On Fri, 15 May 2020 12:37:28 +0000 "Sa, Nuno" <nuno...@analog.com> wrote:
> > From: Ardelean, Alexandru <alexandru.ardel...@analog.com> > > Sent: Freitag, 15. Mai 2020 13:48 > > To: linux-arm-ker...@lists.infradead.org; linux-stm32@st-md- > > mailman.stormreply.com; Sa, Nuno <nuno...@analog.com>; linux- > > ker...@vger.kernel.org; linux-...@vger.kernel.org > > Cc: ludovic.desroc...@microchip.com; nicolas.fe...@microchip.com; > > alexandre.tor...@st.com; a...@it-klinger.de; ji...@kernel.org; > > eugen.hris...@microchip.com; mcoquelin.st...@gmail.com; > > alexandre.bell...@bootlin.com > > 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-ow...@vger.kernel.org <linux-iio- > > ow...@vger.kernel.org> > > > > On Behalf Of Alexandru Ardelean > > > > Sent: Donnerstag, 14. Mai 2020 15:17 > > > > To: linux-...@vger.kernel.org; linux-arm-ker...@lists.infradead.org; > > linux- > > > > st...@st-md-mailman.stormreply.com; linux-kernel@vger.kernel.org > > > > Cc: ludovic.desroc...@microchip.com; eugen.hris...@microchip.com; > > > > ji...@kernel.org; nicolas.fe...@microchip.com; > > > > alexandre.bell...@bootlin.com; alexandre.tor...@st.com; > > > > mcoquelin.st...@gmail.com; a...@it-klinger.de; Ardelean, Alexandru > > > > <alexandru.ardel...@analog.com> > > > > 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.ardel...@analog.com> > > > > --- > > > > 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... Agreed - no need to guarantee alignment of something that doesn't exist :) > > - Nuno Sá >
