Message-ID: <4DEDF2B8.5020502@cam.ac.uk>
Date:	Tue, 07 Jun 2011 10:43:20 +0100
From:	Jonathan Cameron <jic23@....ac.uk>
To:	anish singh <anish198519851985@...il.com>
CC:	Greg KH <gregkh@...e.de>, Joe Perches <joe@...ches.com>,
	devel@...verdev.osuosl.org, arnd@...db.de,
	lucas.demarchi@...fusion.mobi, linux-kernel@...r.kernel.org,
	manuel.stahl@....fraunhofer.de
Subject: Re: [PATCH 1/2] staging: iio replaced kmalloc with local variables.

On 06/07/11 05:56, anish singh wrote:
> 
> 
> On Tue, Jun 7, 2011 at 4:11 AM, Greg KH <gregkh@...e.de <mailto:gregkh@...e.de>> wrote:
> 
>     On Mon, Jun 06, 2011 at 03:28:29PM -0700, Joe Perches wrote:
>     > On Mon, 2011-06-06 at 15:21 -0700, Greg KH wrote:
>     > > On Mon, Jun 06, 2011 at 03:10:57PM -0700, Joe Perches wrote:
>     > > > On Mon, 2011-06-06 at 14:55 -0700, Greg KH wrote:
>     > > > > On Tue, Jun 07, 2011 at 12:37:37AM +0530, anish wrote:
>     > > > > > From: anish kumar <anish198519851985@...il.com <mailto:anish198519851985@...il.com>>
>     > > > > > Replace kmalloc with local variables as it was un-necessary and
>     > > > > > also removed the redudant code after this change.
>     > > > > SPI data, like USB data, has to come from kmalloced data, not from the
>     > > > > stack, or bad things can, and will, happen.
>     > > > Perhaps just add a comment like:
>     > > > +       u8 *tx = kmalloc(2, GFP_KERNEL);  /* can't be on stack */
>     > > You really want to do to that for _EVERY_ SPI and USB driver?  I don't
>     > > think so.
>     >
>     > Nope, only the ones that look especially odd because
>     >       kmalloc(sizeof(struct foo), ...)
>     > or
>     >       kmalloc(sizeof("type), ...)
>     > is not used.
>     >
>     > It might be better to just declare a 2 byte struct.
> 
>     No, this is a very common thing for all USB and SPI drivers.  It's so
>     obvious that once I saw the Subject: line, I knew this patch was going
>     to be wrong.
> 
>     This is something that the USB and SPI developers know all about, it's
>     the way things work, and this driver works, so why are people trying to
>     "clean" it up in ways that will break it, or cause extra work with
>     structures where they are not needed at all?
> 
> Sorry for noise as i didn't the SPI requirements,thought it is similar to I2C and
> in cleaning up below part i wrongly cleaned SPI part also.Below was also part
> of patch.
Not to worry, you are far from the first person to fall into this issue!
Also, you have highlighted a weird corner in that driver, that could do with
tidying up (just not quite the fix you had in mind!).
> static int max1363_write_basic_config(struct i2c_client *client,
>                                      unsigned char d2)
>  {
>        int ret;
> -       u8 *tx_buf = kmalloc(2, GFP_KERNEL);
> +       u8 tx_buf[2];
>        if (!tx_buf)
>                return -ENOMEM;
> @@ -215,7 +215,6 @@ static int max1363_write_basic_config(struct i2c_client *client,
>        tx_buf[1] = d2;
>        ret = i2c_master_send(client, tx_buf, 2);
> -       kfree(tx_buf);
>        return (ret > 0) ? 0 : ret;
>  }
> I think above patch is ok as it is I2C and I2C doesn't have that requirement.
Yes.  I2C bus drivers that do dma do the copy into dma safe memory internally.
Makes for more bouncing around of data, but i2c is slow anyway so it doesn't matter.
Also, based on a quick look this morning, the dma buffers tend to require various
headers to be in place etc which isn't typically the case for spi (a much more 'raw'
bus).

Can you cc linux-iio@...r.kernel.org on that patch when you send it out please.

Jonathan
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