linux-kernel - Re: [PATCH 1/3] drm/rockchip: lvds: add rk3568 support

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Message-ID: <20230120094117.GM24755@pengutronix.de>
Date:   Fri, 20 Jan 2023 10:41:17 +0100
From:   "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>
To:     "Sverdlin, Alexander" <alexander.sverdlin@...mens.com>
Cc:     "a1ba.omarov@...il.com" <a1ba.omarov@...il.com>,
        "heiko@...ech.de" <heiko@...ech.de>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "hjc@...k-chips.com" <hjc@...k-chips.com>,
        "michael.riesch@...fvision.net" <michael.riesch@...fvision.net>,
        "macromorgan@...mail.com" <macromorgan@...mail.com>,
        "daniel@...ll.ch" <daniel@...ll.ch>,
        "frattaroli.nicolas@...il.com" <frattaroli.nicolas@...il.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "pgwipeout@...il.com" <pgwipeout@...il.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "frank-w@...lic-files.de" <frank-w@...lic-files.de>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "ezequiel@...guardiasur.com.ar" <ezequiel@...guardiasur.com.ar>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "airlied@...il.com" <airlied@...il.com>,
        "linux-rockchip@...ts.infradead.org" 
        <linux-rockchip@...ts.infradead.org>
Subject: Re: [PATCH 1/3] drm/rockchip: lvds: add rk3568 support

On Fri, Jan 20, 2023 at 09:31:43AM +0000, Sverdlin, Alexander wrote:
> Hello Sascha,
> 
> On Fri, 2023-01-20 at 10:16 +0100, Sascha Hauer wrote:
> > > +       /* Enable LVDS mode */
> > > +       return regmap_update_bits(lvds->grf, RK3568_GRF_VO_CON2,
> > > +                                 RK3568_LVDS0_MODE_EN(1),
> > > +                                 RK3568_LVDS0_MODE_EN(1));
> > 
> > Isn't this the same as:
> > 
> >         regmap_write(lvds->grf, RK3568_GRF_VO_CON2,
> > RK3568_LVDS0_MODE_EN(1));
> > 
> > Unless I am missing something I find a plain regmap_write() easier to
> > read.
> 
> the former is setting a bit in a RMW operation, the latter is a plain
> write, isn't it?

That's right from the view what the function itself does. Note the
registers that are accessed here are a bit special. They effectively
are 16bit wide, the upper 16bit contain a mask. Only the bits set in the
mask are actually modified in the lower bits. See the register bit
definitions:

#define HIWORD_UPDATE(v, h, l)  ((GENMASK(h, l) << 16) | ((v) << (l)))

#define   RK3568_LVDS0_SELECT(val)             HIWORD_UPDATE(val,  5, 4)

Sascha

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