linux-kernel - Re: [PATCH 3/3] drm/panfrost: Clamp lock region to Bifrost minimum

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Date:   Mon, 23 Aug 2021 10:40:59 +0100
From:   Steven Price <steven.price@....com>
To:     Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        dri-devel@...ts.freedesktop.org
Cc:     Rob Herring <robh@...nel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org,
        Chris Morgan <macromorgan@...mail.com>, stable@...r.kernel.org
Subject: Re: [PATCH 3/3] drm/panfrost: Clamp lock region to Bifrost minimum

On 20/08/2021 22:31, Alyssa Rosenzweig wrote:
> When locking a region, we currently clamp to a PAGE_SIZE as the minimum
> lock region. While this is valid for Midgard, it is invalid for Bifrost,

While the spec does seem to state it's invalid for Bifrost - kbase
didn't bother with a lower clamp for a long time. I actually think this
is in many ways more of a spec bug: i.e. implementation details of the
round-up that the hardware does. But it's much safer following the spec
;) And it seems like kbase eventually caught up too.

> where the minimum locking size is 8x larger than the 4k page size. Add a
> hardware definition for the minimum lock region size (corresponding to
> KBASE_LOCK_REGION_MIN_SIZE_LOG2 in kbase) and respect it.
> 
> Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>
> Tested-by: Chris Morgan <macromorgan@...mail.com>
> Cc: <stable@...r.kernel.org>

Reviewed-by: Steven Price <steven.price@....com>

> ---
>  drivers/gpu/drm/panfrost/panfrost_mmu.c  | 2 +-
>  drivers/gpu/drm/panfrost/panfrost_regs.h | 2 ++
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> index 3a795273e505..dfe5f1d29763 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> @@ -66,7 +66,7 @@ static void lock_region(struct panfrost_device *pfdev, u32 as_nr,
>  	/* The size is encoded as ceil(log2) minus(1), which may be calculated
>  	 * with fls. The size must be clamped to hardware bounds.
>  	 */
> -	size = max_t(u64, size, PAGE_SIZE);
> +	size = max_t(u64, size, AS_LOCK_REGION_MIN_SIZE);
>  	region_width = fls64(size - 1) - 1;
>  	region |= region_width;
>  
> diff --git a/drivers/gpu/drm/panfrost/panfrost_regs.h b/drivers/gpu/drm/panfrost/panfrost_regs.h
> index 1940ff86e49a..6c5a11ef1ee8 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_regs.h
> +++ b/drivers/gpu/drm/panfrost/panfrost_regs.h
> @@ -316,6 +316,8 @@
>  #define AS_FAULTSTATUS_ACCESS_TYPE_READ		(0x2 << 8)
>  #define AS_FAULTSTATUS_ACCESS_TYPE_WRITE	(0x3 << 8)
>  
> +#define AS_LOCK_REGION_MIN_SIZE                 (1ULL << 15)
> +
>  #define gpu_write(dev, reg, data) writel(data, dev->iomem + reg)
>  #define gpu_read(dev, reg) readl(dev->iomem + reg)
>  
>