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Date:   Sat, 22 Aug 2020 20:34:53 +1000
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Leonardo Bras <leobras.c@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        Joel Stanley <joel@....id.au>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Ram Pai <linuxram@...ibm.com>,
        Brian King <brking@...ux.vnet.ibm.com>,
        Murilo Fossa Vicentini <muvic@...ux.ibm.com>,
        David Dai <zdai@...ux.vnet.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 04/10] powerpc/kernel/iommu: Add new
 iommu_table_in_use() helper



On 18/08/2020 09:40, Leonardo Bras wrote:
> Having a function to check if the iommu table has any allocation helps
> deciding if a tbl can be reset for using a new DMA window.
> 
> It should be enough to replace all instances of !bitmap_empty(tbl...).
> 
> iommu_table_in_use() skips reserved memory, so we don't need to worry about
> releasing it before testing. This causes iommu_table_release_pages() to
> become unnecessary, given it is only used to remove reserved memory for
> testing.
> 
> Signed-off-by: Leonardo Bras <leobras.c@...il.com>
> ---
>  arch/powerpc/include/asm/iommu.h |  1 +
>  arch/powerpc/kernel/iommu.c      | 62 ++++++++++++++++++--------------
>  2 files changed, 37 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/iommu.h b/arch/powerpc/include/asm/iommu.h
> index 5032f1593299..2913e5c8b1f8 100644
> --- a/arch/powerpc/include/asm/iommu.h
> +++ b/arch/powerpc/include/asm/iommu.h
> @@ -154,6 +154,7 @@ extern int iommu_tce_table_put(struct iommu_table *tbl);
>   */
>  extern struct iommu_table *iommu_init_table(struct iommu_table *tbl,
>  		int nid, unsigned long res_start, unsigned long res_end);
> +bool iommu_table_in_use(struct iommu_table *tbl);
>  
>  #define IOMMU_TABLE_GROUP_MAX_TABLES	2
>  
> diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
> index 7f603d4e62d4..c5d5d36ab65e 100644
> --- a/arch/powerpc/kernel/iommu.c
> +++ b/arch/powerpc/kernel/iommu.c
> @@ -668,21 +668,6 @@ static void iommu_table_reserve_pages(struct iommu_table *tbl,
>  		set_bit(i - tbl->it_offset, tbl->it_map);
>  }
>  
> -static void iommu_table_release_pages(struct iommu_table *tbl)
> -{
> -	int i;
> -
> -	/*
> -	 * In case we have reserved the first bit, we should not emit
> -	 * the warning below.
> -	 */
> -	if (tbl->it_offset == 0)
> -		clear_bit(0, tbl->it_map);
> -
> -	for (i = tbl->it_reserved_start; i < tbl->it_reserved_end; ++i)
> -		clear_bit(i - tbl->it_offset, tbl->it_map);
> -}
> -
>  /*
>   * Build a iommu_table structure.  This contains a bit map which
>   * is used to manage allocation of the tce space.
> @@ -743,6 +728,38 @@ struct iommu_table *iommu_init_table(struct iommu_table *tbl, int nid,
>  	return tbl;
>  }
>  
> +bool iommu_table_in_use(struct iommu_table *tbl)
> +{
> +	bool in_use;
> +	unsigned long p1_start = 0, p1_end, p2_start, p2_end;
> +
> +	/*ignore reserved bit0*/

s/ignore reserved bit0/ ignore reserved bit0 /  (add spaces)

> +	if (tbl->it_offset == 0)
> +		p1_start = 1;
> +
> +	/* Check if reserved memory is valid*/

A missing space here.

> +	if (tbl->it_reserved_start >= tbl->it_offset &&
> +	    tbl->it_reserved_start <= (tbl->it_offset + tbl->it_size) &&
> +	    tbl->it_reserved_end   >= tbl->it_offset &&
> +	    tbl->it_reserved_end   <= (tbl->it_offset + tbl->it_size)) {


Uff. What if tbl->it_reserved_end is bigger than tbl->it_offset +
tbl->it_size?

The reserved area is to preserve MMIO32 so it is for it_offset==0 only
and the boundaries are checked in the only callsite, and it is unlikely
to change soon or ever.

Rather that bothering with fixing that, may be just add (did not test):

if (WARN_ON((
(tbl->it_reserved_start || tbl->it_reserved_end) && (it_offset != 0))
||
(tbl->it_reserved_start > it_offset && tbl->it_reserved_end < it_offset
+ it_size) && (it_offset == 0)) )
 return true;

Or simply always look for it_offset..it_reserved_start and
it_reserved_end..it_offset+it_size and if there is no reserved area,
initialize it_reserved_start=it_reserved_end=it_offset so the first
it_offset..it_reserved_start becomes a no-op.


> +		p1_end = tbl->it_reserved_start - tbl->it_offset;
> +		p2_start = tbl->it_reserved_end - tbl->it_offset + 1;
> +		p2_end = tbl->it_size;
> +	} else {
> +		p1_end = tbl->it_size;
> +		p2_start = 0;
> +		p2_end = 0;
> +	}
> +
> +	in_use = (find_next_bit(tbl->it_map, p1_end, p1_start) != p1_end);
> +	if (in_use || p2_start == 0)
> +		return in_use;
> +
> +	in_use = (find_next_bit(tbl->it_map, p2_end, p2_start) != p2_end);
> +
> +	return in_use;
> +}
> +
>  static void iommu_table_free(struct kref *kref)
>  {
>  	unsigned long bitmap_sz;
> @@ -759,10 +776,8 @@ static void iommu_table_free(struct kref *kref)
>  		return;
>  	}
>  
> -	iommu_table_release_pages(tbl);
> -
>  	/* verify that table contains no entries */
> -	if (!bitmap_empty(tbl->it_map, tbl->it_size))
> +	if (iommu_table_in_use(tbl))
>  		pr_warn("%s: Unexpected TCEs\n", __func__);
>  
>  	/* calculate bitmap size in bytes */
> @@ -1069,18 +1084,13 @@ int iommu_take_ownership(struct iommu_table *tbl)
>  	for (i = 0; i < tbl->nr_pools; i++)
>  		spin_lock(&tbl->pools[i].lock);
>  
> -	iommu_table_release_pages(tbl);
> -
> -	if (!bitmap_empty(tbl->it_map, tbl->it_size)) {
> +	if (iommu_table_in_use(tbl)) {
>  		pr_err("iommu_tce: it_map is not empty");
>  		ret = -EBUSY;
> -		/* Undo iommu_table_release_pages, i.e. restore bit#0, etc */
> -		iommu_table_reserve_pages(tbl, tbl->it_reserved_start,
> -				tbl->it_reserved_end);
> -	} else {
> -		memset(tbl->it_map, 0xff, sz);
>  	}
>  
> +	memset(tbl->it_map, 0xff, sz);
> +
>  	for (i = 0; i < tbl->nr_pools; i++)
>  		spin_unlock(&tbl->pools[i].lock);
>  	spin_unlock_irqrestore(&tbl->large_pool.lock, flags);
> 

-- 
Alexey

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