Re: [PATCH 2/2] mmc: core: fix performance regression initializing MMC host controllers

Adrian Hunter
2013-04-05T13:24:48Z

On 05/04/13 12:49, Ulf Hansson wrote:
> On 5 April 2013 09:26, Adrian Hunter <adrian.hunter@intel.com> wrote:
>> On 05/04/13 00:02, Ulf Hansson wrote:
>>> On 4 April 2013 15:41, Adrian Hunter <adrian.hunter@intel.com> wrote:
>>>> Commit fa5501890d8974301042e0202d342a6cbe8609f4 introduced a performance
>>>> regression by adding mmc_power_up() to mmc_start_host(). mmc_power_up()
>>>> is not necessary to host controller initialization, it is part of card
>>>> initialization and is performed anyway asynchronously.
>>>
>>> This commit message is a bit miss-leading. In eMMC case, when the host
>>> driver has no possibility of power cycle the VCCQ power supply but
>>> only the VCC, this is the only proper way to prevent violation of the
>>> eMMC spec.
>>
>> But that is not true. The host controller driver can manage the voltages.
>> The patch is a workaround for the regulator_init_complete late initcall.
>
> In my view, the host driver is not responsible for implementing the
> mmc/sd/sdio protocol as such, that is the core layer responsibility.

Having VCC always on is not part of the MMC protocol. For example, you
can't look up EXT_CSD and find out if VCC is always on. Consequently,
the core does not support it. If a platform requires it, it should be
added as a new feature and flagged as such, not introduced as a "fix".

> Moreover I don't think you should consider this as a workaround, it is
> a proper fix to make sure we follow eMMC spec.

It won't work if the host controller driver is loaded as a module.

Also it causes attempts to initialize cards without first powering off
thereby ensuring they are in a known state.

>
> I noticed you V2 patch, but would still like some more clear
> information in there and maybe mention "boot time performance" instead
> of just "performance".

>
> Kind regards
> Ulf Hansson
>
>>
>> I deliberately did not paraphrase the original commit so that people who
>> wanted to know would have to look at it for themselves. I really cannot add
>> things to my commit message that do not make sense to me.
>>
>>>
>>> >From SD-card point of view, it is not needed, so this can be optimized
>>> to be done as before in an asynchronous mode.
>>>
>>>>
>>>> This patch allows a driver to leave the power up in asynchronous code
>>>> (as it was before).
>>>>
>>>> On my current target platform this reduces driver initialization from:
>>>>
>>>> [ 1.313220] initcall sdhci_acpi_driver_init+0x0/0x12 returned 0 after 102008 usecs
>>>>
>>>> to this:
>>>>
>>>> [ 1.217209] initcall sdhci_acpi_driver_init+0x0/0x12 returned 0 after 8331 usecs
>>>>
>>>> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
>>>> ---
>>>> drivers/mmc/core/core.c | 3 ++-
>>>> drivers/mmc/host/sdhci-acpi.c | 2 ++
>>>> include/linux/mmc/host.h | 1 +
>>>> 3 files changed, 5 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
>>>> index 3bf1c46..c1893c9 100644
>>>> --- a/drivers/mmc/core/core.c
>>>> +++ b/drivers/mmc/core/core.c
>>>> @@ -2416,7 +2416,8 @@ void mmc_start_host(struct mmc_host *host)
>>>> {
>>>> host->f_init = max(freqs[0], host->f_min);
>>>> host->rescan_disable = 0;
>>>> - mmc_power_up(host);
>>>> + if (!(host->caps2 & MMC_CAP2_NO_PRESCAN_POWERUP))
>>>> + mmc_power_up(host);
>>>> mmc_detect_change(host, 0);
>>>> }
>>>>
>>>> diff --git a/drivers/mmc/host/sdhci-acpi.c b/drivers/mmc/host/sdhci-acpi.c
>>>> index 2592ddd..7bcf74b 100644
>>>> --- a/drivers/mmc/host/sdhci-acpi.c
>>>> +++ b/drivers/mmc/host/sdhci-acpi.c
>>>> @@ -195,6 +195,8 @@ static int sdhci_acpi_probe(struct platform_device *pdev)
>>>> host->mmc->pm_caps |= c->slot->pm_caps;
>>>> }
>>>>
>>>> + host->mmc->caps2 |= MMC_CAP2_NO_PRESCAN_POWERUP;
>>>> +
>>>> err = sdhci_add_host(host);
>>>> if (err)
>>>> goto err_free;
>>>> diff --git a/include/linux/mmc/host.h b/include/linux/mmc/host.h
>>>> index 17d7148..8873e83 100644
>>>> --- a/include/linux/mmc/host.h
>>>> +++ b/include/linux/mmc/host.h
>>>> @@ -280,6 +280,7 @@ struct mmc_host {
>>>> #define MMC_CAP2_PACKED_WR (1 << 13) /* Allow packed write */
>>>> #define MMC_CAP2_PACKED_CMD (MMC_CAP2_PACKED_RD | \
>>>> MMC_CAP2_PACKED_WR)
>>>> +#define MMC_CAP2_NO_PRESCAN_POWERUP (1 << 14) /* Don't power up before scan */
>>>>
>>>> mmc_pm_flag_t pm_caps; /* supported pm features */
>>>>
>>>> --
>>>> 1.7.11.7
>>>>
>>>
>>>
>>> Some update to the commit msg, then I am happy.
>>>
>>> Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
>>>
>>>
>>
>
>