Re: [PATCH 01/12] mmc: core: Throttle polling rate for CMD6

[Ludovic BARRE <ludovic.barre@st.com>]

hi Ulf

Le 2/4/20 à 9:54 AM, Ulf Hansson a écrit :
> In mmc_poll_for_busy() we loop continuously, either by sending a CMD13 or
> by invoking the ->card_busy() host ops, as to detect when the card stops
> signaling busy. This behaviour is problematic as it may cause CPU hogging,
> especially when the busy signal time reaches beyond a few ms.
> 
> Let's fix the issue by adding a throttling mechanism, that inserts a
> usleep_range() in between the polling attempts. The sleep range starts at
> 16-32us, but increases for each loop by a factor of 2, up until the range

Just to align comment and code: in the code the first usleep range start 
at 32-64us.

> reaches ~32-64ms. In this way, we are able to keep the loop fine-grained
> enough for short busy signaling times, while also not hogging the CPU for
> longer times.
> 
> Note that, this change is inspired by the similar throttling mechanism that
> we already use for mmc_do_erase().
> 
> Reported-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
> Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
> ---
>   drivers/mmc/core/mmc_ops.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/mmc/core/mmc_ops.c b/drivers/mmc/core/mmc_ops.c
> index da425ee2d9bf..446a37cc2a86 100644
> --- a/drivers/mmc/core/mmc_ops.c
> +++ b/drivers/mmc/core/mmc_ops.c
> @@ -456,6 +456,7 @@ static int mmc_poll_for_busy(struct mmc_card *card, unsigned int timeout_ms,
>   	struct mmc_host *host = card->host;
>   	int err;
>   	unsigned long timeout;
> +	unsigned int udelay = 32, udelay_max = 32768;
>   	u32 status = 0;
>   	bool expired = false;
>   	bool busy = false;
> @@ -500,6 +501,13 @@ static int mmc_poll_for_busy(struct mmc_card *card, unsigned int timeout_ms,
>   				mmc_hostname(host), __func__);
>   			return -ETIMEDOUT;
>   		}
> +
> +		/* Throttle the polling rate to avoid hogging the CPU. */
> +		if (busy) {
> +			usleep_range(udelay, udelay * 2);
> +			if (udelay < udelay_max)
> +				udelay *= 2;
> +		}
>   	} while (busy);
>   
>   	return 0;
>