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

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Date:   Tue, 29 Aug 2023 16:37:00 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     will@...nel.org, jgg@...dia.com, joro@...tes.org,
        jean-philippe@...aro.org, apopple@...dia.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux.dev
Subject: Re: [PATCH 1/3] iommu/io-pgtable-arm: Add nents_per_pgtable in struct
 io_pgtable_cfg

On 2023-08-22 17:42, Nicolin Chen wrote:
> On Tue, Aug 22, 2023 at 10:19:21AM +0100, Robin Murphy wrote:
> 
>>>    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?
> 
> bits_per_level is in the private struct arm_lpae_io_pgtable, while
> drivers can only access struct io_pgtable_cfg. Are you suggesting
> to move bits_per_level out of the private struct arm_lpae_io_pgtable
> to the public struct io_pgtable_cfg?
> 
> Or am I missing another bits_per_level?

Bleh, apologies, I always confuse myself trying to remember the fiddly 
design of io-pgtable data. However, I think this then ends up proving 
the opposite point - the number of pages per table only happens to be a 
fixed constant for certain formats like LPAE, but does not necessarily 
generalise. For instance for a single v7s config it would be 1024 or 256 
or 16 depending on what has actually been unmapped.

The mechanism as proposed implicitly assumes LPAE format, so I still 
think we're better off making that assumption explicit. And at that 
point arm-smmu-v3 can then freely admit it already knows the number is 
simply 1/8th of the domain page size.

Thanks,
Robin.