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Message-ID: <20130208093754.GA27859@e106331-lin.cambridge.arm.com>
Date: Fri, 8 Feb 2013 09:37:54 +0000
From: Mark Rutland <mark.rutland@....com>
To: "Manjunathappa, Prakash" <prakash.pm@...com>
Cc: "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
"Porter, Matt" <mporter@...com>,
"davinci-linux-open-source@...ux.davincidsp.com"
<davinci-linux-open-source@...ux.davincidsp.com>,
"cjb@...top.org" <cjb@...top.org>,
"linux@....linux.org.uk" <linux@....linux.org.uk>,
"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
"devicetree-discuss@...ts.ozlabs.org"
<devicetree-discuss@...ts.ozlabs.org>,
"Nori, Sekhar" <nsekhar@...com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
"hs@...x.de" <hs@...x.de>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 2/3] mmc: davinci_mmc: add DT support
Hi,
[...]
> > > diff --git a/Documentation/devicetree/bindings/mmc/davinci_mmc.txt b/Documentation/devicetree/bindings/mmc/davinci_mmc.txt
> > > new file mode 100644
> > > index 0000000..6717ab1
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/mmc/davinci_mmc.txt
> > > @@ -0,0 +1,30 @@
> > > +* TI Highspeed MMC host controller for DaVinci
> > > +
> > > +The Highspeed MMC Host Controller on TI DaVinci family
> > > +provides an interface for MMC, SD and SDIO types of memory cards.
> > > +
> > > +This file documents the properties used by the davinci_mmc driver.
> > > +
> > > +Required properties:
> > > +- compatible:
> > > + Should be "ti,davinci-mmc-da830": for da830, da850, dm365
> > > + Should be "ti,davinci-mmc-dm355": for dm355, dm644x
> > > +
> > > +Optional properties:
> > > +- bus-width: Number of data lines, can be <4>, or <8>, default <1>
> > > +- max-frequency: Maximum operating clock frequency, default 25MHz.
> > > +- mmc-cap-mmc-highspeed: Indicates support for MMC in high speed mode
> > > +- mmc-cap-sd-highspeed: Indicates support for SD in high speed mode
[...]
> > > +static struct davinci_mmc_config
> > > + *mmc_parse_pdata(struct platform_device *pdev)
> > > {
> > > + struct device_node *np;
> > > struct davinci_mmc_config *pdata = pdev->dev.platform_data;
> > > + const struct of_device_id *match =
> > > + of_match_device(of_match_ptr(davinci_mmc_dt_ids), &pdev->dev);
> > > + u32 data;
> > > +
> > > + np = pdev->dev.of_node;
> > > + if (!np)
> > > + return pdata;
> > > +
> > > + pdata = devm_kzalloc(&pdev->dev, sizeof(*pdata), GFP_KERNEL);
> > > + if (!pdata) {
> > > + dev_err(&pdev->dev, "Failed to allocate memory for struct davinci_mmc_config\n");
> > > + goto nodata;
> > > + }
> > > +
> > > + if (match->data)
> > > + pdata->version = (u8)((int)match->data);
> > > +
> > > + of_property_read_u32(np, "max-frequency", &pdata->max_freq);
> > > + if (!pdata->max_freq)
> > > + dev_info(&pdev->dev, "'max-frequency' property not specified, defaulting to 25MHz\n");
> > > +
> > > + if (of_get_property(np, "mmc-cap-mmc-highspeed", NULL))
> > > + pdata->caps |= MMC_CAP_MMC_HIGHSPEED;
> > > + if (of_get_property(np, "mmc-cap-sd-highspeed", NULL))
> > > + pdata->caps |= MMC_CAP_SD_HIGHSPEED;
> >
> > If these aren't derivable from max-frequency, you could use
> > of_property_read_bool to make this clearer.
> >
>
> Correct, I will decide these based on max-frequency.
>
> > > +
> > > + of_property_read_u32(np, "bus-width", &data);
> > > + switch (data) {
> > > + case 0:
> >
> > Judging by the binding doc, should this be 1 rather than 0?
> >
>
> By default driver comes up in 4 bit mode when bus-width is not specified.
> Bus-width is set to 1 bit for invalid bus-widths. Below are the cases
> when bus-width 0 or 4, bus-width is set to 4bit mode
> When bus-width is 8, bus-width is set to 8 bit mode
Why is 0 a special value that means 4? Why not just use 4?
Here you just assign 0 to pdata->wires.
I see the current version of the driver handles this specially in
davinci_mmcsd_probe, but I don't see why this should leak into the binding.
What I was originally trying to get at is that the binding says 8, 4, and 1 are
acceptable values, but you check the cases 8, 4, or 0. So you're accepting
something not documented, and producing a warning in a valid case (1).
>
> I thought that if somebody specifies bus-width as 2, 3, 5, 6, 7..., then
> it should be defaulted to 1 bit mode, so I specified it as 1 bit in binding doc.
If they specify something invalid, falling back to a sane value with a warning
sounds good.
> But I feel that a person who is editing dts file will not make such a mistake.
> I will change binding document to default as 4 bit mode.
There have been and inevitably will be plenty of errors in dts files. I think
it'd be good to provide a warning and either use a value that's guaranteed to
work, or bail out if that can't be done.
If it's always valid to use 1 data line even if the hardware has more, I think
falling back to 1 makes more sense, as it'd be guaranteed to work.
>
> > > + case 4:
> > > + case 8:
> > > + pdata->wires = data;
> > > + break;
> > > + default:
> > > + pdata->wires = 1;
> > > + dev_info(&pdev->dev, "Unsupported buswidth, defaulting to 1 bit\n");
> > > + }
> > > +nodata:
> > > + return pdata;
> > > +}
With the first case changed to "case 1:", this block makes sense to me.
Thanks,
Mark.
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