Re: [PATCH v2 7/8] iio: core: simplify alloc alignment code

On Fri, 15 May 2020 12:37:28 +0000
"Sa, Nuno" [off-list ref] wrote:

quoted

From: Ardelean, Alexandru <redacted>
Sent: Freitag, 15. Mai 2020 13:48
To: linux-arm-kernel@lists.infradead.org; linux-stm32@st-md-
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Subject: Re: [PATCH v2 7/8] iio: core: simplify alloc alignment code

On Fri, 2020-05-15 at 07:12 +0000, Sa, Nuno wrote:  

quoted

Hey Alex,

Just a small question...
 

quoted

From: linux-iio-owner@vger.kernel.org <linux-iio-  

owner@vger.kernel.org>  

quoted

quoted

On Behalf Of Alexandru Ardelean
Sent: Donnerstag, 14. Mai 2020 15:17
To: linux-iio@vger.kernel.org; linux-arm-kernel@lists.infradead.org;  

linux-  

quoted

quoted

stm32@st-md-mailman.stormreply.com; linux-kernel@vger.kernel.org
Cc: ludovic.desroches@microchip.com; eugen.hristev@microchip.com;
jic23@kernel.org; nicolas.ferre@microchip.com;
alexandre.belloni@bootlin.com; alexandre.torgue@st.com;
mcoquelin.stm32@gmail.com; ak@it-klinger.de; Ardelean, Alexandru
[off-list ref]
Subject: [PATCH v2 7/8] iio: core: simplify alloc alignment code

There was a recent discussion about this code:
  https://urldefense.com/v3/__https://lore.kernel.org/linux-
 

iio/20200322165317.0b1f0674@archlinux/__;!!A3Ni8CS0y2Y!pgdUSayJCfxMiE  

quoted

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w8Fpv0LkEZurCSkX0sEcLnXeDSCLmhpu1xont6-vBQj3ZbCw$

This looks like a good time to rework this, since any issues about it
should pop-up under testing, because the iio_dev is having a bit of an
overhaul and stuff being moved to iio_dev_priv.

Signed-off-by: Alexandru Ardelean <redacted>
---
 drivers/iio/industrialio-core.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-

core.c
index a1b29e0f8fd6..7671d36efae7 100644

--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c

@@ -1514,13 +1514,9 @@ struct iio_dev *iio_device_alloc(int  

sizeof_priv)  

quoted

quoted

 	struct iio_dev *dev;
 	size_t alloc_size;

-	alloc_size = sizeof(struct iio_dev_opaque);
-	if (sizeof_priv) {
-		alloc_size = ALIGN(alloc_size, IIO_ALIGN);
-		alloc_size += sizeof_priv;
-	}
-	/* ensure 32-byte alignment of whole construct ? */
-	alloc_size += IIO_ALIGN - 1;
+	alloc_size = ALIGN(sizeof(struct iio_dev_opaque), IIO_ALIGN);
+	if (sizeof_priv)
+		alloc_size += ALIGN(sizeof_priv, IIO_ALIGN);  

Do we actually need to do the `ALIGN` again? It seems to me that  

`alloc_size  

quoted

+= sizeof_priv`
would be enough or am I missing something obvious?  

Well, it's not always clear what value 'sizeof_priv' has, and whether it is
provided already aligned.
The requirement is usually that this data be cacheline aligned.

So, sizeof(struct iio_dev_opaque) is aligned already a few lines above, but
the
private information should also be aligned [given that it's an unknown value
provided by the driver].
I think this is mostly important, if we need to do DMA access to buffers
allocated on the driver's state-struct, which is allocated here, and which is
usually provided as sizeof_priv.  

Yes, AFAIU this is to guarantee that the priv struct will start at an address that is 
DMA safe (cacheline-aligned). Hence, if there is any data in 'priv' that needs to be DMA
safe, we are fine...

Well, I was also misreading the code. Still, I think it should look something like:

````
alloc_size = sizeof(struct iio_dev_opaque)
if (sizeof_priv)
	alloc_size += ALIGN(alloc_size, IIO_ALIGN);
````

If there is no priv, I think we don't need the padding bytes...

Agreed - no need to guarantee alignment of something that doesn't exist :)

- Nuno Sá

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