[3/5] scsi: ufs: qcom: Add a readl() to make sure ref_clk gets enabled

In ufs_qcom_dev_ref_clk_ctrl(), it was noted that the ref_clk needs to be
stable for atleast 1us. Eventhough there is wmb() to make sure the write
gets "completed", there is no guarantee that the write actually reached
the UFS device. There is a good chance that the write could be stored in
a Write Buffer (WB). In that case, eventhough the CPU waits for 1us, the
ref_clk might not be stable for that period.

So lets do a readl() to make sure that the previous write has reached the
UFS device before udelay().

Cc: stable@vger.kernel.org
Fixes: f06fcc7155dc ("scsi: ufs-qcom: add QUniPro hardware support and power optimizations")
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
---
 drivers/scsi/ufs/ufs-qcom.c | 6 ++++++
 1 file changed, 6 insertions(+)

On 4/22/22 06:21, Manivannan Sadhasivam wrote:
> In ufs_qcom_dev_ref_clk_ctrl(), it was noted that the ref_clk needs to be
> stable for atleast 1us. Eventhough there is wmb() to make sure the write
                ^              ^
Some spaces are missing.

> gets "completed", there is no guarantee that the write actually reached
> the UFS device. There is a good chance that the write could be stored in
> a Write Buffer (WB). In that case, eventhough the CPU waits for 1us, the
                                          ^
missing space----------------------------

> ref_clk might not be stable for that period.
> 
> So lets do a readl() to make sure that the previous write has reached the
> UFS device before udelay().
> 
> Cc: stable@vger.kernel.org
> Fixes: f06fcc7155dc ("scsi: ufs-qcom: add QUniPro hardware support and power optimizations")
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> ---
>   drivers/scsi/ufs/ufs-qcom.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/scsi/ufs/ufs-qcom.c b/drivers/scsi/ufs/ufs-qcom.c
> index 5f0a8f646eb5..5b9986c63eed 100644
> --- a/drivers/scsi/ufs/ufs-qcom.c
> +++ b/drivers/scsi/ufs/ufs-qcom.c
> @@ -690,6 +690,12 @@ static void ufs_qcom_dev_ref_clk_ctrl(struct ufs_qcom_host *host, bool enable)
>   		/* ensure that ref_clk is enabled/disabled before we return */
>   		wmb();
>   
> +		/*
> +		 * Make sure the write to ref_clk reaches the destination and
> +		 * not stored in a Write Buffer (WB).
> +		 */
> +		readl(host->dev_ref_clk_ctrl_mmio);
> +
>   		/*
>   		 * If we call hibern8 exit after this, we need to make sure that
>   		 * device ref_clk is stable for at least 1us before the hibern8

The comment above the wmb() call looks wrong to me. How about removing 
that wmb() call?

Thanks,

Bart.

On Fri, Apr 22, 2022 at 10:03:22PM -0700, Bart Van Assche wrote:
> On 4/22/22 06:21, Manivannan Sadhasivam wrote:
> > In ufs_qcom_dev_ref_clk_ctrl(), it was noted that the ref_clk needs to be
> > stable for atleast 1us. Eventhough there is wmb() to make sure the write
>                ^              ^
> Some spaces are missing.
> 
> > gets "completed", there is no guarantee that the write actually reached
> > the UFS device. There is a good chance that the write could be stored in
> > a Write Buffer (WB). In that case, eventhough the CPU waits for 1us, the
>                                          ^
> missing space----------------------------
> 

Kind of used to it ;) Will fix it.

> > ref_clk might not be stable for that period.
> > 
> > So lets do a readl() to make sure that the previous write has reached the
> > UFS device before udelay().
> > 
> > Cc: stable@vger.kernel.org
> > Fixes: f06fcc7155dc ("scsi: ufs-qcom: add QUniPro hardware support and power optimizations")
> > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > ---
> >   drivers/scsi/ufs/ufs-qcom.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> > 
> > diff --git a/drivers/scsi/ufs/ufs-qcom.c b/drivers/scsi/ufs/ufs-qcom.c
> > index 5f0a8f646eb5..5b9986c63eed 100644
> > --- a/drivers/scsi/ufs/ufs-qcom.c
> > +++ b/drivers/scsi/ufs/ufs-qcom.c
> > @@ -690,6 +690,12 @@ static void ufs_qcom_dev_ref_clk_ctrl(struct ufs_qcom_host *host, bool enable)
> >   		/* ensure that ref_clk is enabled/disabled before we return */
> >   		wmb();
> > +		/*
> > +		 * Make sure the write to ref_clk reaches the destination and
> > +		 * not stored in a Write Buffer (WB).
> > +		 */
> > +		readl(host->dev_ref_clk_ctrl_mmio);
> > +
> >   		/*
> >   		 * If we call hibern8 exit after this, we need to make sure that
> >   		 * device ref_clk is stable for at least 1us before the hibern8
> 
> The comment above the wmb() call looks wrong to me. How about removing that
> wmb() call?
> 

Hmm, yes it could be removed as well. readl() on weakly ordered architectures
include a control dependency [1] so there is no way the instructions after it
can be speculated.

Thanks,
Mani

[1] https://www.spinics.net/lists/arm-kernel/msg689858.html

> Thanks,
> 
> Bart.