Re: [PATCH 1/3] iommu/io-pgtable-arm: Add nents_per_pgtable in struct io_pgtable_cfg

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

On 2023-08-22 09:45, Nicolin Chen wrote:
> Add a new nents_per_pgtable in struct io_pgtable_cfg to store the number
> of entries per IO pagetable, so it can be passed back to an IOMMU driver.
> It will be used by one of the following changes to set the maximum number
> of TLBI operations in the arm-smmu-v3 driver.
> 
> Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> ---
>   drivers/iommu/io-pgtable-arm.c | 3 +++
>   include/linux/io-pgtable.h     | 2 ++
>   2 files changed, 5 insertions(+)
> 
> diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
> index 72dcdd468cf3..7583d9ecca2b 100644
> --- a/drivers/iommu/io-pgtable-arm.c
> +++ b/drivers/iommu/io-pgtable-arm.c
> @@ -891,6 +891,7 @@ arm_64_lpae_alloc_pgtable_s1(struct io_pgtable_cfg *cfg, void *cookie)
>   
>   	/* TTBR */
>   	cfg->arm_lpae_s1_cfg.ttbr = virt_to_phys(data->pgd);
> +	cfg->nents_per_pgtable = 1 << data->bits_per_level;
>   	return &data->iop;
>   
>   out_free_data:
> @@ -993,6 +994,7 @@ arm_64_lpae_alloc_pgtable_s2(struct io_pgtable_cfg *cfg, void *cookie)
>   
>   	/* VTTBR */
>   	cfg->arm_lpae_s2_cfg.vttbr = virt_to_phys(data->pgd);
> +	cfg->nents_per_pgtable = 1 << data->bits_per_level;
>   	return &data->iop;
>   
>   out_free_data:
> @@ -1071,6 +1073,7 @@ arm_mali_lpae_alloc_pgtable(struct io_pgtable_cfg *cfg, void *cookie)
>   					  ARM_MALI_LPAE_TTBR_ADRMODE_TABLE;
>   	if (cfg->coherent_walk)
>   		cfg->arm_mali_lpae_cfg.transtab |= ARM_MALI_LPAE_TTBR_SHARE_OUTER;
> +	cfg->nents_per_pgtable = 1 << data->bits_per_level;

The result of this highly complex and expensive calculation is clearly 
redundant with the existing bits_per_level field, so why do we need to 
waste space storing when the driver could simply use bits_per_level?

Thanks,
Robin.

>   	return &data->iop;
>   
> diff --git a/include/linux/io-pgtable.h b/include/linux/io-pgtable.h
> index 1b7a44b35616..4b55a327abc1 100644
> --- a/include/linux/io-pgtable.h
> +++ b/include/linux/io-pgtable.h
> @@ -55,6 +55,7 @@ struct iommu_flush_ops {
>    *                 tables.
>    * @ias:           Input address (iova) size, in bits.
>    * @oas:           Output address (paddr) size, in bits.
> + * @nents_per_pgtable: Number of entries per page table.
>    * @coherent_walk  A flag to indicate whether or not page table walks made
>    *                 by the IOMMU are coherent with the CPU caches.
>    * @tlb:           TLB management callbacks for this set of tables.
> @@ -96,6 +97,7 @@ struct io_pgtable_cfg {
>   	unsigned long			pgsize_bitmap;
>   	unsigned int			ias;
>   	unsigned int			oas;
> +	unsigned int			nents_per_pgtable;
>   	bool				coherent_walk;
>   	const struct iommu_flush_ops	*tlb;
>   	struct device			*iommu_dev;