linux-kernel - Re: [PATCH v1 2/2] mmc: sdhci: Use the SW timer when the HW timer cannot meet the timeout value required by the device

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Message-ID: <79056ca7-bfe3-1b25-b6fd-de8a9388b75f@intel.com>
Date:   Fri, 24 Sep 2021 15:17:44 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Bean Huo <huobean@...il.com>, Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Bean Huo <beanhuo@...ron.com>, linux-mmc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] mmc: sdhci: Use the SW timer when the HW timer
 cannot meet the timeout value required by the device

On 24/09/21 2:45 pm, Bean Huo wrote:
> On Fri, 2021-09-24 at 13:07 +0300, Adrian Hunter wrote:
>> On 24/09/21 12:17 pm, Bean Huo wrote:
>>> On Fri, 2021-09-24 at 08:29 +0300, Adrian Hunter wrote:
>>>>> If the data transmission timeout value required by the device
>>>>> exceeds
>>>>> the maximum timeout value of the host HW timer, we still use
>>>>> the HW
>>>>> timer with the maximum timeout value of the HW timer. This
>>>>> setting
>>>>> is
>>>>> suitable for most R/W situations. But sometimes, the device
>>>>> will
>>>>> complete
>>>>> the R/W task within its required timeout value (greater than
>>>>> the HW
>>>>> timer).
>>>>> In this case, the HW timer for data transmission will time out.
>>>>> Currently, in this condition, we  disable the HW timer and use
>>>>> the
>>>>> SW
>>>>> timer only when the SDHCI_QUIRK2_DISABLE_HW_TIMEOUT quirk is
>>>>> set by
>>>>> the
>>>>> host driver. The patch is to remove this if statement
>>>>> restriction
>>>>> and
>>>>> allow data transmission to use the SW timer when the hardware
>>>>> timer
>>>>> cannot
>>>>> meet the required timeout value.
>>>>
>>>> The reason it is a quirk is because it does not work for all
>>>> hardware.
>>>>
>>>> For some controllers the timeout cannot really be disabled, only
>>>> the
>>>>
>>>> interrupt is disabled, and then the controller never indicates
>>>> completion
>>>>
>>>> if the timeout is exceeded.
>>>
>>> Hi Adrian,
>>> Thanks for your review.
>>>
>>> Yes, you are right. But this quirk prevents disabling the hardware
>>> timeoutIRQ. The purpose of this patch is to disable the hardware
>>> timeout IRQ and
>>> select the software timeout.
>>>
>>> void __sdhci_set_timeout(struct sdhci_host *host, struct
>>> mmc_command
>>> *cmd)
>>> {
>>>         bool too_big = false;
>>>         u8 count = sdhci_calc_timeout(host, cmd, &too_big);
>>>
>>>         if (too_big) {
>>>                 sdhci_calc_sw_timeout(host, cmd);
>>>                 sdhci_set_data_timeout_irq(host, false); // disable
>>> IRQ
>>>         } else if (!(host->ier & SDHCI_INT_DATA_TIMEOUT)) {
>>>                 sdhci_set_data_timeout_irq(host, true);
>>>         }
>>>
>>>         sdhci_writeb(host, count, SDHCI_TIMEOUT_CONTROL);
>>> }
>>>
>>>
>>> The driver has detected that the hardware timer cannot meet the
>>> timeout
>>> requirements of the device, but we still use the hardware timer,
>>> which will
>>> allow potential timeout issuea . Rather than allowing a potential
>>> problem to exist, why can’t software timing be used to avoid this
>>> problem?
>>
>> Timeouts aren't that accurate.  The maximum is assumed still to work.
>> mmc->max_busy_timeout is used to tell the core what the maximum is.
> 
>>
> 
> 
> 
> mmc->max_busy_timeout is still a representation of Host HW timer
> maximum timeout count, isn't it? 

Not necessarily.  For SDHCI_QUIRK2_DISABLE_HW_TIMEOUT it would be
set to zero to indicate no maximum.