Date:   Mon, 21 Jun 2021 16:38:52 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Arnaud POULIQUEN <arnaud.pouliquen@...s.st.com>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ohad Ben-Cohen <ohad@...ery.com>,
        linux-remoteproc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com, julien.massot@....bzh
Subject: Re: [PATCH 3/4] rpmsg: char: Introduce the "rpmsg-raw" channel

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@...s.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. 

> 
> 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"?

> 
> 
> > 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
> >>>>

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