linux-kernel - Re: Issue with sequence to switch to HS400

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Message-ID: <23ce5519-cb4b-4d85-4bfc-3e438212ec7f@intel.com>
Date:   Thu, 25 Jul 2019 10:31:49 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Alan Cooper <alcooperx@...il.com>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Issue with sequence to switch to HS400

On 23/07/19 3:34 PM, Alan Cooper wrote:
> On Tue, Jul 23, 2019 at 1:21 AM Adrian Hunter <adrian.hunter@...el.com> 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 “Timing Interface” 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".

There is no spec for using eMMC with SDHCI.

> 
>>
>> 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.

Yes, I changed my mind.  Does this work:

diff --git a/drivers/mmc/host/sdhci.c b/drivers/mmc/host/sdhci.c
index 59acf8e3331e..f9d241458dcd 100644
--- a/drivers/mmc/host/sdhci.c
+++ b/drivers/mmc/host/sdhci.c
@@ -1849,7 +1849,9 @@ void sdhci_set_uhs_signaling(struct sdhci_host *host, unsigned timing)
 		ctrl_2 |= SDHCI_CTRL_UHS_SDR104;
 	else if (timing == MMC_TIMING_UHS_SDR12)
 		ctrl_2 |= SDHCI_CTRL_UHS_SDR12;
-	else if (timing == MMC_TIMING_UHS_SDR25)
+	else if (timing == MMC_TIMING_SD_HS ||
+		 timing == MMC_TIMING_MMC_HS ||
+		 timing == MMC_TIMING_UHS_SDR25)
 		ctrl_2 |= SDHCI_CTRL_UHS_SDR25;
 	else if (timing == MMC_TIMING_UHS_SDR50)
 		ctrl_2 |= SDHCI_CTRL_UHS_SDR50;