linux-kernel - Re: [PATCH v5 2/9] iommu/arm-smmu-v3: Replace s1_cfg with cdtab

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Date:   Wed, 9 Aug 2023 10:59:33 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Will Deacon <will@...nel.org>
Cc:     Michael Shavit <mshavit@...gle.com>, iommu@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        robin.murphy@....com, nicolinc@...dia.com, jean-philippe@...aro.org
Subject: Re: [PATCH v5 2/9] iommu/arm-smmu-v3: Replace s1_cfg with cdtab_cfg

On Wed, Aug 09, 2023 at 02:49:41PM +0100, Will Deacon wrote:

> > @@ -1360,10 +1357,14 @@ static void arm_smmu_write_strtab_ent(struct arm_smmu_master *master, u32 sid,
> >  		    !master->stall_enabled)
> >  			dst[1] |= cpu_to_le64(STRTAB_STE_1_S1STALLD);
> >  
> > -		val |= (s1_cfg->cdcfg.cdtab_dma & STRTAB_STE_0_S1CTXPTR_MASK) |
> > -			FIELD_PREP(STRTAB_STE_0_CFG, STRTAB_STE_0_CFG_S1_TRANS) |
> > -			FIELD_PREP(STRTAB_STE_0_S1CDMAX, s1_cfg->s1cdmax) |
> > -			FIELD_PREP(STRTAB_STE_0_S1FMT, s1_cfg->s1fmt);
> > +		val |= (cd_table->cdtab_dma & STRTAB_STE_0_S1CTXPTR_MASK) |
> > +		       FIELD_PREP(STRTAB_STE_0_CFG, STRTAB_STE_0_CFG_S1_TRANS) |
> > +		       FIELD_PREP(STRTAB_STE_0_S1CDMAX,
> > +				  cd_table->max_cds_bits) |
> > +		       FIELD_PREP(STRTAB_STE_0_S1FMT,
> > +				  cd_table->l1_desc ?
> > +					  STRTAB_STE_0_S1FMT_64K_L2 :
> > +					  STRTAB_STE_0_S1FMT_LINEAR);
> 
> magically know that we're using 64k tables.
> 
> Why is this an improvement to the driver?

Put the above in a function 

arm_smmu_get_cd_ste(struct arm_smmu_ctx_desc_cfg *cdtab, void *ste)

And it makes more sense.

We don't need the driver to precompute the "s1_cfg" parameters and
store them in a redundant struct along side the ctx_desc_cfg when we
can compute those same values on the fly with no cost.

Jason