Subject: Re: [PATCH v2] scsi: ufs: Fix possible infinite loop in ufshcd_hold

From: Can Guo <[email-masked]>
Date: Wed, 29 Jul 2020 16:43:27 +0800

Hi Stanley,

On 2020-07-29 10:40, Stanley Chu wrote:
> In ufshcd_suspend(), after clk-gating is suspended and link is set
> as Hibern8 state, ufshcd_hold() is still possibly invoked before
> ufshcd_suspend() returns. For example, MediaTek's suspend vops may
> issue UIC commands which would call ufshcd_hold() during the command
> issuing flow.
> 
> Now if UFSHCD_CAP_HIBERN8_WITH_CLK_GATING capability is enabled,
> then ufshcd_hold() may enter infinite loops because there is no
> clk-ungating work scheduled or pending. In this case, ufshcd_hold()
> shall just bypass, and keep the link as Hibern8 state.
> 

The infinite loop is expected as ufshcd_hold is called again after
link is put to hibern8 state, so in QCOM's code, we never do this.
The cap UFSHCD_CAP_HIBERN8_WITH_CLK_GATING means UIC link state
must not be HIBERN8 after ufshcd_hold(async=false) returns.

Instead of bailing out from that loop, which makes the logic of
ufshcd_hold and clk gating even more complex, how about removing
ufshcd_hold/release from ufshcd_send_uic_cmd()? I think they are
redundant and we should never send DME cmds if clocks/powers are
not ready. I mean callers should make sure they are ready to send
DME cmds (and only callers know when), but not leave that job to
ufshcd_send_uic_cmd(). It is convenient to remove ufshcd_hold/
release from ufshcd_send_uic_cmd() as there are not many places
sending DME cmds without holding the clocks, ufs_bsg.c is one.
And I have tested my idea on my setup, it worked well for me.
Another benefit is that it also allows us to use DME cmds
in clk gating/ungating contexts if we need to in the future.

Please let me know your idea, thanks.

Can Guo.

> Signed-off-by: Stanley Chu <[email-masked]>
> Signed-off-by: Andy Teng <[email-masked]>
> 
> ---
> 
> Changes since v1:
> - Fix return value: Use unique bool variable to get the result of
> flush_work(). Thcan prevent incorrect returned value, i.e., rc, if
> flush_work() returns true
> - Fix commit message
> 
> ---
>  drivers/scsi/ufs/ufshcd.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
> index 577cc0d7487f..acba2271c5d3 100644
> --- a/drivers/scsi/ufs/ufshcd.c
> +++ b/drivers/scsi/ufs/ufshcd.c
> @@ -1561,6 +1561,7 @@ static void ufshcd_ungate_work(struct work_struct 
> *work)
>  int ufshcd_hold(struct ufs_hba *hba, bool async)
>  {
>  	int rc = 0;
> +	bool flush_result;
>  	unsigned long flags;
> 
>  	if (!ufshcd_is_clkgating_allowed(hba))
> @@ -1592,7 +1593,9 @@ int ufshcd_hold(struct ufs_hba *hba, bool async)
>  				break;
>  			}
>  			spin_unlock_irqrestore(hba->host->host_lock, flags);
> -			flush_work(&hba->clk_gating.ungate_work);
> +			flush_result = flush_work(&hba->clk_gating.ungate_work);
> +			if (hba->clk_gating.is_suspended && !flush_result)
> +				goto out;
>  			spin_lock_irqsave(hba->host->host_lock, flags);
>  			goto start;
>  		}

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