Re: [PATCH v5 2/2] phy: cadence-torrent: Use kernel PHY API to set PHY attributes

[Vinod Koul <vkoul@kernel.org>]

On 10-09-20, 11:45, Kishon Vijay Abraham I wrote:
> 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.

I am fine with it, we can modify later as the need arises

-- 
~Vinod