Re: [PATCH V4 6/7] iommu/msm: Use writel_relaxed and add a barrier

[Arnd Bergmann <arnd@arndb.de>]

On Monday 16 May 2016 12:19:00 Sricharan R wrote:
> diff --git a/drivers/iommu/msm_iommu.c b/drivers/iommu/msm_iommu.c
> index f7b4c11..1240a5a 100644
> --- a/drivers/iommu/msm_iommu.c
> +++ b/drivers/iommu/msm_iommu.c
> @@ -124,6 +124,7 @@ static void msm_iommu_reset(void __iomem *base, int ncb)
>                 SET_TLBLKCR(base, ctx, 0);
>                 SET_CONTEXTIDR(base, ctx, 0);
>         }
> +       mb(); /* sync */
>  }
>  
>  static void __flush_iotlb(void *cookie)
> @@ -143,6 +144,7 @@ static void __flush_iotlb(void *cookie)
>  
>                 __disable_clocks(iommu);
>         }
> +       mb(); /* sync */
>  fail:
>         return;
>  }

These comments are completely useless. What is the specific race
that you are protecting against, and why are the implicit barriers
not sufficient here? Please find a better way to document what
is going on.

> diff --git a/drivers/iommu/msm_iommu_hw-8xxx.h b/drivers/iommu/msm_iommu_hw-8xxx.h
> index 84ba5739..161036c 100644
> --- a/drivers/iommu/msm_iommu_hw-8xxx.h
> +++ b/drivers/iommu/msm_iommu_hw-8xxx.h
> @@ -24,10 +24,10 @@
>  #define GET_CTX_REG(reg, base, ctx) \
>                                 (readl((base) + (reg) + ((ctx) << CTX_SHIFT)))
>  
> -#define SET_GLOBAL_REG(reg, base, val) writel((val), ((base) + (reg)))
> +#define SET_GLOBAL_REG(reg, base, val) writel_relaxed((val), ((base) + (reg)))
>  
>  #define SET_CTX_REG(reg, base, ctx, val) \
> -                       writel((val), ((base) + (reg) + ((ctx) << CTX_SHIFT)))
> +               writel_relaxed((val), ((base) + (reg) + ((ctx) << CTX_SHIFT)))
>  
>  /* Wrappers for numbered registers */
>  #define SET_GLOBAL_REG_N(b, n, r, v) SET_GLOBAL_REG(b, ((r) + (n << 2)), (v))
> @@ -48,7 +48,8 @@
>  #define SET_FIELD(addr, mask, shift, v) \
>  do { \
>         int t = readl(addr); \
> -       writel((t & ~((mask) << (shift))) + (((v) & (mask)) << (shift)), addr);\
> +       writel_relaxed((t & ~((mask) << (shift))) + \
> +                      (((v) & (mask)) << (shift)), addr);\
>  } while (0)

No, please add a new macro for the relaxed accessors and then add comments
in the places where those are used, to document why you can take shortcuts
in those places.

	Arnd