linux-kernel - Re: [PATCH v2 1/6] iommu/vt-d: Setup scalable mode context entry in probe path

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Message-ID: <20231211173826.GE1489931@ziepe.ca>
Date:   Mon, 11 Dec 2023 13:38:26 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Tian <kevin.tian@...el.com>, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/6] iommu/vt-d: Setup scalable mode context entry in
 probe path

On Tue, Dec 05, 2023 at 09:21:58AM +0800, Lu Baolu wrote:
> +/*
> + * Get the PASID directory size for scalable mode context entry.
> + * Value of X in the PDTS field of a scalable mode context entry
> + * indicates PASID directory with 2^(X + 7) entries.
> + */
> +static unsigned long context_get_sm_pds(struct pasid_table *table)
> +{
> +	unsigned long pds, max_pde;
> +
> +	max_pde = table->max_pasid >> PASID_PDE_SHIFT;
> +	pds = find_first_bit(&max_pde, MAX_NR_PASID_BITS);
> +	if (pds < 7)
> +		return 0;
> +
> +	return pds - 7;

This seems like a convoluted way to write 
  max(ilog2(table-max_pasid) - 7,0)

?

Jason