Re: [RESEND PATCH 1/3] mmc: host: Introduce the request_atomic() for the host

[Adrian Hunter]

On 17/03/20 5:36 am, Baolin Wang wrote:
> On Mon, Mar 16, 2020 at 9:09 PM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>>
>> On 4/03/20 9:42 am, Baolin Wang wrote:
>>> The SD host controller can process one request in the atomic context if
>>> the card is nonremovable, which means we can submit next request in the
>>> irq hard handler when using the MMC software queue to reduce the latency.
>>> Thus this patch adds a new API request_atomic() for the host controller
>>> and implement it for the SD host controller.
>>>
>>> Suggested-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
>>> Signed-off-by: Baolin Wang <baolin.wang7@xxxxxxxxx>
>>> ---
>>>  drivers/mmc/host/sdhci.c | 27 +++++++++++++++++++--------
>>>  drivers/mmc/host/sdhci.h |  1 +
>>>  include/linux/mmc/host.h |  3 +++
>>>  3 files changed, 23 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/drivers/mmc/host/sdhci.c b/drivers/mmc/host/sdhci.c
>>> index 9c37451..4febbcb 100644
>>> --- a/drivers/mmc/host/sdhci.c
>>> +++ b/drivers/mmc/host/sdhci.c
>>> @@ -2016,17 +2016,12 @@ void sdhci_set_power(struct sdhci_host *host, unsigned char mode,
>>>   *                                                                           *
>>>  \*****************************************************************************/
>>>
>>> -void sdhci_request(struct mmc_host *mmc, struct mmc_request *mrq)
>>> +static void sdhci_start_request(struct mmc_host *mmc, struct mmc_request *mrq,
>>> +                             int present)
>>>  {
>>> -     struct sdhci_host *host;
>>> -     int present;
>>> +     struct sdhci_host *host = mmc_priv(mmc);
>>>       unsigned long flags;
>>>
>>> -     host = mmc_priv(mmc);
>>> -
>>> -     /* Firstly check card presence */
>>> -     present = mmc->ops->get_cd(mmc);
>>> -
>>>       spin_lock_irqsave(&host->lock, flags);
>>>
>>>       sdhci_led_activate(host);
>>> @@ -2043,6 +2038,22 @@ void sdhci_request(struct mmc_host *mmc, struct mmc_request *mrq)
>>>
>>>       spin_unlock_irqrestore(&host->lock, flags);
>>>  }
>>> +
>>> +void sdhci_request_atomic(struct mmc_host *mmc, struct mmc_request *mrq)
>>> +{
>>> +     sdhci_start_request(mmc, mrq, 1);
>>> +}
>>> +EXPORT_SYMBOL_GPL(sdhci_request_atomic);
>>> +
>>> +void sdhci_request(struct mmc_host *mmc, struct mmc_request *mrq)
>>> +{
>>> +     int present;
>>> +
>>> +     /* Firstly check card presence */
>>> +     present = mmc->ops->get_cd(mmc);
>>> +
>>> +     sdhci_start_request(mmc, mrq, present);
>>> +}
>>>  EXPORT_SYMBOL_GPL(sdhci_request);
>>>
>>>  void sdhci_set_bus_width(struct sdhci_host *host, int width)
>>> diff --git a/drivers/mmc/host/sdhci.h b/drivers/mmc/host/sdhci.h
>>> index cac2d97..5507a73 100644
>>> --- a/drivers/mmc/host/sdhci.h
>>> +++ b/drivers/mmc/host/sdhci.h
>>> @@ -775,6 +775,7 @@ void sdhci_set_power(struct sdhci_host *host, unsigned char mode,
>>>  void sdhci_set_power_noreg(struct sdhci_host *host, unsigned char mode,
>>>                          unsigned short vdd);
>>>  void sdhci_request(struct mmc_host *mmc, struct mmc_request *mrq);
>>> +void sdhci_request_atomic(struct mmc_host *mmc, struct mmc_request *mrq);
>>>  void sdhci_set_bus_width(struct sdhci_host *host, int width);
>>>  void sdhci_reset(struct sdhci_host *host, u8 mask);
>>>  void sdhci_set_uhs_signaling(struct sdhci_host *host, unsigned timing);
>>> diff --git a/include/linux/mmc/host.h b/include/linux/mmc/host.h
>>> index 562ed06..db5e59c 100644
>>> --- a/include/linux/mmc/host.h
>>> +++ b/include/linux/mmc/host.h
>>> @@ -92,6 +92,9 @@ struct mmc_host_ops {
>>>                           int err);
>>>       void    (*pre_req)(struct mmc_host *host, struct mmc_request *req);
>>>       void    (*request)(struct mmc_host *host, struct mmc_request *req);
>>> +     /* Submit one request to host in atomic context. */
>>> +     void    (*request_atomic)(struct mmc_host *host,
>>> +                               struct mmc_request *req);
>>
>> This doesn't have the flexibility to return "busy".  For example,
>> sdhci_send_command() will potentially wait quite some time if the inhibit
>> bits are set.  That is not good in interrupt context.  It would be better to
>> return immediately in that case and have the caller fall back to a
>> non-atomic context.  Thoughts?
> 
> Yes, I unserstood your concern. But the sdhci_send_command() is
> already under the spin_lock_irqsave() protection, which will also
> disable the interrupt for some time if the inhibit bits are set. That
> is same with moving it in interrupt context.

It is, but I would like to fix that too.

> 
> Moreover, if the previous command complete interrupt and transfer
> complete interrupt are normal, we should not meet this issue of
> polling inhibit bits (I have not met this issue on my platform). So I
> think we can remove the polling here? If the inhibit bits are set, I
> think the command complete interrupt or the transfer complete
> interrupt have been abnormal, so we can just return the error here.
> What do you think? Thanks.
> 

I suspect the inhibit polling might be needed for some host controllers in
some situations.  ie. taking it out would likely break things.