linux-kernel - Re: [PATCH V1] mmc: sdhci-msm: Avoid early clock doubling during HS400 transition

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Message-ID: <5dcd735d-31bf-4c7f-bf12-89a5cae0d628@oss.qualcomm.com>
Date: Fri, 14 Nov 2025 14:02:55 +0530
From: Sarthak Garg <sarthak.garg@....qualcomm.com>
To: Bjorn Andersson <andersson@...nel.org>
Cc: Adrian Hunter <adrian.hunter@...el.com>,
        Ulf Hansson <ulf.hansson@...aro.org>, linux-mmc@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        quic_nguyenb@...cinc.com, quic_rampraka@...cinc.com,
        quic_pragalla@...cinc.com, quic_sayalil@...cinc.com,
        quic_nitirawa@...cinc.com, quic_bhaskarv@...cinc.com,
        kernel@....qualcomm.com
Subject: Re: [PATCH V1] mmc: sdhci-msm: Avoid early clock doubling during
 HS400 transition


On 11/10/2025 8:09 PM, Bjorn Andersson wrote:
> On Mon, Nov 10, 2025 at 12:08:01PM +0530, Sarthak Garg wrote:
>> According to the hardware programming guide, the clock frequency must
>> remain below 52MHz during the transition to HS400 mode.
>>
>> However,in the current implementation, the timing is set to HS400 (a
>> DDR mode) before adjusting the clock. This causes the clock to double
>> prematurely to 104MHz during the transition phase, violating the
>> specification and potentially resulting in CRC errors or CMD timeouts.
>>
>> This change ensures that clock doubling is avoided during intermediate
>> transitions and is applied only when the card requires a 200MHz clock
>> for HS400 operation.
>>
>> Signed-off-by: Sarthak Garg <sarthak.garg@....qualcomm.com>
>> ---
>>   drivers/mmc/host/sdhci-msm.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/mmc/host/sdhci-msm.c b/drivers/mmc/host/sdhci-msm.c
>> index 4e5edbf2fc9b..eca6a09a4547 100644
>> --- a/drivers/mmc/host/sdhci-msm.c
>> +++ b/drivers/mmc/host/sdhci-msm.c
>> @@ -355,7 +355,8 @@ static unsigned int msm_get_clock_mult_for_bus_mode(struct sdhci_host *host)
>>   	 */
>>   	if (ios.timing == MMC_TIMING_UHS_DDR50 ||
>>   	    ios.timing == MMC_TIMING_MMC_DDR52 ||
>> -	    ios.timing == MMC_TIMING_MMC_HS400 ||
>> +	    (ios.timing == MMC_TIMING_MMC_HS400 &&
>> +	    ios.clock == MMC_HS200_MAX_DTR) ||
> It's a bit ugly that sdhci_msm_execute_tuning() passes ios.clock as an
> argument to msm_set_clock_rate_for_bus_mode(), which then calls
> msm_get_clock_mult_for_bus_mode() where you reach back into ios.clock.
>
> In fact, having msm_get_clock_mult_for_bus_mode() reach into
> host->mmc->ios to get ios.timing, seems a violation of the original
> intent of the prototype.
>
>
> How about cleaning this up and passing "timing" as an argument to
> msm_set_clock_rate_for_bus_mode(), and then pass host, clock, and timing
> to msm_get_clock_mult_for_bus_mode()?
>
> That way we avoid this mix of passing parameters to the functions in
> both arguments and in state at the same time.
>
> Regards,
> Bjorn


Sure will update in V2.

Regards,
Sarthak


>>   	    host->flags & SDHCI_HS400_TUNING)
>>   		return 2;
>>   	return 1;
>> -- 
>> 2.34.1
>>
>>