Date:   Fri, 18 Jun 2021 13:35:43 +0200
From:   Arnaud POULIQUEN <arnaud.pouliquen@...s.st.com>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
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

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.

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.


> 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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