Re: Issue with sequence to switch to HS400

[Alan Cooper]

On Tue, Jul 23, 2019 at 1:21 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>
> On 23/07/19 1:31 AM, Alan Cooper wrote:
> > I'm having a problem with a new SD/MMC controller and PHY in our
> > latest SoC's. The issue I'm seeing is that I can't switch into HS400
> > mode. This looks like something the driver is doing that doesn't meet
> > the JEDEC spec. In the "HS400 timing mode selection" section of the
> > JEDEC spec , in step 7 it states:
> >
> > 7) Set the âTiming Interfaceâ parameter in the HS_TIMING [185] field
> > of the Extended CSD register to 0x1 to switch to High Speed mode and
> > then set the clock frequency to a value not greater than 52 MHz.
> >
> > In the function mmc_select_hs400() in mmc.c, I see that a switch
> > command is done to set the eMMC device to HS mode and then
> > mmc_set_timing(card->host, MMC_TIMING_MMC_HS) is used to change the
> > controller to HS mode. The problem is that the "SD Host Controller
> > Standard Specification" states that "UHS Mode Select" field of the
> > "Host Control 2 Register" controls the mode when the "1.8V Signaling
> > Enable" bit in the same register is set, so mmc_set_timing() is
> > actually leaving the controller in SDR12 mode and mmc_select_hs400()
> > will then set the clock to 52MHz. This causes our PHY to detect an
> > illegal combination and return an error.
> >
> > I think the easiest fix would be to change mmc_set_timing(card->host,
> > MMC_TIMING_MMC_HS) to mmc_set_timing(card->host,
> > MMC_TIMING_UHS_SDR25). The other possibility would be to change
> > mmc_set_timing to handle the "1.8V Signaling Enable" bit properly.
> > I'll submit a patch based on the feedback I get.
>
> eMMC is governed by JEDEC specs not SD specs.

My understanding is that JEDEC does not have a host controller spec so
this driver uses the "SD Host Controller Standard Specification".

>
> Please consider making a change in your driver instead.  For example, hook
> ->set_ios() and if 1.8V is enabled and timing is set to MMC_TIMING_MMC_HS
> then change it to MMC_TIMING_UHS_SDR25.

That's an easy fix, but it still leaves all other drivers/systems
temporarily using SDR12 at 52MHz during the switch to HS400.