> From: Ardelean, Alexandru <alexandru.Ardelean@xxxxxxxxxx> > Sent: Freitag, 15. Mai 2020 13:48 > To: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-stm32@st-md- > mailman.stormreply.com; Sa, Nuno <Nuno.Sa@xxxxxxxxxx>; linux- > kernel@xxxxxxxxxxxxxxx; linux-iio@xxxxxxxxxxxxxxx > Cc: ludovic.desroches@xxxxxxxxxxxxx; nicolas.ferre@xxxxxxxxxxxxx; > alexandre.torgue@xxxxxx; ak@xxxxxxxxxxxxx; jic23@xxxxxxxxxx; > eugen.hristev@xxxxxxxxxxxxx; mcoquelin.stm32@xxxxxxxxx; > alexandre.belloni@xxxxxxxxxxx > Subject: Re: [PATCH v2 7/8] iio: core: simplify alloc alignment code > > On Fri, 2020-05-15 at 07:12 +0000, Sa, Nuno wrote: > > Hey Alex, > > > > Just a small question... > > > > > From: linux-iio-owner@xxxxxxxxxxxxxxx <linux-iio- > owner@xxxxxxxxxxxxxxx> > > > On Behalf Of Alexandru Ardelean > > > Sent: Donnerstag, 14. Mai 2020 15:17 > > > To: linux-iio@xxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; > linux- > > > stm32@xxxxxxxxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx > > > Cc: ludovic.desroches@xxxxxxxxxxxxx; eugen.hristev@xxxxxxxxxxxxx; > > > jic23@xxxxxxxxxx; nicolas.ferre@xxxxxxxxxxxxx; > > > alexandre.belloni@xxxxxxxxxxx; alexandre.torgue@xxxxxx; > > > mcoquelin.stm32@xxxxxxxxx; ak@xxxxxxxxxxxxx; Ardelean, Alexandru > > > <alexandru.Ardelean@xxxxxxxxxx> > > > 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 > > > 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 <alexandru.ardelean@xxxxxxxxxx> > > > --- > > > 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) > > > 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 > > += 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... - Nuno Sá
