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Subject: Re: [PATCH 02/10] phy: Add configuration interface
To:     Maxime Ripard <maxime.ripard@bootlin.com>
CC:     Boris Brezillon <boris.brezillon@bootlin.com>,
        Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        <linux-media@vger.kernel.org>,
        Archit Taneja <architt@codeaurora.org>,
        Andrzej Hajda <a.hajda@samsung.com>,
        Chen-Yu Tsai <wens@csie.org>, <linux-kernel@vger.kernel.org>,
        <dri-devel@lists.freedesktop.org>,
        <linux-arm-kernel@lists.infradead.org>,
        Krzysztof Witos <kwitos@cadence.com>,
        Rafal Ciepiela <rafalc@cadence.com>
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From:   Kishon Vijay Abraham I <kishon@ti.com>
Message-ID: <b5ebd9a5-f43b-728b-1d7f-0f7b08021f39@ti.com>
Date:   Mon, 24 Sep 2018 17:25:26 +0530
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Hi Maxime,

On Monday 24 September 2018 03:24 PM, Maxime Ripard wrote:
> Hi,
> 
> On Mon, Sep 24, 2018 at 02:18:35PM +0530, Kishon Vijay Abraham I wrote:
>> On Wednesday 19 September 2018 05:44 PM, Maxime Ripard wrote:
>>> Hi,
>>>
>>> On Fri, Sep 14, 2018 at 02:18:37PM +0530, Kishon Vijay Abraham I wrote:
>>>>>>>>> +/**
>>>>>>>>> + * phy_validate() - Checks the phy parameters
>>>>>>>>> + * @phy: the phy returned by phy_get()
>>>>>>>>> + * @mode: phy_mode the configuration is applicable to.
>>>>>>>>> + * @opts: Configuration to check
>>>>>>>>> + *
>>>>>>>>> + * Used to check that the current set of parameters can be handled by
>>>>>>>>> + * the phy. Implementations are free to tune the parameters passed as
>>>>>>>>> + * arguments if needed by some implementation detail or
>>>>>>>>> + * constraints. It will not change any actual configuration of the
>>>>>>>>> + * PHY, so calling it as many times as deemed fit will have no side
>>>>>>>>> + * effect.
>>>>>>>>> + *
>>>>>>>>> + * Returns: 0 if successful, an negative error code otherwise
>>>>>>>>> + */
>>>>>>>>> +int phy_validate(struct phy *phy, enum phy_mode mode,
>>>>>>>>> +		  union phy_configure_opts *opts)
>>>>>>>>
>>>>>>>> IIUC the consumer driver will pass configuration options (or PHY parameters)
>>>>>>>> which will be validated by the PHY driver and in some cases the PHY driver can
>>>>>>>> modify the configuration options? And these modified configuration options will
>>>>>>>> again be given to phy_configure?
>>>>>>>>
>>>>>>>> Looks like it's a round about way of doing the same thing.
>>>>>>>
>>>>>>> Not really. The validate callback allows to check whether a particular
>>>>>>> configuration would work, and try to negotiate a set of configurations
>>>>>>> that both the consumer and the PHY could work with.
>>>>>>
>>>>>> Maybe the PHY should provide the list of supported features to the consumer
>>>>>> driver and the consumer should select a supported feature?
>>>>>
>>>>> It's not really about the features it supports, but the boundaries it
>>>>> might have on those features. For example, the same phy integrated in
>>>>> two different SoCs will probably have some limit on the clock rate it
>>>>> can output because of the phy design itself, but also because of the
>>>>> clock that is fed into that phy, and that will be different from one
>>>>> SoC to the other.
>>>>>
>>>>> This integration will prevent us to use some clock rates on the first
>>>>> SoC, while the second one would be totally fine with it.
>>>>
>>>> If there's a clock that is fed to the PHY from the consumer, then the consumer
>>>> driver should model a clock provider and the PHY can get a reference to it
>>>> using clk_get(). Rockchip and Arasan eMMC PHYs has already used something like
>>>> that.
>>>
>>> That would be doable, but no current driver has had this in their
>>> binding. So that would prevent any further rework, and make that whole
>>> series moot. And while I could live without the Allwinner part, the
>>> Cadence one is really needed.
>>
>> We could add a binding and modify the driver to to register a clock provider.
>> That could be included in this series itself.
> 
> That wouldn't work for device whose bindings need to remain backward
> compatible. And the Allwinner part at least is in that case.

Er..
> 
> I think we should aim at making it a norm for newer bindings, but we
> still have to support the old ones that cannot be changed.

There are drivers which support both the old and new bindings. Allwinner could
be in that category.
> 
>>>> Assuming the PHY can get a reference to the clock provided by the consumer,
>>>> what are the parameters we'll be able to get rid of in struct
>>>> phy_configure_opts_mipi_dphy?
>>>
>>> hs_clock_rate and lp_clock_rate. All the other ones are needed.
>>
>> For a start we could use that and get rid of hs_clock_rate and lp_clock_rate in
>> phy_configure_opts_mipi_dphy.
> 
> As I was saying above, I'm not sure we can do that.
> 
>> We could also use phy_set_bus_width() for lanes.
> 
> I overlooked this function somehow, it indeed looks like we can remove
> the lanes part in favor of this function.
> 
>>>
>>>> I'm sorry but I'm not convinced a consumer driver should have all the details
>>>> that are added in phy_configure_opts_mipi_dphy.
>>>
>>> If it can convince you, here is the parameters that are needed by all
>>> the MIPI-DSI drivers currently in Linux to configure their PHY:
>>>
>>>   - cdns-dsi (drivers/gpu/drm/bridge/cdns-dsi.c)
>>>     - hs_clk_rate
>>>     - lanes
>>>     - videomode
>>>
>>>   - kirin (drivers/gpu/drm/hisilicon/kirin/dw_drm_dsi.c)
>>>     - hs_exit
>>>     - hs_prepare
>>>     - hs_trail
>>>     - hs_zero
>>>     - lpx
>>>     - ta_get
>>>     - ta_go
>>>     - wakeup
>>>
>>>   - msm (drivers/gpu/drm/msm/dsi/*)
>>>     - clk_post
>>>     - clk_pre
>>>     - clk_prepare
>>>     - clk_trail
>>>     - clk_zero
>>>     - hs_clk_rate
>>>     - hs_exit
>>>     - hs_prepare
>>>     - hs_trail
>>>     - hs_zero
>>>     - lp_clk_rate
>>>     - ta_get
>>>     - ta_go
>>>     - ta_sure
>>>
>>>   - mtk (drivers/gpu/drm/mediatek/mtk_dsi.c)
>>>     - hs_clk_rate
>>>     - hs_exit
>>>     - hs_prepare
>>>     - hs_trail
>>>     - hs_zero
>>>     - lpx
>>>
>>>   - sun4i (drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c)
>>>     - clk_post
>>>     - clk_pre
>>>     - clk_prepare
>>>     - clk_zero
>>>     - hs_prepare
>>>     - hs_trail
>>>     - lanes
>>>     - lp_clk_rate
>>>
>>>   - tegra (drivers/gpu/drm/tegra/dsi.c)
>>>     - clk_post
>>>     - clk_pre
>>>     - clk_prepare
>>>     - clk_trail
>>>     - clk_zero
>>>     - hs_exit
>>>     - hs_prepare
>>>     - hs_trail
>>>     - hs_zero
>>>     - lpx
>>>     - ta_get
>>>     - ta_go
>>>     - ta_sure
>>>
>>>   - vc4 (drivers/gpu/drm/vc4/vc4_dsi.c)
>>>     - hs_clk_rate
>>>     - lanes
>>>
>>> Now, for MIPI-CSI receivers:
>>>
>>>   - marvell-ccic (drivers/media/platform/marvell-ccic/mcam-core.c)
>>>     - clk_term_en
>>>     - clk_settle
>>>     - d_term_en
>>>     - hs_settle
>>>     - lp_clk_rate
>>>
>>>   - omap4iss (drivers/staging/media/omap4iss/iss_csiphy.c)
>>>     - clk_miss
>>>     - clk_settle
>>>     - clk_term
>>>     - hs_settle
>>>     - hs_term
>>>     - lanes
>>>
>>>   - rcar-vin (drivers/media/platform/rcar-vin/rcar-csi2.c)
>>>     - hs_clk_rate
>>>     - lanes
>>>
>>>   - ti-vpe (drivers/media/platform/ti-vpe/cal.c)
>>>     - clk_term_en
>>>     - d_term_en
>>>     - hs_settle
>>>     - hs_term
>>
>> Thank you for providing the exhaustive list.
>>
>>> So the timings expressed in the structure are the set of all the ones
>>> currently used in the tree by DSI and CSI drivers. I would consider
>>> that a good proof that it would be useful.
>>
>> The problem I see here is each platform (PHY) will have it's own set of
>> parameters and we have to keep adding members to phy_configure_opts which is
>> not scalable. We should try to find a correlation between generic PHY modes and
>> these parameters (at-least for a subset).
> 
> I definitely understand you skepticism towards someone coming in and
> dropping such a big list of obscure parameters :)

That's part of my concern. The other concern is consumer driver having to know
so much internal details of the PHY. None of the other consumers (PCIe, USB,
etc) had to know so much internals of the PHY.
> 
> However, those values are actually the whole list of parameters
> defined by the MIPI-DPHY standard, so I really don't expect drivers to
> need more than that. As you can see, most drivers allow less
> parameters to be configured, but all of them are defined within those
> parameters. So I'm not sure we need to worry about an ever-expanding
> list of parameters: we have a limited set of parameters defined, and
> from an authoritative source, so we can also push back if someone
> wants to add a parameter that is implementation specific.

Your commit log mentioned there are a few parameters in addition to what is
specified in the MIPI D-PHY spec :-/

"The parameters added here are the one defined in the MIPI D-PHY spec, plus
some parameters that were used by a number of PHY drivers currently found
in the linux kernel."

Thanks
Kishon