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

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

On Fri, Nov 13, 2015 at 06:54:14PM +0000, Trent Piepho wrote:
> On Fri, 2015-11-13 at 11:28 +0100, Sascha Hauer wrote:
> > > 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?
> 
> Don't have the problem on Linux.  The OF parsing works the same way, but
> the dw driver stats with 0 bus-width caps, like how mxs and tegra-sdmmc
> work.
> 
> The linux dw_mmc driver allows "sub-drivers" to supply caps, and the
> Exynos4/5 driver adds CAP_8_BIT.  Which is probably broken, as it
> doesn't add 4_BIT as well and will have the same problem with the device
> tree not being able to remove bus widths.

Ok, it seems the rationale behind the MMC_CAP_x_BIT_DATA is another one
than thought it is. The host driver cannot know how a card is actually
connected, so it should rely purely on the device tree bus-width
property. So dw_mmc.c should start with host->mci.host_caps = 0 in the
device tree case.
If this is changed we'll also have to fix the platform_data for dw_mmc,
it now needs a bus_width field to let ./arch/arm/mach-socfpga/xload.c
work in 4 bit mode.

Sascha

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