Re: [PATCH v2 8/8] drm/panfrost: Remove unnecessary flushing from tlb_inv_context

[Robin Murphy <robin.murphy@arm.com>]

On 23/08/2019 03:12, Rob Herring wrote:
> tlb_inv_context() hook is only called when freeing the page tables. There's
> no point in flushing only to free the page tables immediately following.

FWIW, in general the point of flushing is *because* we're about to free 
the pagetables - if there's any possibility that the hardware could 
continue to issue translation table walks (speculative or otherwise) 
after those pages have been reused by someone else, TLB badness may ensue.

For panfrost in particular I suspect we can probably get away without 
it, at least for the moment, but it might be worth moving the flush to 
mmu_disable() for complete peace of mind (which kind of preempts the 
sort of thing that per-process AS switching will want anyway).

Robin.

> There is also a problem that we could be accessing the h/w when suspended.
> 
> Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
> Cc: Steven Price <steven.price@arm.com>
> Cc: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Signed-off-by: Rob Herring <robh@kernel.org>
> ---
> v2: new patch
> 
>   drivers/gpu/drm/panfrost/panfrost_mmu.c | 6 +-----
>   1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> index ccf671a9c3fb..9f85275a896c 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> @@ -328,11 +328,7 @@ void panfrost_mmu_unmap(struct panfrost_gem_object *bo)
>   }
>   
>   static void mmu_tlb_inv_context_s1(void *cookie)
> -{
> -	struct panfrost_file_priv *priv = cookie;
> -
> -	mmu_hw_do_operation(priv->pfdev, &priv->mmu, 0, ~0UL, AS_COMMAND_FLUSH_MEM);
> -}
> +{}
>   
>   static void mmu_tlb_inv_range_nosync(unsigned long iova, size_t size,
>   				     size_t granule, bool leaf, void *cookie)
> 
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