Date:   Mon, 30 Mar 2020 15:19:13 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Chris Lew <clew@...eaurora.org>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        gregkh@...uxfoundation.org, davem@...emloft.net,
        smohanad@...eaurora.org, jhugo@...eaurora.org,
        kvalo@...eaurora.org, hemantk@...eaurora.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH v3 6/7] net: qrtr: Add MHI transport layer

Hi Chris,

On Thu, Mar 26, 2020 at 03:54:42PM -0700, Chris Lew wrote:
> 
> 
> On 3/25/2020 3:37 AM, Manivannan Sadhasivam wrote:
> > Hi Bjorn,
> > 
> > + Chris Lew
> > 
> > On Tue, Mar 24, 2020 at 01:39:52PM -0700, Bjorn Andersson wrote:
> > > On Mon 23 Mar 23:10 PDT 2020, Manivannan Sadhasivam wrote:
> > > 
> > > > MHI is the transport layer used for communicating to the external modems.
> > > > Hence, this commit adds MHI transport layer support to QRTR for
> > > > transferring the QMI messages over IPC Router.
> > > > 
> > > > Cc: "David S. Miller" <davem@...emloft.net>
> > > > Cc: netdev@...r.kernel.org
> > > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > > > ---
> > > >   net/qrtr/Kconfig  |   7 ++
> > > >   net/qrtr/Makefile |   2 +
> > > >   net/qrtr/mhi.c    | 208 ++++++++++++++++++++++++++++++++++++++++++++++
> > > >   3 files changed, 217 insertions(+)
> > > >   create mode 100644 net/qrtr/mhi.c
> > > > 
> > > > diff --git a/net/qrtr/Kconfig b/net/qrtr/Kconfig
> > > > index 63f89cc6e82c..8eb876471564 100644
> > > > --- a/net/qrtr/Kconfig
> > > > +++ b/net/qrtr/Kconfig
> > > > @@ -29,4 +29,11 @@ config QRTR_TUN
> > > >   	  implement endpoints of QRTR, for purpose of tunneling data to other
> > > >   	  hosts or testing purposes.
> > > > +config QRTR_MHI
> > > > +	tristate "MHI IPC Router channels"
> > > > +	depends on MHI_BUS
> > > > +	help
> > > > +	  Say Y here to support MHI based ipcrouter channels. MHI is the
> > > > +	  transport used for communicating to external modems.
> > > > +
> > > >   endif # QRTR
> > > > diff --git a/net/qrtr/Makefile b/net/qrtr/Makefile
> > > > index 1c6d6c120fb7..3dc0a7c9d455 100644
> > > > --- a/net/qrtr/Makefile
> > > > +++ b/net/qrtr/Makefile
> > > > @@ -5,3 +5,5 @@ obj-$(CONFIG_QRTR_SMD) += qrtr-smd.o
> > > >   qrtr-smd-y	:= smd.o
> > > >   obj-$(CONFIG_QRTR_TUN) += qrtr-tun.o
> > > >   qrtr-tun-y	:= tun.o
> > > > +obj-$(CONFIG_QRTR_MHI) += qrtr-mhi.o
> > > > +qrtr-mhi-y	:= mhi.o
> > > > diff --git a/net/qrtr/mhi.c b/net/qrtr/mhi.c
> > > > new file mode 100644
> > > > index 000000000000..90af208f34c1
> > > > --- /dev/null
> > > > +++ b/net/qrtr/mhi.c
> > > > @@ -0,0 +1,208 @@
> > > > +// SPDX-License-Identifier: GPL-2.0
> > > > +/*
> > > > + * Copyright (c) 2018-2020, The Linux Foundation. All rights reserved.
> > > > + */
> > > > +
> > > > +#include <linux/mhi.h>
> > > > +#include <linux/mod_devicetable.h>
> > > > +#include <linux/module.h>
> > > > +#include <linux/skbuff.h>
> > > > +#include <net/sock.h>
> > > > +
> > > > +#include "qrtr.h"
> > > > +
> > > > +struct qrtr_mhi_dev {
> > > > +	struct qrtr_endpoint ep;
> > > > +	struct mhi_device *mhi_dev;
> > > > +	struct device *dev;
> > > > +	spinlock_t ul_lock;		/* lock to protect ul_pkts */
> > > > +	struct list_head ul_pkts;
> > > > +	atomic_t in_reset;
> > > > +};
> > > > +
> > > > +struct qrtr_mhi_pkt {
> > > > +	struct list_head node;
> > > > +	struct sk_buff *skb;
> > > > +	struct kref refcount;
> > > > +	struct completion done;
> > > > +};
> > > > +
> > > > +static void qrtr_mhi_pkt_release(struct kref *ref)
> > > > +{
> > > > +	struct qrtr_mhi_pkt *pkt = container_of(ref, struct qrtr_mhi_pkt,
> > > > +						refcount);
> > > > +	struct sock *sk = pkt->skb->sk;
> > > > +
> > > > +	consume_skb(pkt->skb);
> > > > +	if (sk)
> > > > +		sock_put(sk);
> > > > +
> > > > +	kfree(pkt);
> > > > +}
> > > > +
> > > > +/* From MHI to QRTR */
> > > > +static void qcom_mhi_qrtr_dl_callback(struct mhi_device *mhi_dev,
> > > > +				      struct mhi_result *mhi_res)
> > > > +{
> > > > +	struct qrtr_mhi_dev *qdev = dev_get_drvdata(&mhi_dev->dev);
> > > > +	int rc;
> > > > +
> > > > +	if (!qdev || mhi_res->transaction_status)
> > > > +		return;
> > > > +
> > > > +	rc = qrtr_endpoint_post(&qdev->ep, mhi_res->buf_addr,
> > > > +				mhi_res->bytes_xferd);
> > > > +	if (rc == -EINVAL)
> > > > +		dev_err(qdev->dev, "invalid ipcrouter packet\n");
> > > 
> > > Perhaps this should be a debug print, perhaps rate limited. But either
> > > way it's relevant for any transport, so I think you should skip it here
> > > - and potentially move it into qrtr_endpoint_post() in some form.
> > > 
> > 
> > I agree with moving this to qrtr_endpoint_post() but I don't think it is a
> > good idea to make it as a debug print. It is an error log and should stay
> > as it is. Only in this MHI transport driver, the return value is not getting
> > used but in others it is.
> > 
> 
> This print has been useful for catching transport errors. Only issue I see
> with moving this to qrtr_endpoint_post() is that there are multiple points
> of failure in qrtr_endpoint_post() and logging it here makes it easier.
> 
> > > > +}
> > > > +
> > > > +/* From QRTR to MHI */
> > > > +static void qcom_mhi_qrtr_ul_callback(struct mhi_device *mhi_dev,
> > > > +				      struct mhi_result *mhi_res)
> > > > +{
> > > > +	struct qrtr_mhi_dev *qdev = dev_get_drvdata(&mhi_dev->dev);
> > > > +	struct qrtr_mhi_pkt *pkt;
> > > > +	unsigned long flags;
> > > > +
> > > > +	spin_lock_irqsave(&qdev->ul_lock, flags);
> > > > +	pkt = list_first_entry(&qdev->ul_pkts, struct qrtr_mhi_pkt, node);
> > > > +	list_del(&pkt->node);
> > > > +	complete_all(&pkt->done);
> > > 
> > > You should be able to release the lock after popping the item off the
> > > list, then complete and refcount it.
> > > 
> > 
> > Okay.
> > 
> > > > +
> > > > +	kref_put(&pkt->refcount, qrtr_mhi_pkt_release);
> > > > +	spin_unlock_irqrestore(&qdev->ul_lock, flags);
> > > > +}
> > > > +
> > > > +static void qcom_mhi_qrtr_status_callback(struct mhi_device *mhi_dev,
> > > > +					  enum mhi_callback mhi_cb)
> > > > +{
> > > > +	struct qrtr_mhi_dev *qdev = dev_get_drvdata(&mhi_dev->dev);
> > > > +	struct qrtr_mhi_pkt *pkt;
> > > > +	unsigned long flags;
> > > > +
> > > > +	if (mhi_cb != MHI_CB_FATAL_ERROR)
> > > > +		return;
> > > > +
> > > > +	atomic_inc(&qdev->in_reset);
> > > 
> > > You have ul_lock close at hand in both places where you access in_reset,
> > > so I think it would be better to just use that, instead of an atomic.
> > > 
> > 
> > Okay.
> > 
> 
> Does this version of MHI give the MHI_CB_FATAL_ERROR as a status callback?
> We added this as an early notifier workaround. The in_reset code probably
> isn't required with the current feature set.
> 

Err, NO. I got confused with mhi_cntrl->status_cb which gives MHI_CB_FATAL_ERROR
to the controller drivers. But there is no MHI_CB_FATAL_ERROR provided for the
client drivers like this one.

Even in downstream (msm-4.14), I can't find where the early notifier is getting
called.

> > > > +	spin_lock_irqsave(&qdev->ul_lock, flags);
> > > > +	list_for_each_entry(pkt, &qdev->ul_pkts, node)
> > > > +		complete_all(&pkt->done);
> > 
> > Chris, shouldn't we require list_del(&pkt->node) here?
> > 
> 
> No this isn't a full cleanup, with the "early notifier" we just unblocked
> any threads waiting for the ul_callback. Those threads will wake, check
> in_reset, return an error back to the caller. Any list cleanup will be done
> in the ul_callbacks that the mhi bus will do for each queued packet right
> before device remove.
> 
> Again to simplify the code, we can probable remove the in_reset handling
> since it's not required with the current feature set.
> 

So since we are not getting status_cb for fatal errors, I think we should just
remove status_cb, in_reset and timeout code.

> > > > +	spin_unlock_irqrestore(&qdev->ul_lock, flags);
> > > > +}
> > > > +
> > > > +/* Send data over MHI */
> > > > +static int qcom_mhi_qrtr_send(struct qrtr_endpoint *ep, struct sk_buff *skb)
> > > > +{
> > > > +	struct qrtr_mhi_dev *qdev = container_of(ep, struct qrtr_mhi_dev, ep);
> > > > +	struct qrtr_mhi_pkt *pkt;
> > > > +	int rc;
> > > > +
> > > > +	rc = skb_linearize(skb);
> > > > +	if (rc) {
> > > > +		kfree_skb(skb);
> > > > +		return rc;
> > > > +	}
> > > > +
> > > > +	pkt = kzalloc(sizeof(*pkt), GFP_KERNEL);
> > > > +	if (!pkt) {
> > > > +		kfree_skb(skb);
> > > > +		return -ENOMEM;
> > > > +	}
> > > > +
> > > > +	init_completion(&pkt->done);
> > > > +	kref_init(&pkt->refcount);
> > > > +	kref_get(&pkt->refcount);
> > > > +	pkt->skb = skb;
> > > > +
> > > > +	spin_lock_bh(&qdev->ul_lock);
> > > > +	list_add_tail(&pkt->node, &qdev->ul_pkts);
> > > > +	rc = mhi_queue_skb(qdev->mhi_dev, DMA_TO_DEVICE, skb, skb->len,
> > > > +			   MHI_EOT);
> > > 
> > > Do you want to continue doing this when qdev->in_reset? Wouldn't it be
> > > better to bail early if the remote end is dying?
> > > 
> > 
> > Now I'm thinking why we are not decrementing in_reset anywhere! Incase of
> > SYS_ERR, the status_cb will get processed and in_reset will be set but
> > it will stay so even when after MHI gets reset.
> > 
> > Chris, can you please clarify?
> > 
> 
> Early notify of reset, if we do get SYS_ERR, we expect remove to be called
> *soon*.
> 
> > > > +	if (rc) {
> > > > +		list_del(&pkt->node);
> > > > +		/* Reference count needs to be dropped 2 times */
> > > > +		kref_put(&pkt->refcount, qrtr_mhi_pkt_release);
> > > > +		kref_put(&pkt->refcount, qrtr_mhi_pkt_release);
> > > > +		kfree_skb(skb);
> > > > +		spin_unlock_bh(&qdev->ul_lock);
> > > > +		return rc;
> > > > +	}
> > > > +
> > > > +	spin_unlock_bh(&qdev->ul_lock);
> > > > +	if (skb->sk)
> > > > +		sock_hold(skb->sk);
> > > > +
> > > > +	rc = wait_for_completion_interruptible_timeout(&pkt->done, HZ * 5);
> > > > +	if (atomic_read(&qdev->in_reset))
> > > > +		rc = -ECONNRESET;
> > > > +	else if (rc == 0)
> > > > +		rc = -ETIMEDOUT;
> > > 
> > > Is this recoverable? The message will remain on the list and may be
> > > delivered at a later point(?), but qrtr and the app will learn that the
> > > message was lost - which is presumably considered fatal.
> > > 
> > > Is it guaranteed that qcom_mhi_qrtr_ul_callback() will happen later and
> > > find the head of the list?
> > > 
> > 
> > There are 2 scenarios:
> > 
> > 1. If the completion interrupt happens after timeout, ul_callback()
> > will be called. But it will only try to fetch the current head of ul_pkts.
> > In most cases, we can hope that the completion interrupt will happen before
> > next queue_skb().
> > 
> > 2. If we don't get completion interrupt, timeout will happen and at the final
> > stage (during mhi_driver_remove()), MHI stack will go over the pending TREs
> > for all channels in queue and call ul_callback() with -ENOTCONN. But in the
> > callback, we don't have any idea of the pkt which was not successfully
> > transferred to the device and currently just fetching first entry.
> > 
> > Now I'm seeing some issue here which I missed earlier. If the completion
> > interrupt never happens then the corresponding pkt will never get freed and
> > therefore we have a leak. Eventhough the ul_callback() will get called during
> > mhi_driver_remove() for pending TREs, we don't exactly fetch the right pkt.
> > 
> > Chris, our assumption of the ul_callback() gets called irrespective of
> > transfer status is wrong. I think this code needs a bit of rework.
> > 

All fine here. I overlooked the list_del() part where the sent packet
gets popped out of list and when the MHI stack calls ul_callback() during
mhi_driver_remove() the leftover packets will get handled correctly.

> > > 
> > > The reason for my question is that without this you have one of two
> > > scenarios;
> > > 1) the message is put on the list, decremented in
> > > qcom_mhi_qrtr_ul_callback() then we get back here and decrement it
> > > again.
> > > 2) the message is put on the list, then qcom_mhi_qrtr_status_callback()
> > > happens and all messages are released - presumably then
> > > qcom_mhi_qrtr_ul_callback() won't happen.
> > > 
> > > 
> > > So if the third case (where we return here and then later
> > > qcom_mhi_qrtr_ul_callback() must find this particular packet at the
> > > front of the queue) can't happen, then you can just skip the entire
> > > refcounting.
> > > 
> > > Further more, you could carry qrtr_mhi_pkt on the stack.
> > > 
> > > 
> > > ...or to flip this around, is there a reason to wait here at all? What
> > > would happen if you just return immediately after calling
> > > mhi_queue_skb()? Wouldn't that provide you better throughput?
> > > 
> > 
> > Chris would be best person to answer this question.
> > 
> 
> Yea - after working with MHI for a bit now, I think we can definitely return
> after queueing the skb.
> 
> The IPC-Router Protocol, wasn't really designed with the ability to recover
> from dropped packets since SMD/GLINK are "Lossless". I figured it would be
> better for the client to definitely know that the packet reached the other
> side which is why it blocks until ul callback.
> 

Agree.

> I thought having the client get an error on timeout and resend the packet
> would be better than silently dropping it. In practice, we've really only
> seen the timeout or ul_callback errors on unrecoverable errors so I think
> the timeout handling can definitely be redone.
> 

You mean we can just remove the timeout handling part and return after
kref_put()?

Thanks,
Mani

> Thanks,
> Chris
> 
> > > > +	else if (rc > 0)
> > > > +		rc = 0;
> > > > +
> > > > +	kref_put(&pkt->refcount, qrtr_mhi_pkt_release);
> > > > +
> > > > +	return rc;
> > > > +}
> > > > +
> > > > +static int qcom_mhi_qrtr_probe(struct mhi_device *mhi_dev,
> > > > +			       const struct mhi_device_id *id)
> > > > +{
> > > > +	struct qrtr_mhi_dev *qdev;
> > > > +	u32 net_id;
> > > > +	int rc;
> > > > +
> > > > +	qdev = devm_kzalloc(&mhi_dev->dev, sizeof(*qdev), GFP_KERNEL);
> > > > +	if (!qdev)
> > > > +		return -ENOMEM;
> > > > +
> > > > +	qdev->mhi_dev = mhi_dev;
> > > > +	qdev->dev = &mhi_dev->dev;
> > > > +	qdev->ep.xmit = qcom_mhi_qrtr_send;
> > > > +	atomic_set(&qdev->in_reset, 0);
> > > > +
> > > > +	net_id = QRTR_EP_NID_AUTO;
> > > 
> > > Just pass QRTR_EP_NID_AUTO directly in the function call below.
> > > 
> > 
> > Okay.
> > 
> > Thanks,
> > Mani
> > 
> > > Regards,
> > > Bjorn
> > > 
> > > > +
> > > > +	INIT_LIST_HEAD(&qdev->ul_pkts);
> > > > +	spin_lock_init(&qdev->ul_lock);
> > > > +
> > > > +	dev_set_drvdata(&mhi_dev->dev, qdev);
> > > > +	rc = qrtr_endpoint_register(&qdev->ep, net_id);
> > > > +	if (rc)
> > > > +		return rc;
> > > > +
> > > > +	dev_dbg(qdev->dev, "Qualcomm MHI QRTR driver probed\n");
> > > > +
> > > > +	return 0;
> > > > +}
> > > > +
> > > > +static void qcom_mhi_qrtr_remove(struct mhi_device *mhi_dev)
> > > > +{
> > > > +	struct qrtr_mhi_dev *qdev = dev_get_drvdata(&mhi_dev->dev);
> > > > +
> > > > +	qrtr_endpoint_unregister(&qdev->ep);
> > > > +	dev_set_drvdata(&mhi_dev->dev, NULL);
> > > > +}
> > > > +
> > > > +static const struct mhi_device_id qcom_mhi_qrtr_id_table[] = {
> > > > +	{ .chan = "IPCR" },
> > > > +	{}
> > > > +};
> > > > +MODULE_DEVICE_TABLE(mhi, qcom_mhi_qrtr_id_table);
> > > > +
> > > > +static struct mhi_driver qcom_mhi_qrtr_driver = {
> > > > +	.probe = qcom_mhi_qrtr_probe,
> > > > +	.remove = qcom_mhi_qrtr_remove,
> > > > +	.dl_xfer_cb = qcom_mhi_qrtr_dl_callback,
> > > > +	.ul_xfer_cb = qcom_mhi_qrtr_ul_callback,
> > > > +	.status_cb = qcom_mhi_qrtr_status_callback,
> > > > +	.id_table = qcom_mhi_qrtr_id_table,
> > > > +	.driver = {
> > > > +		.name = "qcom_mhi_qrtr",
> > > > +	},
> > > > +};
> > > > +
> > > > +module_mhi_driver(qcom_mhi_qrtr_driver);
> > > > +
> > > > +MODULE_DESCRIPTION("Qualcomm IPC-Router MHI interface driver");
> > > > +MODULE_LICENSE("GPL v2");
> > > > -- 
> > > > 2.17.1
> > > > 
> 
> -- 
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
> Foundation Collaborative Project

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