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Subject: Re: [PATCH v5 2/2] phy: cadence-torrent: Use kernel PHY API to set
 PHY attributes
To:     Milind Parab <mparab@cadence.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>
CC:     Swapnil Kashinath Jakhade <sjakhade@cadence.com>,
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        Yuti Suresh Amonkar <yamonkar@cadence.com>,
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From:   Kishon Vijay Abraham I <kishon@ti.com>
Message-ID: <634823a7-1a4d-6418-575b-484998afa7ae@ti.com>
Date:   Thu, 10 Sep 2020 11:45:21 +0530
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Hi Milind,

On 08/09/20 7:45 pm, Milind Parab wrote:
> Hi Kishon,
> 
>> -----Original Message-----
>> From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>> Sent: Thursday, September 3, 2020 9:00 PM
>> To: Kishon Vijay Abraham I <kishon@ti.com>
>> Cc: Swapnil Kashinath Jakhade <sjakhade@cadence.com>; vkoul@kernel.org;
>> linux-kernel@vger.kernel.org; maxime@cerno.tech; Milind Parab
>> <mparab@cadence.com>; Yuti Suresh Amonkar <yamonkar@cadence.com>;
>> nsekhar@ti.com; tomi.valkeinen@ti.com; jsarha@ti.com; praneeth@ti.com
>> Subject: Re: [PATCH v5 2/2] phy: cadence-torrent: Use kernel PHY API to set
>> PHY attributes
>>
>> EXTERNAL MAIL
>>
>>
>> Hi Kishon,
>>
>> On Thu, Sep 03, 2020 at 05:00:14PM +0530, Kishon Vijay Abraham I wrote:
>>> On 9/3/2020 4:29 PM, Swapnil Kashinath Jakhade wrote:
>>>> On Wednesday, September 2, 2020 5:47 PM, Laurent Pinchart wrote:
>>>>> On Wed, Sep 02, 2020 at 07:09:21AM +0000, Swapnil Kashinath Jakhade
>> wrote:
>>>>>> On Wednesday, September 2, 2020 6:00 AM Laurent Pinchart wrote:
>>>>>>> On Mon, Aug 24, 2020 at 08:28:31PM +0200, Swapnil Jakhade wrote:
>>>>>>>> Use generic PHY framework function phy_set_attrs() to set number
>>>>>>>> of lanes and maximum link rate supported by PHY.
>>>>>>>>
>>>>>>>> Signed-off-by: Swapnil Jakhade <sjakhade@cadence.com>
>>>>>>>> Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
>>>>>>>> ---
>>>>>>>>   drivers/phy/cadence/phy-cadence-torrent.c | 7 +++++++
>>>>>>>>   1 file changed, 7 insertions(+)
>>>>>>>>
>>>>>>>> diff --git a/drivers/phy/cadence/phy-cadence-torrent.c
>>>>>>>> b/drivers/phy/cadence/phy-cadence-torrent.c
>>>>>>>> index 7116127358ee..eca71467c4a8 100644
>>>>>>>> --- a/drivers/phy/cadence/phy-cadence-torrent.c
>>>>>>>> +++ b/drivers/phy/cadence/phy-cadence-torrent.c
>>>>>>>> @@ -1710,6 +1710,7 @@ static int cdns_torrent_phy_probe(struct
>> platform_device *pdev)
>>>>>>>>   	struct cdns_torrent_phy *cdns_phy;
>>>>>>>>   	struct device *dev = &pdev->dev;
>>>>>>>>   	struct phy_provider *phy_provider;
>>>>>>>> +	struct phy_attrs torrent_attr;
>>>>>>>>   	const struct of_device_id *match;
>>>>>>>>   	struct cdns_torrent_data *data;
>>>>>>>>   	struct device_node *child;
>>>>>>>> @@ -1852,6 +1853,12 @@ static int cdns_torrent_phy_probe(struct
>> platform_device *pdev)
>>>>>>>>   				 cdns_phy->phys[node].num_lanes,
>>>>>>>>   				 cdns_phy->max_bit_rate / 1000,
>>>>>>>>   				 cdns_phy->max_bit_rate % 1000);
>>>>>>>> +
>>>>>>>> +			torrent_attr.bus_width = cdns_phy-
>>> phys[node].num_lanes;
>>>>>>>> +			torrent_attr.max_link_rate = cdns_phy-
>>> max_bit_rate;
>>>>>>>> +			torrent_attr.mode = PHY_MODE_DP;
>>>>>>>> +
>>>>>>>> +			phy_set_attrs(gphy, &torrent_attr);
>>>>>>>
>>>>>>> Why is this better than accessing the attributes manually as follows ?
>>>>>>>
>>>>>>> 			gphy->attrs.bus_width = cdns_phy-
>>> phys[node].num_lanes;
>>>>>>> 			gphy->attrs.max_link_rate = cdns_phy-
>>> max_bit_rate;
>>>>>>> 			gphy->attrs.mode = PHY_MODE_DP;
>>>>>>>
>>>>>>> This is called in cdns_torrent_phy_probe(), before the PHY
>>>>>>> provider is registered, so nothing can access the PHY yet. What
>>>>>>> race condition are you trying to protect against with usage of
>> phy_set_attrs() ?
>>>>>>
>>>>>> I agree that for Cadence DP bridge driver and Torrent PHY driver
>>>>>> use case, it would not matter even if we set the attributes in
>>>>>> Torrent PHY driver in a way you suggested above.
>>>>>> But as per the discussion in [1], phy_set_attrs/phy_get_attrs APIs
>>>>>> in future could maybe used by other drivers replacing existing
>>>>>> individual functions for attributes bus_width and mode which are
>>>>>> phy_set_bus_width/phy_get_bus_width and
>> phy_set_mode/phy_get_mode.
>>>>>> So this usage in Torrent PHY driver is an example implementation of the
>> API.
>>>>>>
>>>>>> [1]
>>>>>> https://urldefense.com/v3/__https://lkml.org/lkml/2020/5/18/472__;
>>>>>> !!EH
>>>>>> scmS1ygiU1lA!QKTTI7BS1R35a_zoMfJsY4A4yCtEKrQNtiAXTyIZ-
>> SYIEEibYdpBM
>>>>>> JTll
>>>>>> Yrd-00$
>>>>>
>>>>> This doesn't seem a very good API to me :-S It will require callers
>>>>> to always call phy_get_attrs() first, modify the attributes they
>>>>> want to set, and then call phy_set_attrs(). Not only will be copy
>>>>> the whole phy_attrs structure needlessly, it will also not be an
>>>>> atomic operation as someone else could modify attributes between the
>> get and set calls.
>>>>> The lack of atomicity may not be an issue in practice if there's a
>>>>> single user of the PHY at all times, but in that case no mutex is needed.
>>>
>>> What if the consumer tries to set an attribute at the middle of a
>>> phy_power_on() operation? That is still a valid operation and phy core
>>> layer should try to prevent it no?
>>
>> I see multiple questions here.
>>
>> First of all, unless I'm mistaken, the attributes set here are static properties,
>> set by the PHY driver at probe time, and only read by PHY consumers. There
>> should be no need for any kind of protection or special API to access them.
>>
>> Then, there's the question of how to handle dynamic attributes. In theory a
>> dynamic attribute could be changed at any time, and thus race wit, for
>> instance phy_power_on(). However, the proposed API won't help much
>> address this issue. Using a mutex will indeed ensure that the attribute change
>> will be serialized with other operations, but it won't give any guarantee to the
>> PHY consumer on whether the attribute will be set before or after
>> phy_power_on() is processed. The consumer will not know if the new value
>> of the attribute has been taken into account.
>>
>> The question is thus whether we want to make the PHY consumer API thread-
>> safe (note that due to the usage of a mutex, we don't support calling most of
>> the API functions from an interrupt handler, so it really requires the consumer
>> to use a work queue, a thread, or possibly a threaded interrupt). If the answer
>> is yes, the API should define what use cases are valid, and how the PHY has to
>> behave. This includes documenting when new attribute values can be set, and
>> when they are taken into account. If we had to document this as part of this
>> patch series, we would have to state that the new values are taken into
>> account at an undefined point of time if the attribute set call is concurrent
>> with other API calls, which makes the API ill-defined in my opinion. I expect
>> that we would need to turn attribute setting into a callback to the PHY driver
>> in that case, or at least make it a more complex operation handled by the PHY
>> core that would use the existing PHY ops to reconfigure the PHY.
>>
>> Is it worth it allowing drivers to call the PHY API from different threads as
>> opposed to requiring consumers to serialize calls if their use cases require so ?
>> I would expect most consumers to only try to reconfigure a PHY when it's
>> stopped, or to manually stop, reconfigure and restart the PHY.
>>
>>>>> I think this series tries to fix a problem that doesn't exist.
>>>>
>>>> Thanks Laurent for your comments.
>>>>
>>>> Hi Kishon,
>>>>
>>>> Could you please suggest what would be the better approach regarding
>>>> this PHY attributes series. Should we add individual get/set
>>>> functions for new attribute max_link_rate just like mode and
>>>> bus_width, or should we use phy_get_attrs() and phy_set_attrs()
>> functions removing mutex.  Your suggestions would really help.
>>>
>>> I think Laurent's point is not having an API at all for configuring
>>> attributes and access them manually?
>>
>> If the answer to the above question is that a thread-safe API isn't worth it as
>> we wouldn't have good use cases for it, then I think accessing the attributes
>> manually is all we need.
>>
> 
> Should we proceed accessing attribute manually

yeah, let's deal with dynamic attributes later when the use cases arise 
unless Vinod disagrees.

Thanks
Kishon