linux-kernel - Re: [PATCH v2 8/9] phy: Add Cadence D-PHY support

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Date:   Thu, 22 Nov 2018 14:16:58 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Maxime Ripard <maxime.ripard@...tlin.com>
CC:     Boris Brezillon <boris.brezillon@...tlin.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        <linux-media@...r.kernel.org>,
        Archit Taneja <architt@...eaurora.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Chen-Yu Tsai <wens@...e.org>, <linux-kernel@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        Krzysztof Witos <kwitos@...ence.com>,
        Rafal Ciepiela <rafalc@...ence.com>
Subject: Re: [PATCH v2 8/9] phy: Add Cadence D-PHY support

Hi,

On 21/11/18 7:17 PM, Maxime Ripard wrote:
> Hi Kishon,
> 
> On Wed, Nov 21, 2018 at 03:59:43PM +0530, Kishon Vijay Abraham I wrote:
>> On 21/11/18 3:41 PM, Maxime Ripard wrote:
>>> Hi Kishon,
>>>
>>> On Tue, Nov 20, 2018 at 11:02:34AM +0530, Kishon Vijay Abraham I wrote:
>>>>>> +static int cdns_dphy_config_from_opts(struct phy *phy,
>>>>>> +				      struct phy_configure_opts_mipi_dphy *opts,
>>>>>> +				      struct cdns_dphy_cfg *cfg)
>>>>>> +{
>>>>>> +	struct cdns_dphy *dphy = phy_get_drvdata(phy);
>>>>>> +	unsigned int dsi_hfp_ext = 0;
>>>>>> +	int ret;
>>>>>> +
>>>>>> +	ret = phy_mipi_dphy_config_validate(opts);
>>>>>> +	if (ret)
>>>>>> +		return ret;
>>>>>> +
>>>>>> +	ret = cdns_dsi_get_dphy_pll_cfg(dphy, cfg,
>>>>>> +					opts, &dsi_hfp_ext);
>>>>>> +	if (ret)
>>>>>> +		return ret;
>>>>>> +
>>>>>> +	opts->wakeup = cdns_dphy_get_wakeup_time_ns(dphy);
>>>>
>>>> Is the wakeup populated here used by the consumer driver?
>>>
>>> It's supposed to, if it can yes.
>>
>> But I guess right now it's not using. I'm thinking the usefulness of validate
>> callback (only from this series point of view). Why should a consumer driver
>> invoke validate if it doesn't intend to configure the PHY?
>>
>> The 3 steps required are
>> 	* The consumer driver gets the default config
>> 	* The consumer driver changes some of the configuration and
>> 	* The consumer driver invokes phy configure callback.
>>
>> phy_configure anyways can validate the config before actually configuring the phy.
> 
> If you only want to configure the PHY, then yes, sure.
> 
> However, the point here is that validate helps negotiating what the
> source device (DSI controller for example) and what the sink device
> (say a panel) are capable of.
> 
> A panel for example might very well have multiple resolutions that it
> supports, and the DSI controller will have some as well. And the PHY
> will only be able to operate within certain boundaries too. However,
> they don't necessarily match, since there's so many panels, and so
> many SoCs.
> 
> Let's say our (very weird) panel is able to do 640x480, 1024x768 and
> 1280x800. Our DSI driver can only operate with 1024 pixels in both
> dimensions, and the PHY can only reach the bus frequency needed for
> around 800x600.
> 
> We'll want to filter out 1280x800 (because the DSI controller can't
> provide that) and 1024x768 (because the PHY can go fast enough), to
> only provide the 640x480 option to the user.
> 
> That's what validate bring you. The option to test whether a given
> configuration *would* work, without actually wanting to apply it right
> now.
> 
> Does that make sense?

Thank you for the explanation. It's clear now.

Thanks
Kishon