linux-kernel - RE: [PATCH v1 2/2] phy: phy-cadence-torrent: Use PHY kernel APIs to set PHY attributes

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Message-ID: <BY5PR07MB6982568DE9F5A4BF00433CA6C5B80@BY5PR07MB6982.namprd07.prod.outlook.com>
Date:   Mon, 18 May 2020 06:54:36 +0000
From:   Swapnil Kashinath Jakhade <sjakhade@...ence.com>
To:     Kishon Vijay Abraham I <kishon@...com>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Maxime Ripard <maxime@...no.tech>
CC:     Yuti Suresh Amonkar <yamonkar@...ence.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "jsarha@...com" <jsarha@...com>,
        "praneeth@...com" <praneeth@...com>,
        Milind Parab <mparab@...ence.com>,
        Vinod Koul <vkoul@...nel.org>
Subject: RE: [PATCH v1 2/2] phy: phy-cadence-torrent: Use PHY kernel APIs to
 set PHY attributes

Hi Kishon,

> -----Original Message-----
> From: Kishon Vijay Abraham I <kishon@...com>
> Sent: Wednesday, May 13, 2020 8:08 AM
> To: Tomi Valkeinen <tomi.valkeinen@...com>; Maxime Ripard
> <maxime@...no.tech>; Swapnil Kashinath Jakhade
> <sjakhade@...ence.com>
> Cc: Yuti Suresh Amonkar <yamonkar@...ence.com>; linux-
> kernel@...r.kernel.org; mark.rutland@....com; jsarha@...com;
> praneeth@...com; Milind Parab <mparab@...ence.com>; Vinod Koul
> <vkoul@...nel.org>
> Subject: Re: [PATCH v1 2/2] phy: phy-cadence-torrent: Use PHY kernel APIs to
> set PHY attributes
> 
> EXTERNAL MAIL
> 
> 
> Hi,
> 
> On 5/8/2020 1:20 PM, Tomi Valkeinen wrote:
> > On 07/05/2020 20:17, Maxime Ripard wrote:
> >
> >>> Actually, for this particular case, consumer driver will be the
> >>> Cadence MHDP bridge driver for DisplayPort which is also under
> >>> review process for upstreaming [1]. So this DRM bridge driver will
> >>> make use of the PHY APIs
> >>> phy_get_bus_width() and phy_get_max_link_rate() during execution of
> >>> probe function to get the number of lanes and maximum link rate
> >>> supported by Cadence Torrent PHY. This information is required to
> >>> set the host capabilities in the DRM bridge driver, based on which
> >>> initial values for DisplayPort link training will be determined.
> >>>
> >>> The changes in this PHY patch series are based on suggestions in the
> >>> review comments in [1] which asks to use kernel PHY APIs to read
> >>> these properties instead of directly accessing PHY device node. The
> >>> complete driver and actual use of these APIs can be found in [2].
> >>> This is how we are planning to use these APIs.
> >>
> >> I haven't really looked into the displayport spec, but I'd assume
> >> that there's a lot more parameters that would need to be negociated
> >> between the phy and the DP block? If so, then it would make more
> >> sense to follow the path we did for MIPI-DSI where the parameters can
> >> be negociated through the phy_configure / phy_validate interface.
> >
> > I don't think this is negotiation, but just exposing the (max)
> > capabilities of PHY, inside which the configure can work. Maybe all
> > the capabilities could handled with a struct (struct phy_attrs),
> > instead of adding separate functions for each, though.
> 
> yeah, that makes sense. Just that users should take care not to over-write all
> the phy attributes with partial information.

It would be really helpful if you could clarify a bit regarding how to handle this
exactly. What I could understand from Tomi' suggestion is that all PHY attributes
in struct phy_attrs should have single pair of functions to get and set all the PHY
attributes (e.g. phy_get_attrs / phy_set_attrs), instead of separate get/set pair of
functions for individual attribute (bus_width, mode, max_link_rate etc). Is this
understanding correct? If so, how should the existing functions for bus_width and
mode be used?

Thanks & regards,
Swapnil

> 
> Thanks
> Kishon