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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Date:   Fri, 24 Sep 2021 13:07:14 +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 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.