Message-ID: <6c38682a-0ae0-408a-87a7-cce33b3e550e@collabora.com>
Date: Fri, 7 Jun 2024 11:27:28 +0200
From: Julien Massot <julien.massot@...labora.com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>,
 Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: sakari.ailus@....fi, devicetree@...r.kernel.org, kernel@...labora.com,
 linux-kernel@...r.kernel.org, mchehab@...nel.org, robh+dt@...nel.org,
 krzysztof.kozlowski+dt@...aro.org, linux-media@...r.kernel.org,
 conor+dt@...nel.org
Subject: Re: [PATCH v7 0/5] Add support for MAX96714/F and MAX96717/F GMSL2
 ser/des

Hi Laurent,

On 6/6/24 5:24 PM, Laurent Pinchart wrote:
> On Thu, Jun 06, 2024 at 04:34:19PM +0300, Tomi Valkeinen wrote:
>> Hi,
>>
>> On 30/04/2024 16:19, Julien Massot wrote:
>>> Change since v6:
>>>     - Remove mention of C-PHY for MAX96717, this serializer is D-PHY only
>>>     - Remove bus-type requirement for MAX96717
>>>     - Minor changes requested by Sakari
>>>     - Workaround a MAX96717 issue, which occurs when stopping
>>>       the CSI source before stopping the MAX96717 CSI receiver.
>>>
>>> Power management is not included in this patchset. The GMSL link is
>>> not always resuming when the deserializer is suspended without
>>> suspending the serializer.
>>>
>>> Change since v5:
>>>    - Reverse fallback logic: max9671{4,7} can fallback to max9671{4,7}F
>>>    - use const instead of enum for max9671{4,7}f compatible as suggested
>>>
>>> Change since v4:
>>>    - Add support for MAX96717 and MAX96714 and use them as a fallback for
>>>      MAX96717F and MAX96714F respectively
>>>    - The drivers are now compatible with MAX96717 and MAX96714 since no change in
>>>      the logic is needed
>>>    - Reference 'i2c-gate' instead of 'i2c-controller' in the bindings
>>>
>>> Change since v3:
>>> - bindings
>>>     - Renamed bindings to drop the 'f' suffix
>>>     - Add bus type to MAX96717 and remove from MAX9674
>>>     - Add lane-polarities to both bindings
>>>
>>> - drivers
>>>     - Address changes requested by Sakari in v3
>>>     - use v4l2_subdev_s_stream_helper for MAX96714
>>>     - do not init regmap twice in the MAX96714 driver
>>>     - Fix compilations on 32 bits platforms
>>>
>>> Change since v2:
>>> - Convert drivers to use CCI helpers
>>> - Use generic node name
>>> - Use 'powerdown' as gpio name instead of 'enable'
>>> - Add pattern generator support for MAX96714
>>>
>>> These patches add support for Maxim MAX96714F deserializer and
>>> MAX96717F serializer.
>>>
>>> MAX96714F has one GMSL2 input port and one CSI2 4 lanes output port,
>>> MAX96717F has one CSI2 input port and one GMSL2 output port.
>>>
>>> The drivers support the tunnel mode where all the
>>> CSI2 traffic coming from an imager is replicated through the deserializer
>>> output port.
>>>
>>> Both MAX96714F and MAX96717F are limited to a 3Gbps forward link rate
>>> leaving a maximum of 2.6Gbps for the video payload.
>>
>> (I see this mail turned out to be a collection of thoughts rather than
>> clear questions... Bear with me =))
>>
>> I know I'm very late to this party, and perhaps these topics have
>> already been discussed, but as I will most likely be doing some GMSL
>> work in the future I wanted to ask these questions. My main
>> questions/concerns are related to the i2c and the representation of the
>> links in the DT.
>>
>> First, I know these particular devices are one input, one output
>> serializer and deserializer, so there's not much to do wrt. i2c
>> translation/gating. But even here I wonder how does one support a case
>> where a single local i2c bus would have two deserializer devices (with
>> different i2c addresses), connected to two identical camera modules?
>>
>> Controlling the deserializers would work fine, but as the serializers
>> and the remote peripherals (sensor) would answer to identical i2c
>> addresses, it would conflict and not work.
>>
>> If I understand the HW docs right, a way (maybe there are others?) to
>> handle this would be:
>> - deser probes, but keeps the link disabled by default
> 
> I don't know if the GMSL2 deserializers typically start with the link
> enabled or disabled by default, but I assume you mean here that early in
> the probe sequence the driver would disable the link if it's enabled by
> default.
> 
> Note that the forward (from serializer to deserializer, carrying video
> and I2C "replies") and reverse (from deserializer to serializer,
> carrying I2C "requests") can be controlled separately in GMSL1. I don't
> know if GMSL2 allows doing the same. It would be good to be precise in
> the discussions.
> 
>> - deser reads the initial serializer i2c address from the DT, but also a
>> new address which we want the serializer to have (which doesn't conflict
>> with the other serializer)
> 
> There's also the devices behind the serializer that we need to consider.
> There will typically be one (a camera sensor), but possibly multiple
> (microcontrollers, EEPROMs, ISPs, ...) such devices. With GMSL1, the
> serializer has the ability to perform address translation for up to two
> addresses, plus the ability to reprogram the serializer address. If we
> end up having more than two devices behind the serializers, address
> translation won't be good enough.
> 
> For GMSL1, we decided not to reprogram the serializer address, but
> instead to implement an I2C mux in the deserializer driver. The
> deserializer would disable all reverse links, and enable them
> selectively through the I2C mux API. This ensured that only one reverse
> link would be enabled at a time per deserializer. It didn't address the
> issue of multiple deserializers on the same I2C bus.
> 
> There's also the issue of power management to consider. Power to the
> cameras and deserializers could be cut when they're unused. We need to
> ensure they can come back up with I2C conflicts, as they would be reset
> to their default address. I don't know if this is a solvable problem in
> the generic case with GMSL1 and GMSL2.
I don't know either for GMSL1 serializers, but for the GMSL2 one there is a
sleep mode on the serializer which retains some register configuration such
as the I2C address. I don't know how this mode can be used since we need 
a close
relationship between the des and ser to wake it up.

> 
>> - deser enables the link and immediately (how to be sure the other deser
>> driver doesn't do this at the same time?) sends a write to the
>> serializer's DEV_ADDR, changing the serializer's i2c address.
> 
> We faced a similar issue when we started working on the MAX9286 driver
> (a quad GMSL1 deserializer). Our test platform had two MAX9286 on the
> same I2C bus (for a total of 8 cameras), and all cameras were identical.
> 
> The initial driver implementation posted to the list ([1]) included a
> mechanism to handle this problem:
> 
> 	/*
> 	 * We can have multiple MAX9286 instances on the same physical I2C
> 	 * bus, and I2C children behind ports of separate MAX9286 instances
> 	 * having the same I2C address. As the MAX9286 starts by default with
> 	 * all ports enabled, we need to disable all ports on all MAX9286
> 	 * instances before proceeding to further initialize the devices and
> 	 * instantiate children.
> 	 *
> 	 * Start by just disabling all channels on the current device. Then,
> 	 * if all other MAX9286 on the parent bus have been probed, proceed
> 	 * to initialize them all, including the current one.
> 	 */
> 	max9286_i2c_mux_close(dev);
>     
> 	/*
> 	 * The MAX9286 initialises with auto-acknowledge enabled by default.
> 	 * This means that if multiple MAX9286 devices are connected to an I2C
> 	 * bus, another MAX9286 could ack I2C transfers meant for a device on
> 	 * the other side of the GMSL links for this MAX9286 (such as a
> 	 * MAX9271). To prevent that disable auto-acknowledge early on; it
> 	 * will be enabled later as needed.
> 	 */
> 	max9286_configure_i2c(dev, false);
>     
> 	ret = device_for_each_child(client->dev.parent, &client->dev,
> 				    max9286_is_bound);
> 	if (ret)
> 		return 0;
>     
> 	dev_dbg(&client->dev,
> 		"All max9286 probed: start initialization sequence\n");
> 	ret = device_for_each_child(client->dev.parent, NULL,
> 				    max9286_init);
> 
> [1] https://lore.kernel.org/all/20180605233435.18102-3-kieran.bingham+renesas@ideasonboard.com/
> 
> This was considered as a hack and dropped, limiting support to a single
> MAX9286 on a given I2C bus. I think we should revive that discussion,
> and implement a generic mechanism to handle synchronized initialization
> at probe time, synchronized operation of muxes across multiple
> deserializers (if we decide to go that way for GMSL1), and synchronized
> power up at runtime (again if we decide we can handle runtime power
> management).
> 
>> - deser can now add the serializer linux i2c device, so that the
>> serializer can probe
> 
> I'm a bit concerned about having the deserializer driver writing to a
> serializer register. If possible, I'd like that to be performed by the
> serializer driver when it probes. Power management needs to be taken
> into account here (if we decide to support it).
> 
>> - the serializer should prevent any remote i2c transactions until it has
>> written the SRC_A/B and DST_A/B registers, to get translation for the
>> remote peripherals (or maybe the deser driver should do this part too).
>>
>> Am I on the right track with the above?
> 
> As explained above, we went the I2C mux way. I think address translation
> would make sense to explore, but we may need to support falling back to
> a mux if there are too many devices behind the serializers.
> 
>> Now, maybe having such a HW config, two deserializers on a single i2c
>> bus, doesn't happen in real life,
> 
> It did, we cried about it, and the world didn't care. Maybe we didn't
> sacrifice the right goat to the right god, but I'm pretty sure we'll run
> out of goats and/or gods before we run out of "interesting" hardware
> designs.
> 
>> but this issue comes up with
>> multi-port deserializers. And while those deserializers are different
>> devices than what's added in this series, the serializers used may be
>> the same as here. This means the serializer drivers and DT bindings
>> should be such that multi-port deserializers can be supported.
> 
> I fully agree, the DT bindings need to consider more than just the
> particular serializers and deserializers that this series covers.
> 
>> As I said, I'm late (and new) to this party, and struggling to consume
>> and understand all the related specs and drivers, so I hope you can give
>> some insight into how all this might be implemented in the future =).
>>
>> Have you looked at the FPD-Link drivers (ds90ub9xx)? The i2c management
>> is a bit different with those (with my current understanding, a bit
>> saner...), but I wonder if similar style would help here, or if the
>> i2c-atr could be utilized. It would be nice (but I guess not really
>> mandatory in any way) to have similar style in DT bindings for all
>> ser-des solutions.
>>
>> To summarize the i2c management on both FPD-Link and GMSL (if I have
>> understood it right):
>>
>> In FPD-Link the deserializer does it all: it has registers for the
>> serializer i2c aliases, and for i2c address translation (per port). So
>> when the deser probes, it can program suitable i2c addresses (based on
>> data from DT), which will be the addresses visible on the main i2c bus,
>> and thus there are never any conflicts.
> 
> That's much nicer than the GMSL architecture in my opinion.
> 
>> In addition to that, the drivers utilize i2c-atr, which means that new
>> linux i2c busses are created for each serializer. E.g. the deser might
>> be, say, on i2c bus 4, and also the serializers, via their i2c aliases,
>> would be accessible bus 4. When the serializer drivers probe they will
>> create new i2c busses with i2c-atr. So with a 4 port deserializer we
>> might get i2c busses 5, 6, 7 and 8. The linux i2c devices for remote
>> peripherals (sensors mainly) would be created on these busses with their
>> real i2c addresses. When a sensor driver does an i2c write to its
>> device, the i2c-atr will catch the write, change the address according
>> to the translation table, and do an actual write on the i2c bus 4. This
>> would result in the deser HW to catch this write, switch the address
>> back to the "real" one, and send it to the appropriate serializer, which
>> would then send the i2c transaction on its i2c bus.
>>
>> In GMSL the deser just forwards everything it sees on the i2c bus, if a
>> port is enabled. The deser has no other support related to i2c. The
>> serializers have DEV_ADDR register which can be used to change the
>> address the serializers respond to, and the serializers also have i2c
>> translation for two remote peripherals.
> 
> In addition to that, the deserializer (at least the MAX9286) has support
> for auto-ack. When enabled, it will automatically ack any I2C write,
> when the I2C reverse channel is available for the forward channel isn't.
> It's a plug-and-pray approach, used to write serializer registers
> related to the I2C forward channel configuration. One issue with this is
> that any I2C write on the bus seen by the deserializer will be acked by
> it, even if it is for a completely unrelated device.
> 
>> But if the i2c translation is used, it would mean that, say, the sensor
>> driver would need to use the "virtual" address, not the real one to
>> communicate with the sensor device, which doesn't sound right...
> 
> How so ? With FPD-Link, with ATR is enabled, doesn't the sensor driver
> also use the "virtual" (as in host-visible) I2C address instead of the
> real one (as in the address used on the bus physically connected to the
> sensor) ?
> 
>> You have used i2c-gate for both the deser and the ser. I don't have
>> experience with i2c-gate, but how can we manage the serializer i2c
>> address and the i2c address translation with it?
>>
>> One difference with the FPD-Link and this series' DT bindings is that I
>> have a "links" node in the deser, instead of just adding the serializers
>> under an i2c node. In FPD-Link case this allowed me to better represent
>> the hardware and the configuration needed.
>>
>> So... Perhaps my bottom line question is: do we need something similar
>> to what the FPD-Link uses (links, i2c-atr) to fully support GMSL
>> devices? And also, if we merge the DT bindings in this series, will we
>> have gone into a corner wrt. how we can manage the i2c?
> 
> For consistency, I would like to keep the bindings as close as possible
> to each other when there is no reason to do otherwise. Of course, we
> already have GMSL1 and FPD-Link bindings that are not identical... Given
> the backward compatibility of GMSL2 with GMSL1, we may need to stay
> closer to the GMSL1 bindings than to the FPD-Link bindings. Of course,
> any feature not available in the GMSL1 bindings that we would need to
> design and implement can mimick the FPD-Link bindings.
The difference I'm seeing with the max9286 binding is that the deserializer
declare the serializer in the i2c-gate node instead of a i2c-mux.
But this device has only one port, and we don't have a way to only transmit
the i2c requests on a particular link.


-- 
Julien

Powered by blists - more mailing lists