linux-kernel - Re: [PATCH] mmc: block: Add new ioctl to send multi commands

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Date:	Thu, 10 Sep 2015 19:20:54 +0100
From:	Jon Hunter <jonathanh@...dia.com>
To:	Grant Grundler <grundler@...gle.com>
CC:	Arnd Bergmann <arnd@...db.de>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Olof Johansson <olofj@...omium.org>,
	Seshagiri Holi <sholi@...dia.com>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mmc: block: Add new ioctl to send multi commands


On 10/09/15 18:10, Grant Grundler wrote:
> On Thu, Sep 10, 2015 at 1:24 AM, Jon Hunter <jonathanh@...dia.com> wrote:
> ...
>>> - you have some implicit padding after the structure and should replace that
>>>   with explictit pad bytes to extend the structure to a multiple of its
>>>   alignment (8 bytes).
>>
>> Would padding with __u32 at the end be sufficient here? I assume the
>> __u32 would be 32-bit aligned. However, was not sure if this would
>> always be the case.
> 
> Is there something wrong with implicit padding?
> Only one copy of the structure is passed to the kernel for any given call.
> 
>>>>> struct mmc_ioc_multi_cmd {
>>>>>     __u64 num_of_cmds;
>>>>>     struct mmc_ioc_cmd cmds[0];
>>>>> };
> 
> I think this would work just as well. But doesn't "pointer to an
> array" require 32-bit ioctl compat handling?
> We were trying to avoid a 32-bit user space compatibility handler.

I think that this is fine as it is a zero length array [0] and not a
pointer.

Cheers
Jon

[0] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
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