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:   Tue, 28 Sep 2021 13:18:49 +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 25/09/2021 00:33, Bean Huo wrote:
> On Fri, 2021-09-24 at 16:26 +0300, Adrian Hunter wrote:
>> On 24/09/21 4:08 pm, Bean Huo wrote:
>>> On Fri, 2021-09-24 at 15:17 +0300, Adrian Hunter wrote:
>>>>>>>          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.
>>>
>>> yes, this is the purpose of the patch, for the host controller
>>> without
>>> quirk SDHCI_QUIRK2_DISABLE_HW_TIMEOUT, if the timeout count
>>> required by
>>> device is beyond the HW timer max count, we choose SW timer to
>>> avoid the HW timer timeout IRQ.
>>>
>>> I don't know if I get it correctly.
>>
>> Why can't drivers that want the behaviour just set the quirk?
>>
>> Drivers that do not work with the quirk, do not have to set it.
> 
> 
> Adrian,
> 
> We cannot add this quirk to every host driver.

I was suggesting only the ones for which it works.

>  This is the difference
> on the device side.

It is the host controller that has the problem, not the device.
Hence the quirk.

> In addition, I don't know what the maximum hardware
> timer budget for each host is.

mmc->max_busy_timeout is calculated by sdhci.c, or the driver can
override the maximum count via ->get_max_timeout_count() or the driver
can override mmc->max_busy_timeout.

With the quirk, sdhci.c will usually set mmc->max_busy_timeout to zero.
That allows timeouts greater than the hardware can support, and then,
with the quirk, the driver will switch to a software timeout when needed.

However, that won't work for every host controller, because some do not
provide a completion interrupt after the timeout, even if the timeout
interrupt is disabled.  That means they should set mmc->max_busy_timeout
to the hardware value.  Hence the quirk is needed to tell the difference.

> Even if you use the same SOC, the
> maximum time budget on different platforms may be different.

The mmc core calculates timeouts based on the relevant standards and
values provided by the device itself.

> Assume that the maximum timeout time supported by the hardware timer is
> 100 milliseconds

I realise it is an example, but 100 milliseconds is a bit low. Legacy
host controllers have always had to deal with standard SD card and
MMC card timeouts.  SD card write timeout of 500 milliseconds for instance.

> but the maximum data transmission time required by
> the device is 150 milliseconds. In most cases, data transfer will not
> take so long. 150 is the maximum time under extreme conditions. This
> means that the device just needs to complete a data transfer within
>> 100ms and keep the data line busy. If we still use the HW timer, it
> will trigger a DATA LINE timeout interrupt.
> 
> This patch does not affect scenarios where the hardware timer meets the
> max data transmission time requirements of the device. It will still
> use the hardware timer. Only when the device changes, will it switch to
> using the SW timer.

Which is what the quirk does.  So I am very confused why the quirk is
no good?