Re: MCI bus-width host caps via device tree don't work properly for dw and imx

[Sascha Hauer <s.hauer@pengutronix.de>]

Hi Trent,

On Fri, Nov 13, 2015 at 12:36:22AM +0000, Trent Piepho wrote:
> I found my 4-bit connected eMMC chip wasn't working because barebox was
> trying to run it in 8-bit mode.  I had the proper bus-width property in
> the device tree, but this doesn't work.
> 
> The problem is that mci_of_parse() uses bus-width to _add_ 8-bit and/or
> 4-bit caps to the host.  It doesn't remove any host_caps the driver
> already has set.  So if the driver supports 8-bit but the DT bus-width
> is 4, you still get a driver that thinks 8-bit will work.
> 
> You can see this in dw_mmc.c, dw_mmc_probe() sets:
> host->mci.host_caps = MMC_CAP_4_BIT_DATA | MMC_CAP_8_BIT_DATA
> then later
> mci_of_parse(&host->mci);
> return mci_register(&host->mci);
> 
> The mci_of_parse call will not remove the 4 and 8 bits caps.  There is
> the same problem in imx-esdhc.c.  The other two users of mci_of_parse,
> mxs.c and tegra-sdmmc.c, don't have this problem, since they don't set
> any bus-width host_caps in the driver and depend on the DT or
> platform_data to have this info.
> 
> With drivers split 50/50, which is the right way?  Don't have the driver
> report what bits it supports and depend on the device tree to have that
> information?  Or have of_mci_parse remove widths that aren't indicated
> as supported in the DT, so that the DT can remove widths the hardware
> indicates it supports.

I think the resulting flags should be the subset of what all components
can do, that is (driver_flags & devicetree_flags & card_flags).

So mci_of_parse() should IMO clear the MMC_CAP_8_BIT_DATA flag if the
bus-width property is set to 4.

You should encounter the same problem under Linux, right?

Sascha


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