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Date:   Fri, 8 Jan 2021 16:52:10 +0000
From:   Cristian Marussi <cristian.marussi@....com>
To:     Thara Gopinath <thara.gopinath@...aro.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        sudeep.holla@....com, lukasz.luba@....com,
        james.quinlan@...adcom.com, Jonathan.Cameron@...wei.com,
        f.fainelli@...il.com, etienne.carriere@...aro.org,
        vincent.guittot@...aro.org, souvik.chakravarty@....com
Subject: Re: [PATCH v4 37/37] firmware: arm_scmi: add dynamic scmi devices
 creation

On Fri, Jan 08, 2021 at 11:32:17AM -0500, Thara Gopinath wrote:
> 
> 
> On 1/8/21 9:42 AM, Cristian Marussi wrote:
> > On Thu, Jan 07, 2021 at 09:28:07AM -0500, Thara Gopinath wrote:
> > > Hi Christian,
> > > 
> > > On 1/6/21 3:16 PM, Cristian Marussi wrote:
> > > > Having added the support for SCMI protocols as modules in order to let
> > > > vendors extend the SCMI core with their own additions it seems odd to
> > > > then force SCMI drivers built on top to use a static device table to
> > > > declare their devices since this way any new SCMI drivers addition
> > > > would need the core SCMI device table to be updated too.
> > > > 
> > > > Remove the static core device table and let SCMI drivers to simply declare
> > > > which device/protocol pair they need at initialization time: the core will
> > > > then take care to generate such devices dynamically during platform
> > > > initialization or at module loading time, as long as the requested
> > > > underlying protocol is defined in the DT.
> > > > 
> > > > Signed-off-by: Cristian Marussi <cristian.marussi@....com>
> > > > ---
> > > 	
> > > [snip]
> > > 
> > > > -static inline void
> > > > -scmi_create_protocol_devices(struct device_node *np, struct scmi_info *info,
> > > > -			     int prot_id)
> > > > +	for (; rdev; rdev = rdev->next)
> > > > +		scmi_create_protocol_device(np, info, prot_id,
> > > > +					    rdev->id_table->name);
> > > > +}
> > > > +
> > > > +/**
> > > > + * scmi_request_protocol_device  - Helper to request a device
> > > > + *
> > > > + * @id_table: A protocol/name pair descriptor for the device to be created.
> > > > + *
> > > > + * This helper let an SCMI driver request specific devices identified by the
> > > > + * @id_table to be created for each active SCMI instance.
> > > > + *
> > > > + * The requested device name MUST NOT be already existent for any protocol;
> > > > + * at first the freshly requested @id_table is annotated in the IDR table
> > > > + * @scmi_requested_devices, then a matching device is created for each already
> > > > + * active SCMI instance. (if any)
> > > > + *
> > > > + * This way the requested device is created straight-away for all the already
> > > > + * initialized(probed) SCMI instances (handles) but it remains instead pending
> > > > + * for creation if the requesting SCMI driver is loaded before some instance
> > > > + * and related transports was available: when such late SCMI instance is probed
> > > > + * it will take care to scan the list of pending requested devices and create
> > > > + * those on its own (see @scmi_create_protocol_devices and its enclosing loop)
> > > > + *
> > > > + * Return: 0 on Success
> > > > + */
> > > > +int scmi_request_protocol_device(const struct scmi_device_id *id_table)
> > > >    {
> > > > -	int loop, cnt;
> > > > +	int ret = 0;
> > > > +	unsigned int id = 0;
> > > > +	struct scmi_requested_dev *rdev, *proto_rdev = NULL;
> > > > +	struct scmi_info *info;
> > > > -	for (loop = 0; loop < ARRAY_SIZE(devnames); loop++) {
> > > > -		if (devnames[loop].protocol_id != prot_id)
> > > > -			continue;
> > > > +	pr_debug("Requesting SCMI device (%s) for protocol %x\n",
> > > > +		 id_table->name, id_table->protocol_id);
> > > > -		for (cnt = 0; cnt < ARRAY_SIZE(devnames[loop].names); cnt++) {
> > > > -			const char *name = devnames[loop].names[cnt];
> > > > +	/*
> > > > +	 * Search for the matching protocol rdev list and then search
> > > > +	 * of any existent equally named device...fails if any duplicate found.
> > > > +	 */
> > > > +	mutex_lock(&scmi_requested_devices_mutex);
> > > > +	idr_for_each_entry(&scmi_requested_devices, rdev, id) {
> > > > +		if (rdev->id_table->protocol_id == id_table->protocol_id)
> > > > +			proto_rdev = rdev;
> > > > +		for (; rdev; rdev = rdev->next) {
> > > > +			if (!strcmp(rdev->id_table->name, id_table->name)) {
> > > > +				pr_err("Ignoring duplicate request [%d] %s\n",
> > > > +				       rdev->id_table->protocol_id,
> > > > +				       rdev->id_table->name);
> > > > +				ret = -EINVAL;
> > > > +				goto out;
> > > > +			}
> > > 	Shouldn't there be proto_rdev = rdev here as well ?
> > > 
> > 
> > No, because each IDR entry points to one or more linked rdev descriptors
> > for the same protocol: while scanning each list in the IDR table I'm
> > searching for the proto_rdev representing the head of that protocol list
> > (if any already exist) and also scan all the lists fully to check for
> > duplicates, in such a case we give up.
> > The IDR map containing list resembles a lot a Linux hash implementation
> > but I decided not to use it because it seemed cumbersome to use an
> > hash given most of the time each IDR entry will contain just one single
> > element and this lookup happens really very infrequently (just at driver
> > loading time)
> 
> I agree that using hash might be overkill here.
> I still think you need proto_rdev = rdev so that proto_rdev points to the
> last element and not the head. Else later on, below when you do
> 	proto_rdev->next = rdev;
> you will lose devices.
> 
> Basically in the current implementation if there are more than two devices
> for a protocol, you will end up losing devices since you are adding the new
> device as the second device always.
> 
Ah right I missed that sorry...now I'm definitely convinced to use klist heads
in the IDRs and avoid all of this :D

> I think like you mentioned this should be a klist instead of a custom linked
> list. And idr can keep track of head of each list.
> 
> > 
> > > > +		}
> > > > +	}
> > > > +
> > > > +	/*
> > > > +	 * No duplicate found for requested id_table, so let's create a new
> > > > +	 * requested device entry for this new valid request.
> > > > +	 */
> > > > +	rdev = kzalloc(sizeof(*rdev), GFP_KERNEL);
> > > > +	if (!rdev) {
> > > > +		ret = -ENOMEM;
> > > > +		goto out;
> > > > +	}
> > > > +	rdev->id_table = id_table;
> > > > +
> > > > +	/*
> > > > +	 * Append the new requested device table descriptor to the head of the
> > > > +	 * related protocol chain, eventually creating such chain if not already
> > > > +	 * there.
> > > > +	 */
> > > > +	if (!proto_rdev) {
> > > > +		ret = idr_alloc(&scmi_requested_devices, (void *)rdev,
> > > > +				rdev->id_table->protocol_id,
> > > > +				rdev->id_table->protocol_id + 1, GFP_KERNEL);
> > > > +		if (ret != rdev->id_table->protocol_id) {
> > > > +			pr_err("Failed to save SCMI device - ret:%d\n", ret);
> > > > +			kfree(rdev);
> > > > +			ret = -EINVAL;
> > > > +			goto out;
> > > > +		}
> > > > +		ret = 0;
> > > > +	} else {
> > > > +		proto_rdev->next = rdev;
> > > > +	}
> > > > +
> > > > +	/*
> > > > +	 * Now effectively create and initialize the requested device for every
> > > > +	 * already initialized SCMI instance which has registered the requested
> > > > +	 * protocol as a valid active one: i.e. defined in DT and supported by
> > > > +	 * current platform FW.
> > > > +	 */
> > > > +	mutex_lock(&scmi_list_mutex);
> > > > +	list_for_each_entry(info, &scmi_list, node) {
> > > > +		struct device_node *child;
> > > > +
> > > > +		child = idr_find(&info->active_protocols,
> > > > +				 id_table->protocol_id);
> > > > +		if (child) {
> > > > +			struct scmi_device *sdev;
> > > > +
> > > > +			sdev = scmi_get_protocol_device(child, info,
> > > > +							id_table->protocol_id,
> > > > +							id_table->name);
> > > > +			/* Set handle if not already set (device existed) */
> > > > +			if (sdev && !sdev->handle)
> > > > +				sdev->handle = scmi_handle_get_from_info(info);
> > > > +		} else {
> > > > +			dev_err(info->dev,
> > > > +				"Failed. SCMI protocol %d not active.\n",
> > > > +				id_table->protocol_id);
> > > > +		}
> > > > +	}
> > > > +	mutex_unlock(&scmi_list_mutex);
> > > > +
> > > > +out:
> > > > +	mutex_unlock(&scmi_requested_devices_mutex);
> > > > +
> > > > +	return ret;
> > > > +}
> > > > +
> > > > +/**
> > > > + * scmi_unrequest_protocol_device  - Helper to unrequest a device
> > > > + *
> > > > + * @id_table: A protocol/name pair descriptor for the device to be unrequested.
> > > > + *
> > > > + * An helper to let an SCMI driver release its request about devices; note that
> > > > + * devices are created and initialized once the first SCMI driver request them
> > > > + * but they destroyed only on SCMI core unloading/unbinding.
> > > > + *
> > > > + * The current SCMI transport layer uses such devices as internal references and
> > > > + * as such they could be shared as same transport between multiple drivers so
> > > > + * that cannot be safely destroyed till the whole SCMI stack is removed.
> > > > + * (unless adding further burden of refcounting.)
> > > > + */
> > > > +void scmi_unrequest_protocol_device(const struct scmi_device_id *id_table)
> > > > +{
> > > > +	struct scmi_requested_dev *victim, *prev, *head;
> > > > +
> > > > +	pr_debug("Unrequesting SCMI device (%s) for protocol %x\n",
> > > > +		 id_table->name, id_table->protocol_id);
> > > > -			if (name)
> > > > -				scmi_create_protocol_device(np, info, prot_id,
> > > > -							    name);
> > > > +	head = idr_find(&scmi_requested_devices, id_table->protocol_id);
> > > > +	if (!head)
> > > > +		return;
> > > > +
> > > > +	/*
> > > > +	 * Scan the protocol list of requested device name searching
> > > > +	 * for the victim.
> > > > +	 */
> > > > +	victim = head;
> > > > +	for (prev = victim; victim; prev = victim, victim = victim->next)
> > > 
> > > 	The initial assignment for the for loop is wrong. With this when you break
> > > prev will be equal to victim. You want prev to be the one pointing to the
> > > victim. Or am I missing something?
> > > 
> > 
> > Yes prev is the one preceding the victim, if any, but if it was the head
> > I'll remove the head and not use at all the prev really.
> > I think is right as it is, it is the naming that is misleading, because
> > yes in the initial assignment prev = victim BUT victim = head, so if I bail
> > out immediately I'm really removing the head.
> > It would be clearer like
> > 
> >           prev = victim = head;
> >           for (; victim; prev = victim, victim = victim->next)
> > 	 ...
> > 
> > But it's better that I review this whole loop in deep to simplify it; I
> > avoided using klist because seemed easier enough to handle a singly
> > linked list which most of the time is one element deep, buut maybe I
> > should just stick with well known and proven kists.
> 
> Yes you are right. No bug here. Like I mentioned above, klists are something
> to consider here.
> 

Thanks, I'll switch to klist.

Cristian

> > 
> > Thanks
> > 
> > Cristian
> > 
> > > 
> > > -- 
> > > Warm Regards
> > > Thara
> 
> -- 
> Warm Regards
> Thara

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