linux-kernel - Re: [PATCH v2 2/6] iommu/io-pgtable: Allow a pgtable implementation to skip TLB operations

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Date:   Tue, 7 Jul 2020 12:34:26 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Jordan Crouse <jcrouse@...eaurora.org>,
        linux-arm-msm@...r.kernel.org
Cc:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        iommu@...ts.linux-foundation.org,
        John Stultz <john.stultz@...aro.org>,
        freedreno@...ts.freedesktop.org, Joerg Roedel <jroedel@...e.de>,
        Will Deacon <will@...nel.org>, Yong Wu <yong.wu@...iatek.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/6] iommu/io-pgtable: Allow a pgtable implementation
 to skip TLB operations

On 2020-06-26 21:04, Jordan Crouse wrote:
> Allow a io-pgtable implementation to skip TLB operations by checking for
> NULL pointers in the helper functions. It will be up to to the owner
> of the io-pgtable instance to make sure that they independently handle
> the TLB correctly.

I don't really understand what this is for - tricking the IOMMU driver 
into not performing its TLB maintenance at points when that maintenance 
has been deemed necessary doesn't seem like the appropriate way to 
achieve anything good :/

Robin.

> Signed-off-by: Jordan Crouse <jcrouse@...eaurora.org>
> ---
> 
>   include/linux/io-pgtable.h | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/io-pgtable.h b/include/linux/io-pgtable.h
> index 53d53c6c2be9..bbed1d3925ba 100644
> --- a/include/linux/io-pgtable.h
> +++ b/include/linux/io-pgtable.h
> @@ -210,21 +210,24 @@ struct io_pgtable {
>   
>   static inline void io_pgtable_tlb_flush_all(struct io_pgtable *iop)
>   {
> -	iop->cfg.tlb->tlb_flush_all(iop->cookie);
> +	if (iop->cfg.tlb)
> +		iop->cfg.tlb->tlb_flush_all(iop->cookie);
>   }
>   
>   static inline void
>   io_pgtable_tlb_flush_walk(struct io_pgtable *iop, unsigned long iova,
>   			  size_t size, size_t granule)
>   {
> -	iop->cfg.tlb->tlb_flush_walk(iova, size, granule, iop->cookie);
> +	if (iop->cfg.tlb)
> +		iop->cfg.tlb->tlb_flush_walk(iova, size, granule, iop->cookie);
>   }
>   
>   static inline void
>   io_pgtable_tlb_flush_leaf(struct io_pgtable *iop, unsigned long iova,
>   			  size_t size, size_t granule)
>   {
> -	iop->cfg.tlb->tlb_flush_leaf(iova, size, granule, iop->cookie);
> +	if (iop->cfg.tlb)
> +		iop->cfg.tlb->tlb_flush_leaf(iova, size, granule, iop->cookie);
>   }
>   
>   static inline void
> @@ -232,7 +235,7 @@ io_pgtable_tlb_add_page(struct io_pgtable *iop,
>   			struct iommu_iotlb_gather * gather, unsigned long iova,
>   			size_t granule)
>   {
> -	if (iop->cfg.tlb->tlb_add_page)
> +	if (iop->cfg.tlb && iop->cfg.tlb->tlb_add_page)
>   		iop->cfg.tlb->tlb_add_page(gather, iova, granule, iop->cookie);
>   }
>   
>