linux-kernel - Re: Is iommu_num

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Message-ID: <m24nohsuz8.fsf@linux-m68k.org>
Date:	Sun, 05 Aug 2012 08:39:39 +0200
From:	Andreas Schwab <schwab@...ux-m68k.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Joerg Roedel <joerg.roedel@....com>,
	Anton Blanchard <anton@....ibm.com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: Is iommu_num_pages() broken ?

Benjamin Herrenschmidt <benh@...nel.crashing.org> writes:

> static inline unsigned long iommu_num_pages(unsigned long addr,
> 					    unsigned long len,
> 					    unsigned long io_page_size)
> {
> 	unsigned long size = (addr & (io_page_size - 1)) + len;
>
> 	return DIV_ROUND_UP(size, io_page_size);
> }
>  
>
> That doesn't look right to me...
>
> The powerpc iommu code at least uses that with an addr which may not be
> page aligned (ie, result of sg_virt() which include the offset).
>
> The above code will align the start before adding the len which is wrong

addr & (io_page_size - 1) computes the offset into the page pointed to
by addr.  Looks right to me.

addr & ~(io_page_size - 1) would round addr down to the start of the
page.

Andreas.

-- 
Andreas Schwab, schwab@...ux-m68k.org
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