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Date:   Tue, 22 Jun 2021 14:30:19 -0600
From:   Mathieu Poirier <mathieu.poirier@linaro.org>
To:     Arnaud POULIQUEN <arnaud.pouliquen@foss.st.com>
Cc:     Bjorn Andersson <bjorn.andersson@linaro.org>,
        Ohad Ben-Cohen <ohad@wizery.com>,
        linux-remoteproc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-stm32@st-md-mailman.stormreply.com, julien.massot@iot.bzh
Subject: Re: [PATCH 3/4] rpmsg: char: Introduce the "rpmsg-raw" channel
Message-ID: <20210622203019.GB1006507@p14s>
References: <20210607173032.30133-1-arnaud.pouliquen@foss.st.com>
 <20210607173032.30133-4-arnaud.pouliquen@foss.st.com>
 <20210615200102.GE604521@p14s>
 <b55cd4e5-fb9d-a0ab-03a9-3a771898db04@foss.st.com>
 <20210617213154.GA790564@p14s>
 <d8e81ecd-c77d-9d16-7e43-218bd54a9f83@foss.st.com>
 <20210621223852.GA980846@p14s>
 <b293bc1c-5f80-6a4c-4d9b-57bbada01958@foss.st.com>
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On Tue, Jun 22, 2021 at 10:21:02AM +0200, Arnaud POULIQUEN wrote:
> 
> 
> On 6/22/21 12:38 AM, Mathieu Poirier wrote:
> > On Fri, Jun 18, 2021 at 01:35:43PM +0200, Arnaud POULIQUEN wrote:
> >> Hi Mathieu,
> >>
> >> On 6/17/21 11:31 PM, Mathieu Poirier wrote:
> >>> On Wed, Jun 16, 2021 at 02:38:26PM +0200, Arnaud POULIQUEN wrote:
> >>>> Hi Mathieu,
> >>>>
> >>>> On 6/15/21 10:01 PM, Mathieu Poirier wrote:
> >>>>> On Mon, Jun 07, 2021 at 07:30:31PM +0200, Arnaud Pouliquen wrote:
> >>>>>> Allows to probe the endpoint device on a remote name service announcement,
> >>>>>> by registering a rpmsg_driverfor the "rpmsg-raw" channel.
> >>>>>>
> >>>>>> With this patch the /dev/rpmsgX interface can be instantiated by the remote
> >>>>>> firmware.
> >>>>>>
> >>>>>> Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
> >>>>>> ---
> >>>>>>  drivers/rpmsg/rpmsg_char.c | 54 ++++++++++++++++++++++++++++++++++++--
> >>>>>>  1 file changed, 52 insertions(+), 2 deletions(-)
> >>>>>>
> >>>>>> diff --git a/drivers/rpmsg/rpmsg_char.c b/drivers/rpmsg/rpmsg_char.c
> >>>>>> index 4199ac1bee10..3b850b218eb0 100644
> >>>>>> --- a/drivers/rpmsg/rpmsg_char.c
> >>>>>> +++ b/drivers/rpmsg/rpmsg_char.c
> >>>>>> @@ -25,6 +25,8 @@
> >>>>>>  
> >>>>>>  #include "rpmsg_char.h"
> >>>>>>  
> >>>>>> +#define RPMSG_CHAR_DEVNAME "rpmsg-raw"
> >>>>>> +
> >>>>>>  static dev_t rpmsg_major;
> >>>>>>  static struct class *rpmsg_class;
> >>>>>>  
> >>>>>> @@ -416,6 +418,40 @@ int rpmsg_chrdev_eptdev_create(struct rpmsg_device *rpdev, struct device *parent
> >>>>>>  }
> >>>>>>  EXPORT_SYMBOL(rpmsg_chrdev_eptdev_create);
> >>>>>>  
> >>>>>> +static int rpmsg_chrdev_probe(struct rpmsg_device *rpdev)
> >>>>>> +{
> >>>>>> +	struct rpmsg_channel_info chinfo;
> >>>>>> +
> >>>>>> +	memcpy(chinfo.name, RPMSG_CHAR_DEVNAME, sizeof(RPMSG_CHAR_DEVNAME));
> >>>>>> +	chinfo.src = rpdev->src;
> >>>>>> +	chinfo.dst = rpdev->dst;
> >>>>>> +
> >>>>>> +	return __rpmsg_chrdev_eptdev_create(rpdev, &rpdev->dev, chinfo, true);
> >>>>>
> >>>>> I am a little puzzled here as to why we need different modes... Why can't we
> >>>>> simply call rpmsg_chrdev_eptdev_create() and let the endpoint be created on
> >>>>> open() and destroyed on release() as per the current implementation?
> >>>>
> >>>> The main reason is the support of the NS announcement
> >>>> a NS announcement is received from the remote processor:
> >>>> channel name: "rpmsg-raw"
> >>>> remote address (dst address): 0x400
> >>>> local address (scr address) : RPMSG_ADDR_ANY
> >>>> => no default endpoint, and not local address.
> >>>>
> >>>> case 1) if we use legacy implementation ( no default endpoint)
> >>>> => create/destroy endpoint on open/stop
> >>>> - on first open: created endpoint is bound to scr address 0x406
> >>>> - a first message is sent to the remote side, the address 0x406 is stored as
> >>>> default channel dst address on remote side.
> >>>> - on close: endpoint is closed and associated address 0x406 is free.
> >>>> - another driver create an enpoint the address 0x406 is reserved for this new
> >>>> endpoint.
> >>>> - on new open:  scr address is set to next value 0x407
> >>>> => how to inform remote processor that the address has changed?
> >>>> => no reservation mechanism that ensure that you can reuse the same address
> >>>>
> >>>> case 2) relying on use_default_ept
> >>>> => Ensure that both side have always the same addresses to communicate.
> >>>
> >>> I see the problem and your solution is adequate - I think the code simply needs
> >>> to be moved around a little.  Here is what I suggest:
> >>>
> >>> 1) Create the endpoint in rpmsg_chrdev_probe(), just before calling
> >>> rpmsg_chrdev_eptdev_create().  That way changes to rpmsg_eptdev_open() can be
> >>> kept to a minimum.  I don't think we'll be needing
> >>> __rpmsg_chrdev_eptdev_create() anymore.
> >>
> >> Yes i could, but this will break a concept of the rpmsg_char that creates the
> >> endpoint on open, meaning that application is ready to communicate.
> > 
> > In my opinion creating and destorying an endpoint on open/close is something we
> > want to move away from. 
> 
> Not simple to answer... As discussed a mechanism is requested by some developer
> to be able on a ns announcement to inform the remote side that the user
> application or the client driver is ready to communicate, the endpoint creation
> could be the trigger.
> 
> That said, let's go by steps. For this patchset I will try to come back to my
> first implementation where i created the endpoint on probe.

Going back to that conversation I realise the directions I gave out on that
front was not optimal.  As you mention above please go back to creating the
endpoint on probe().

> 
> > 
> >>
> >> I would rather preserve this behavior.
> >>
> >>>
> >>> 2) We can get rid of use_default_ept by taking advantage of the fact that the
> >>> rpmsg_char driver does not use rpmsg_device::ept.  If we create the endpoint in
> >>> rpmsg_chrdev_probe() we know that if rpdev->ept exists, we must not create
> >>> or destroy the endpoint in rpmsg_eptdev_open() and rpmsg_eptdev_release().
> >>>
> >>> 3) Function rpmsg_eptdev_open() doesn't change much.  If rpdev->ept is NULL
> >>> than
> >>> an endpoint is created as the current implementation.  Otherwise we simply do:
> >>>
> >>>         eptdev->ept = rpdev->ept;
> >>>
> >>
> >> In qcom_glink_create_chrdev, a rpmsg_ctrl rpdev with a default endpoint is
> >> created and used as parameter of the  pmsg_ctrldev_register_device [1]
> >> => rpdev->ept is not NULL.
> >>
> >> So the rpmsg_char has to differentiate 2 cases on rpmsg_eptdev_open:
> >> - A enpdoint has to be created as requested by RPMSG_CREATE_EPT_IOCTL
> >> (regardless of the rpdev->ept value)
> >> - for a rpmsg device created by an NS announcement: A default endpoint has to be
> >> reused (or created if rpdev->ept is null).
> >>
> >> so the rpdev->ept test is not relevant for decision, the use_default_ept ( or
> >> another flag) is mandatory.
> > 
> > Yes, we need a flag.  May I suggest "fixed_ept" rather than "used_default_ept"?
> 
> "fixed_ept" could be miss-understood . It can be interpreted as an endpoint with
> a fixed address (not set to RPMSG_ADDR_ANY).
> What about "default_ept" or "static_ept"?

I'll take static_ept.

> 
> Thanks
> Arnaud
> 
> > 
> >>
> >>
> >>> 4) Make sure the teardown path works as well.  From what I can see, it should.
> >>>
> >>> 5) Add a __lot__ of comments.
> >>>
> >>> If the above all works this entire patchset should become really small.
> >>
> >> Thanks,
> >> Arnaud
> >>
> >>>
> >>>>
> >>>>>
> >>>>> I'd rather keep things simple for the refactoring and introduce new features
> >>>>> later if need be.
> >>>>
> >>>> Yes I agree with you, but here it could become a nightmare for the remote
> >>>> processor if the Linux endpoint address is not stable.
> >>>>
> >>>> Anyway we can consider this as a workaround waiting the extension of the NS
> >>>> announcement to have a better management of the address exchange on channel
> >>>> initialization.
> >>>>
> >>>> Thanks
> >>>> Arnaud
> >>>>
> >>>>>
> >>>>> As I said, it may be that I don't understand the usecase.
> >>>>>
> >>>>> Thanks,
> >>>>> Mathieu
> >>>>>
> >>>>>> +}
> >>>>>> +
> >>>>>> +static void rpmsg_chrdev_remove(struct rpmsg_device *rpdev)
> >>>>>> +{
> >>>>>> +	int ret;
> >>>>>> +
> >>>>>> +	ret = device_for_each_child(&rpdev->dev, NULL, rpmsg_chrdev_eptdev_destroy);
> >>>>>> +	if (ret)
> >>>>>> +		dev_warn(&rpdev->dev, "failed to destroy endpoints: %d\n", ret);
> >>>>>> +}
> >>>>>> +
> >>>>>> +static struct rpmsg_device_id rpmsg_chrdev_id_table[] = {
> >>>>>> +	{ .name	= RPMSG_CHAR_DEVNAME },
> >>>>>> +	{ },
> >>>>>> +};
> >>>>>> +
> >>>>>> +static struct rpmsg_driver rpmsg_chrdev_driver = {
> >>>>>> +	.probe = rpmsg_chrdev_probe,
> >>>>>> +	.remove = rpmsg_chrdev_remove,
> >>>>>> +	.id_table = rpmsg_chrdev_id_table,
> >>>>>> +	.drv = {
> >>>>>> +		.name = "rpmsg_chrdev",
> >>>>>> +	},
> >>>>>> +};
> >>>>>> +
> >>>>>>  static int rpmsg_chrdev_init(void)
> >>>>>>  {
> >>>>>>  	int ret;
> >>>>>> @@ -429,16 +465,30 @@ static int rpmsg_chrdev_init(void)
> >>>>>>  	rpmsg_class = class_create(THIS_MODULE, "rpmsg");
> >>>>>>  	if (IS_ERR(rpmsg_class)) {
> >>>>>>  		pr_err("failed to create rpmsg class\n");
> >>>>>> -		unregister_chrdev_region(rpmsg_major, RPMSG_DEV_MAX);
> >>>>>> -		return PTR_ERR(rpmsg_class);
> >>>>>> +		ret = PTR_ERR(rpmsg_class);
> >>>>>> +		goto free_region;
> >>>>>> +	}
> >>>>>> +
> >>>>>> +	ret = register_rpmsg_driver(&rpmsg_chrdev_driver);
> >>>>>> +	if (ret < 0) {
> >>>>>> +		pr_err("rpmsg: failed to register rpmsg raw driver\n");
> >>>>>> +		goto free_class;
> >>>>>>  	}
> >>>>>>  
> >>>>>>  	return 0;
> >>>>>> +
> >>>>>> +free_class:
> >>>>>> +	class_destroy(rpmsg_class);
> >>>>>> +free_region:
> >>>>>> +	unregister_chrdev_region(rpmsg_major, RPMSG_DEV_MAX);
> >>>>>> +
> >>>>>> +	return ret;
> >>>>>>  }
> >>>>>>  postcore_initcall(rpmsg_chrdev_init);
> >>>>>>  
> >>>>>>  static void rpmsg_chrdev_exit(void)
> >>>>>>  {
> >>>>>> +	unregister_rpmsg_driver(&rpmsg_chrdev_driver);
> >>>>>>  	class_destroy(rpmsg_class);
> >>>>>>  	unregister_chrdev_region(rpmsg_major, RPMSG_DEV_MAX);
> >>>>>>  }
> >>>>>> -- 
> >>>>>> 2.17.1
> >>>>>>