linux-kernel - Re: [PATCH v2] PL330: Add PL330 DMA controller driver

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Message-id: <4BAB4D74.7070909@samsung.com>
Date:	Thu, 25 Mar 2010 20:48:04 +0900
From:	Joonyoung Shim <jy0922.shim@...sung.com>
To:	Marc Zyngier <maz@...terjones.org>
Cc:	linus.ml.walleij@...il.com, dan.j.williams@...el.com,
	kyungmin.park@...sung.com, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] PL330: Add PL330 DMA controller driver

On 3/25/2010 7:32 PM, Marc Zyngier wrote:
> On Thu, 25 Mar 2010 19:05:47 +0900, Joonyoung Shim
> <jy0922.shim@...sung.com> wrote:
>> On 3/25/2010 6:32 PM, Marc Zyngier wrote:
>>> On Thu, 25 Mar 2010 18:01:00 +0900, Joonyoung Shim
>>> <jy0922.shim@...sung.com> wrote:
>>>> On 3/25/2010 2:44 PM, Marc Zyngier wrote:
>>>>> On Thu, 25 Mar 2010 12:17:15 +0900
>>>>> Joonyoung Shim <jy0922.shim@...sung.com> wrote:
>>>>>
>>>>>> +	writew(imm, desc_pool_virt);
>>>> Right. The write[bwl] is api for address ioremapped of io device. I
> will
>>>> change these.
>>>>
>>>>> Does anything ensure that this won't generate an unaligned access?
>>>>>
>>>> PL330 DMA controller fetches variable length instructions that consist
>>> of
>>>> one to six bytes, so i think unaligned access is no problem.
>>> I'm not too concerned about the device side of things. I'm more worried
>>> about the CPU access when writing the 'imm' value to memory.
>>>
>>> Consider desc_pool_virt 16bit aligned when entering the function.
> Writing
>>> the opcode makes it unaligned and then writing the 'imm' value will
>>> result
>>> as an unaligned access.
>>>
>> Why desc_pool_virt should be aligned more than 16bit?
> 
> There is reason for desc_pool_virt to be 16bit aligned. It's just that you
> have 50% chance that it will.
> In such case, you will write 'imm' to a non 16bit-aligned address. In my
> book, that's bad.
> 
> Same for pl330_dmamov(), which tries to write a 32bit value without
> checking the proper alignment.
> In such case, please use the put_unaligned macro to handle the possible
> unaligned access.
> 

OK. i will use put_unaligned.
Thanks.
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