linux-kernel - Re: [PATCH v2 4/9] phy: dphy: Add configuration helpers

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Message-ID: <20181204154847.f2cclpxa6a7v2ffx@flea>
Date:   Tue, 4 Dec 2018 16:48:47 +0100
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Kishon Vijay Abraham I <kishon@...com>
Cc:     Sakari Ailus <sakari.ailus@....fi>,
        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 4/9] phy: dphy: Add configuration helpers

On Tue, Dec 04, 2018 at 11:28:37AM +0530, Kishon Vijay Abraham I wrote:
> Hi Maxime,
> 
> On 21/11/18 3:03 PM, Maxime Ripard wrote:
> > Hi Sakari,
> > 
> > Thanks for your review.
> > 
> > On Mon, Nov 19, 2018 at 03:43:57PM +0200, Sakari Ailus wrote:
> >>> +/*
> >>> + * Minimum D-PHY timings based on MIPI D-PHY specification. Derived
> >>> + * from the valid ranges specified in Section 6.9, Table 14, Page 41
> >>> + * of the D-PHY specification (v2.1).
> >>
> >> I assume these values are compliant with the earlier spec releases.
> > 
> > I have access to the versions 1.2 and 2.1 of the spec and as far as I
> > can tell, they match here. I can't really say for other releases, but
> > I wouldn't expect any changes (and it can always be adjusted later on
> > if needed).
> > 
> >>> + */
> >>> +int phy_mipi_dphy_get_default_config(unsigned long pixel_clock,
> >>
> >> How about using the bus frequency instead of the pixel clock? Chances are
> >> that the caller already has that information, instead of calculating it
> >> here?
> > 
> > I went for the pixel clock since it's something that all drivers will
> > have access too without any computation. The bus frequency can be
> > available as well in v4l2, but won't be in DRM, and that would require
> > for all drivers to duplicate that computation, which doesn't seem like
> > a good choice.
> > 
> >>> +				     unsigned int bpp,
> >>> +				     unsigned int lanes,
> >>> +				     struct phy_configure_opts_mipi_dphy *cfg)
> >>> +{
> >>> +	unsigned long hs_clk_rate;
> >>> +	unsigned long ui;
> >>> +
> >>> +	if (!cfg)
> >>> +		return -EINVAL;
> >>> +
> >>> +	hs_clk_rate = pixel_clock * bpp / lanes;
> >>> +	ui = DIV_ROUND_UP(NSEC_PER_SEC, hs_clk_rate);
> >>
> >> Nanoseconds may not be precise enough for practical computations on these
> >> values. At 1 GHz, this ends up being precisely 1. At least Intel hardware
> >> has some more precision, I presume others do, too. How about using
> >> picoseconds instead?
> > 
> > Sounds like a good idea.
> 
> Would you be fixing this? Or this can be a later patch?

I have fixed this locally, but I wanted to wait a bit for more
feedback. I can send a new version if you prefer.

Maxime

-- 
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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