linux-kernel - Re: [PATCH RFC v3 08/11] scsi: ufs: core: Perform read back after disabling interrupts

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Message-ID: <8f02738d-1b0d-4288-a971-c56d9fca0843@acm.org>
Date: Thu, 21 Dec 2023 11:30:09 -0800
From: Bart Van Assche <bvanassche@....org>
To: Andrew Halaney <ahalaney@...hat.com>, Andy Gross <agross@...nel.org>,
 Bjorn Andersson <andersson@...nel.org>,
 Konrad Dybcio <konrad.dybcio@...aro.org>,
 Manivannan Sadhasivam <mani@...nel.org>,
 "James E.J. Bottomley" <jejb@...ux.ibm.com>,
 "Martin K. Petersen" <martin.petersen@...cle.com>,
 Hannes Reinecke <hare@...e.de>, Janek Kotas <jank@...ence.com>,
 Alim Akhtar <alim.akhtar@...sung.com>, Avri Altman <avri.altman@....com>,
 Can Guo <quic_cang@...cinc.com>
Cc: Will Deacon <will@...nel.org>, linux-arm-msm@...r.kernel.org,
 linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v3 08/11] scsi: ufs: core: Perform read back after
 disabling interrupts

On 12/21/23 11:09, Andrew Halaney wrote:
> Currently, interrupts are cleared and disabled prior to registering the
> interrupt. An mb() is used to complete the clear/disable writes before
> the interrupt is registered.
> 
> mb() ensure that the write completes, but completion doesn't mean that
> it isn't stored in a buffer somewhere. The recommendation for
> ensuring these bits have taken effect on the device is to perform a read
> back to force it to make it all the way to the device. This is
> documented in device-io.rst and a talk by Will Deacon on this can
> be seen over here:
> 
>      https://youtu.be/i6DayghhA8Q?si=MiyxB5cKJXSaoc01&t=1678
> 
> Let's do that to ensure these bits hit the device. Because the mb()'s
> purpose wasn't to add extra ordering (on top of the ordering guaranteed
> by writel()/readl()), it can safely be removed.
> 
> Fixes: 199ef13cac7d ("scsi: ufs: avoid spurious UFS host controller interrupts")
> Signed-off-by: Andrew Halaney <ahalaney@...hat.com>
> ---
>   drivers/ufs/core/ufshcd.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
> index 7bfb556e5b8e..bb603769b029 100644
> --- a/drivers/ufs/core/ufshcd.c
> +++ b/drivers/ufs/core/ufshcd.c
> @@ -10568,7 +10568,7 @@ int ufshcd_init(struct ufs_hba *hba, void __iomem *mmio_base, unsigned int irq)
>   	 * Make sure that UFS interrupts are disabled and any pending interrupt
>   	 * status is cleared before registering UFS interrupt handler.
>   	 */
> -	mb();
> +	ufshcd_readl(hba, REG_INTERRUPT_ENABLE);
>   
>   	/* IRQ registration */
>   	err = devm_request_irq(dev, irq, ufshcd_intr, IRQF_SHARED, UFSHCD, hba);

Reviewed-by: Bart Van Assche <bvanassche@....org>