linux-kernel - Re: Proper way to allocate DMA buffer within a single 4GB block?

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Message-ID: <20170924102046.357c00ca@gmail.com>
Date:   Sun, 24 Sep 2017 10:20:46 -0400
From:   Chuck Ebbert <cebbert.lkml@...il.com>
To:     Timur Tabi <timur@...eaurora.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: Proper way to allocate DMA buffer within a single 4GB block?

On Wed, 20 Sep 2017 17:18:09 -0500
Timur Tabi <timur@...eaurora.org> wrote:

> I have a device that requires I allocated a few buffers for DMA. The 
> problem is that this device has only one register for the upper 32
> bits of all of the buffers. That is, all of buffers must reside
> within the same 4GB block of memory. In order words,
> 
> 	end = start + size - 1;
> 	if (upper_32_bits(start) != upper_32_bits(end))
> 		// Oh no, the buffer spans across a 4GB boundary!
> 
> The buffer is typically less than 16KB in size, so we've never seen
> it actually span across a 4GB boundary.  However, I want to ensure
> that it's impossible.  I wrote this function that re-tries the
> allocation if the first one is invalid, but I suspect that it's too
> hackish.  Is there a better way?
> 

Allocate a buffer twice as big as what you really need. If the first
half doesn't work, the second half will.