tre)->dword[1])
> 
> #define	TRE_FLAG_CHAIN	BIT(0)
> 
> Then just call
> 	bei = FIELD_GET(TRE_FLAG_CHAIN, tre->dword[1]);
> 
> But I haven't looked at the code where this is used yet.
> 
> > +#define MHI_EP_TRE_GET_IEOB(tre) FIELD_GET(BIT(8), (tre)->dword[1])
> > +#define MHI_EP_TRE_GET_IEOT(tre) FIELD_GET(BIT(9), (tre)->dword[1])
> > +#define MHI_EP_TRE_GET_BEI(tre) FIELD_GET(BIT(10), (tre)->dword[1])
> > +
> 
> These macros should be shared/shareable between the host and endpoint.
> They operate on external interfaces and so should be byte swapped
> (where used) when updating actual memory.  Unlike the patches from
> Paul Davey early in this series, this does *not* byte swap the
> values in the right hand side of these definitions, which is good.
> 
> I'm pretty sure I mentioned this before...  I don't really like these
> "DWORD" macros that simply write compute register values to write
> out to the TREs.  A TRE is a structure, not a set of registers.  And
> a whole TRE can be written or read in a single ARM instruction in
> some cases--but most likely you need to define it as a structure
> for that to happen.
> 
> struct mhi_tre {
> 	__le64 addr;
> 	__le16 len_opcode
> 	__le16 reserved;
> 	__le32 flags;
> };

Changing the TRE structure requires changes to both host and endpoint
stack. So I'll tackle this as an improvement later.

Added to TODO list.

> 
> Which reminds me, this shared memory area should probably be mapped
> using memremap() rather than ioremap().  I haven't checked whether
> it is...
> 
> >   enum mhi_ep_ring_type {
> >   	RING_TYPE_CMD = 0,
> >   	RING_TYPE_ER,
> > @@ -131,6 +143,11 @@ union mhi_ep_ring_ctx {
> >   	struct mhi_generic_ctx generic;
> >   };
> > +struct mhi_ep_ring_item {
> > +	struct list_head node;
> > +	struct mhi_ep_ring *ring;
> > +};
> > +
> >   struct mhi_ep_ring {
> >   	struct mhi_ep_cntrl *mhi_cntrl;
> >   	int (*ring_cb)(struct mhi_ep_ring *ring, struct mhi_ep_ring_element *el);
> > @@ -143,6 +160,9 @@ struct mhi_ep_ring {
> >   	u32 db_offset_h;
> >   	u32 db_offset_l;
> >   	u32 ch_id;
> > +	u32 er_index;
> > +	u32 irq_vector;
> > +	bool started;
> >   };
> >   struct mhi_ep_cmd {
> > @@ -168,6 +188,19 @@ struct mhi_ep_chan {
> >   	bool skip_td;
> >   };
> > +/* MHI Ring related functions */
> > +void mhi_ep_ring_init(struct mhi_ep_ring *ring, enum mhi_ep_ring_type type, u32 id);
> > +void mhi_ep_ring_reset(struct mhi_ep_cntrl *mhi_cntrl, struct mhi_ep_ring *ring);
> > +int mhi_ep_ring_start(struct mhi_ep_cntrl *mhi_cntrl, struct mhi_ep_ring *ring,
> > +		      union mhi_ep_ring_ctx *ctx);
> > +size_t mhi_ep_ring_addr2offset(struct mhi_ep_ring *ring, u64 ptr);
> > +int mhi_ep_process_ring(struct mhi_ep_ring *ring);
> > +int mhi_ep_ring_add_element(struct mhi_ep_ring *ring, struct mhi_ep_ring_element *element);
> > +void mhi_ep_ring_inc_index(struct mhi_ep_ring *ring);
> > +int mhi_ep_process_cmd_ring(struct mhi_ep_ring *ring, struct mhi_ep_ring_element *el);
> > +int mhi_ep_process_tre_ring(struct mhi_ep_ring *ring, struct mhi_ep_ring_element *el);
> > +int mhi_ep_update_wr_offset(struct mhi_ep_ring *ring);
> > +
> >   /* MMIO related functions */
> >   u32 mhi_ep_mmio_read(struct mhi_ep_cntrl *mhi_cntrl, u32 offset);
> >   void mhi_ep_mmio_write(struct mhi_ep_cntrl *mhi_cntrl, u32 offset, u32 val);
> > 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

> > +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.

Thanks,
Mani