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

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Message-ID: <ZOTlcFs2NG6nJEPN@Asurada-Nvidia>
Date:   Tue, 22 Aug 2023 09:42:24 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Robin Murphy <robin.murphy@....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 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?

Thanks
Nicolin