linux-kernel - Re: [PATCH 2/2] dma: of: Remove check on always true condition

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Date:	Mon, 22 Apr 2013 17:13:22 -0500
From:	Jon Hunter <jon-hunter@...com>
To:	Lars-Peter Clausen <lars@...afoo.de>
CC:	Vinod Koul <vinod.koul@...el.com>, Arnd Bergmann <arnd@...db.de>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] dma: of: Remove check on always true condition


On 04/22/2013 04:00 PM, Lars-Peter Clausen wrote:
> On 04/22/2013 10:52 PM, Jon Hunter wrote:
>>
>> On 04/22/2013 03:33 AM, Lars-Peter Clausen wrote:
>>> Both of_dma_nbcells field of the of_dma_controller and the args_count field of
>>> the dma_spec are initialized by parsing the #dma-cells attribute of their device
>>> tree node. So if the device tree nodes of a DMA controller and the dma_spec
>>> match this means that of_dma_nbcells and args_count will also match. So the
>>> second test in the of_dma_find_controller loop is redundant because given the
>>> first test yields true the second test will also yield true. So we can safely
>>> remove the test whether of_dma_nbcells matches args_count. Since this was the
>>> last user of the of_dma_nbcells field we can remove it altogether.
>>
>> This assumes that someone has correctly added the dma information to the
>> dma slave binding. I could see systems where different dma controllers
>> have different of_dma_nbcells and so someone could put the enter wrong
>> number of cells for a dma slave binding. Its really to catch user error.
> 
> No, this assumes nothing. The condition will _always_ be true.
> 
> dma_spec->args_count is initialized by parsing the #dma-cells attribute of
> dma_sepc->np. of_dma->of_dma_nbcells is initialized by parsing the #dma-cells
> attribute of of_dma->of_node. If ofdma->of_node equals dma_spec->np then
> dma_spec->args_count will also equal of_dma->of_dma_nbcells.

Thanks for the clarification. I should have looked more closely at
of_parse_phandle_with_args().

Yes I agree it will always be true if count is less than or equal to
MAX_PHANDLE_ARGS (which defaults to 8). It is very unlikely that someone
would use more than 8 and I guess it does warn on this condition.

if (out_args) {
	int i;
	if (WARN_ON(count > MAX_PHANDLE_ARGS))
		count = MAX_PHANDLE_ARGS;
	out_args->np = node;
	out_args->args_count = count;
	for (i = 0; i < count; i++)
		out_args->args[i] = be32_to_cpup(list++);
}

Cheers
Jon
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