Date:   Thu, 17 Jun 2021 15:31:54 -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 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.

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;

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.

> 
> > 
> > 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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