linux-kernel - Re: [PATCH 3/3] drm/panel: add panel driver for Xinpeng XPP055C272 panels

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Message-ID: <1744285.zQlJhejOUX@phil>
Date:   Sun, 15 Dec 2019 10:13:29 +0100
From:   Heiko Stuebner <heiko@...ech.de>
To:     Sam Ravnborg <sam@...nborg.org>
Cc:     thierry.reding@...il.com, mark.rutland@....com,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, robh+dt@...nel.org,
        christoph.muellner@...obroma-systems.com
Subject: Re: [PATCH 3/3] drm/panel: add panel driver for Xinpeng XPP055C272 panels

Hi Sam,

Am Sonntag, 15. Dezember 2019, 09:29:16 CET schrieb Sam Ravnborg:
> Hi Heiko.
> 
> > Am Samstag, 14. Dezember 2019, 09:17:30 CET schrieb Sam Ravnborg:
> > > > +#define dsi_generic_write_seq(dsi, cmd, seq...) do {			\
> > > > +		static const u8 d[] = { seq };				\
> > > > +		int ret;						\
> > > > +		ret = mipi_dsi_dcs_write(dsi, cmd, d, ARRAY_SIZE(d));	\
> > > > +		if (ret < 0)						\
> > > > +			return ret;					\
> > > > +	} while (0)
> > > This macro return an error code if a write fails.
> > > 
> > > > +
> > > > +static int xpp055c272_init_sequence(struct xpp055c272 *ctx)
> > > > +{
> > > > +	struct mipi_dsi_device *dsi = to_mipi_dsi_device(ctx->dev);
> > > > +	struct device *dev = ctx->dev;
> > > > +	int ret;
> > > > +
> > > > +	/*
> > > > +	 * Init sequence was supplied by the panel vendor without much
> > > > +	 * documentation.
> > > > +	 */
> > > > +	dsi_generic_write_seq(dsi, XPP055C272_CMD_SETEXTC, 0xf1, 0x12, 0x83);
> > > But all uses of the macro here ignore the error.
> > 
> > hmm, am I way off track here?
> > 
> > 	dsi_generic_write_seq(dsi, XPP055C272_CMD_SETEXTC, 0xf1, 0x12, 0x83);
> > 	dsi_generic_write_seq(dsi, XPP055C272_CMD_SETMIPI,
> > 			      0x33, 0x81, 0x05, 0xf9, 0x0e, 0x0e, 0x00, 0x00,
> > 			      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x44, 0x25,
> > 			      0x00, 0x91, 0x0a, 0x00, 0x00, 0x02, 0x4f, 0x01,
> > 			      0x00, 0x00, 0x37);
> > 	...
> > 
> > should just expand to
> > 
> > do {
> > 		static const u8 d[] = { 0xf1, 0x12, 0x83 };
> > 		int ret;
> > 		ret = mipi_dsi_dcs_write(dsi, XPP055C272_CMD_SETEXTC, d, ARRAY_SIZE(d));
> > 		if (ret < 0)
> > 			return ret;
> > } while (0)
> > do {
> > 		static const u8 d[] = { 0x33, 0x81, 0x05, 0xf9, 0x0e, 0x0e, 0x00, 0x00,
> > 			      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x44, 0x25,
> > 			      0x00, 0x91, 0x0a, 0x00, 0x00, 0x02, 0x4f, 0x01,
> > 			      0x00, 0x00, 0x37 };
> > 		int ret;
> > 		ret = mipi_dsi_dcs_write(dsi, XPP055C272_CMD_SETMIPI, d, ARRAY_SIZE(d));
> > 		if (ret < 0)
> > 			return ret;
> > } while (0)
> > ...
> > 
> > so every write instance will actually return an error if it happens and not
> > continue on with the next init item.
> The idea was that if a write returned an error then do not even attempt
> more writes. So if a write fails we do not loose the original error
> code, assuming subsequent write would also fail.

Shouldn't the code above do exactly that? ... Because it's like

	ret = dcs_write(...)
	if (ret <0)
		return ret;

So if any of the dcs_writes goes wrong it should just return the
error code from that write from the function and not try any more
writes. (or I'm blind and do not see something ;-) )


Heiko

> I have looked through all the panel drivers now, and the majority does
> not check if the write goes wrong.
> So following the pattern on the other panels you can also decide to just
> ignore the return value of mipi_dsi_dcs_write() rahter than trying to
> invent the check I tried to explain.
> 
> 	Sam
>