linux-kernel - Re: [PATCH v6 1/3] spi/pl022: Add chip select handling via GPIO

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Date:	Sun, 2 Sep 2012 09:18:29 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	shiraz hashim <shiraz.linux.kernel@...il.com>
Cc:	Roland Stigge <stigge@...com.de>, aletes.xgr@...il.com,
	broonie@...nsource.wolfsonmicro.com, grant.likely@...retlab.ca,
	rob.herring@...xeda.com, rob@...dley.net,
	devicetree-discuss@...ts.ozlabs.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	spi-devel-general@...ts.sourceforge.net,
	gabriel.fernandez@...ricsson.com, lee.jones@...aro.org,
	viresh.kumar@...aro.org, sachin.verma@...co
Subject: Re: [PATCH v6 1/3] spi/pl022: Add chip select handling via GPIO

On Sat, Sep 1, 2012 at 1:14 PM, shiraz hashim
<shiraz.linux.kernel@...il.com> wrote:
> Hi Roland,
>
> On Wed, Aug 22, 2012 at 7:19 PM, Roland Stigge <stigge@...com.de> wrote:
>> @@ -2016,6 +2030,8 @@ pl022_probe(struct amba_device *adev, co
>>         pl022->master_info = platform_info;
>>         pl022->adev = adev;
>>         pl022->vendor = id->data;
>> +       /* Point chipselects to allocated memory beyond the main struct */
>> +       pl022->chipselects = (int *) pl022 + sizeof(struct pl022);
>
> This is going beyond memory allocated for chipselects
> as it adds 4 * sizeof(struct pl022) bytes to pl022.

Yes that is why the allocation looks like this:

+       master = spi_alloc_master(dev, sizeof(struct pl022) + sizeof(int) *
+                                 platform_info->num_chipselect);

> pl022->chipselects = (int *) &pl022[1];
> can be musch safer.

I see absolutely no sematic difference between these two
methods to reach the first position beyond the first struct.

If we're gonna be debating this it's a safe sign that this is
not a good design pattern at all, so then it is better to simply
devm_kzalloc(sizeof(int) * platform_info->num_chipselect);
separately.

(But I'm happy with the patch as it is. And the other way
too, since I'm not very picky.)

Yours,
Linus Walleij
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