On Sun, 29 Nov 2020 22:26:46 +0200 Alexandru Ardelean <ardeleanalex@xxxxxxxxx> wrote: > On Sun, Nov 29, 2020 at 8:33 PM Jonathan Cameron <jic23@xxxxxxxxxx> wrote: > > > > On Sun, 29 Nov 2020 18:15:28 +0200 > > Alexandru Ardelean <ardeleanalex@xxxxxxxxx> wrote: > > > > > On Sun, Nov 29, 2020 at 3:24 PM Jonathan Cameron <jic23@xxxxxxxxxx> wrote: > > > > > > > > Dear All, > > > > > > > > Whilst I'm reasonably confident this series is correct (famous last words) > > > > I don't like applying anything non trivial without having had at least one > > > > set of additional eyes on it. > > > > > > > > As such, if anyone has a chance to do a quick sanity check that would be > > > > much appreciated! > > > > > > This looks good AFAICT. > > > > Thanks. If you are comfortable giving a tag that would be great. > > If not, no problem! > > Sure. Apologies for not adding one sooner. > > Reviewed-by: Alexandru Ardelean <alexandru.ardelean@xxxxxxxxxx> > Thanks, Applied to the togreg branch of iio.git and marked for stable. Didn't send these the faster route as we are near the next merge window anyway and these have been there for a very long time. > > > > > But it also looks like it could do with some re-design/re-thinking. > > > Maybe somehow moving more of this buffer + timestamp management > > > somewhere in IIO core. > > > > > > It would change the paradigm a bit, in the sense that a driver would > > > ask IIO core for a buffer/scratchpad area, where to put the data read > > > from the device. > > > This buffer pool management could be interesting [in the long run]. > > > Maybe with some zero-copy mechanism. > > > > Whilst an interesting idea, it would run into various challenges. > > 1) Need to be DMA safe for SPI drivers, or they would still need to have > > bounce buffers. Current cases where we have to copy twice are a bit > > annoying of course. > > 2) Need to show sufficient performance benefit to be worth the churn to > > make the change + the potential complexity. > > 3) That interface isn't necessarily just going to one place as there may > > be multiple consumers. There are probably still ways that could be > > dealt with but it's another level of complexity. > > > > So perhaps worth exploring but the performance vs complexity question is > > where I suspect it would come unstuck. > > I still feel there may be some reasonably simple mechanisms, that > would benefit drivers that use iio_push_to_buffers_with_timestamp(). > I don't have anything clear in mind yet. > I still need to dig through other backlog stuff. > But, I'll try to make a note of this and see later if I can get to it. Cool. There might be stuff that is worth doing in this area. Will see what you come up with! Jonathan > > > > > Thanks, > > > > Jonathan > > > > > > > > > > > > > Thanks > > > > > > > > Jonathan > > > > > > > > +CC a few additional helpful souls :) > > > > > > > > On Sun, 20 Sep 2020 12:27:34 +0100 > > > > Jonathan Cameron <jic23@xxxxxxxxxx> wrote: > > > > > > > > > From: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> > > > > > > > > > > Took me a while to get back to these. We have 2 new patches in here to > > > > > fix issues unrelated to the main topic, but which effect the buffer lengths. > > > > > I've done those as precursors so it is clear what is going on. > > > > > > > > > > Note there are still a few outstanding drivers to be fixed before we can > > > > > think about adding a warning if unaligned buffers are provided. Naturally > > > > > they are the hardest ones, or the ones where I couldn't work out how > > > > > the code is working today, so may take a little while. > > > > > > > > > > Changes since v3: > > > > > * Applied all the ones where only minor comment changes were needed. > > > > > * rpr0521: Fixed typo. Also added to patch description Mikko's information > > > > > on why it would be costly to split off the interrupt read. > > > > > * st_uvis: Drop the pointless masking. > > > > > * mag3110: Rename element to temperature > > > > > * bmi160: Add fix to length of buffer. > > > > > * bmi160: Improve comments and carry forwards shorter length. > > > > > * mpl3115: Sufficiently unusual to need a 'special' comment and another review. > > > > > * ti-ads124s08: Add fix to length of buffer. > > > > > * ti-ads124s08: Expand comment to express the buffer length not all needed if > > > > > not all channels are enabled. > > > > > > > > > > Changes since v2: > > > > > * bmc150-accel: Use sizeof() for channel size (Andy Shevchenko) > > > > > * st_uvis25: Use local variable for regmap call (Andy Shevchenko) > > > > > * st_lsm6dsx: Use array of scan[] rather than 3 structures (Lorenzo Bianconi) > > > > > * inv_mpu6050: Add patch switching to a regmap_noinc_read (Jean-Baptiste Maneyrol) > > > > > * ina2xx: Use a structure (previously failed to notice that works here) > > > > > * I've added clarifying notes to patch descriptions based on questions asked. > > > > > These were mainly about why we didn't use the structure approach everywhere > > > > > and why I've forced alignment in places it wasn't strictly needed. > > > > > > > > > > Previous cover letter: > > > > > A few notes based on questions on v1. > > > > > > > > > > 1. Why not use put_unaligned to avoid the whole thing? > > > > > This interface can pass directly to a variety of in kernel users > > > > > who would reasonably assume that we have true natural alignment. > > > > > When it gets passed directly is subtle as it depends on whether > > > > > the demux has to realign the data or not. So enabling an extra > > > > > channel could result in a previously working alignment no longer > > > > > being true. > > > > > > > > > > Even if this is fine for existing usecases we are likely to > > > > > store up subtle long term issues if we don't fix it explicitly. > > > > > It's also worth noting that the data channel sometimes suffered > > > > > the same problem as the timestamp. > > > > > > > > > > 2. Why not specify explicit padding? > > > > > In my view this is error prone in comparisom with relying on > > > > > c to do the hard work for us. > > > > > > > > > > 3. Why not move the timestamp to the start? > > > > > ABI breakage and as timestamp is optional (no obvious from the > > > > > iio_push_to_buffers_with_timestamp call) we can end up having > > > > > to shift the rest of the data within that call. > > > > > > > > > > Changes since v1. > > > > > > > > > > Andy Schevchenko pointed out that on x86_32 s64 elements are only > > > > > aligned to 4 bytes. Where I had tried to use a structure to avoid > > > > > explicit need to list the padding, there were some cases where > > > > > this results in insufficient padding being inserted. > > > > > > > > > > This doesn't affect the few patches that had already been applied and > > > > > sent upstream. (which was lucky ;) > > > > > > > > > > The fix was to take advantage of __aligned(8) which (according to > > > > > my reading of the c spec and the gcc docs) enforces the alignment of > > > > > both the element within a structure and the structure itself. > > > > > The kernel now requires a recent enough version of GCC to ensure this > > > > > works both on the stack and heap. This is done in lots of other > > > > > userspace interfaces. In some cases iio_push_to_buffers_with_ts > > > > > is aligning data for passing to userspace, be it via a kfifo > > > > > so it is sensible we should use the same solution. > > > > > > > > > > Note that we could have used u64_aligned but there is no equivalent > > > > > for s64 and explicit use of __aligned(8) is common in > > > > > the kernel so we adopt this here. > > > > > > > > > > Note that there were about 8 drivers that would have been broken with > > > > > v1 of the patch. I have also forced alignment of timestamps in cases > > > > > where (mostly by coincidence) we would have been fine (padding was > > > > > less than 4 bytes anyway. I did this partly to reduce fragility if > > > > > other elements are added in future and also to avoid cut and paste > > > > > errors in new drivers. > > > > > > > > > > There were a few other minor tidying up changes inline with reviews > > > > > of v1. > > > > > > > > > > I've kept tags given for v1 on basis the changes are minor. Shout if > > > > > you disagree. > > > > > > > > > > Version 1 part 1 cover letter. > > > > > > > > > > Lars noted in a recent review [1] of the adis16475 that we had an issue around > > > > > the alignment requirements of iio_push_to_buffers_with_timestamp. > > > > > Whilst it's not documented, that function assumes that the overall buffer > > > > > is 8 byte aligned, to ensure the timestamp is itself naturally aligned. > > > > > We have drivers that use arrays (typically on the stack) that do > > > > > not guarantee this alignment. > > > > > > > > > > We could have fixed this by using a put_unaligned to write the timestamp > > > > > but I think that just pushes the problem down the line. If we were to > > > > > have a consumer buffer wanting all the channels in the current > > > > > active_scanmask then it will get the raw buffer from the driver passed > > > > > straight through. It seems odd to me if we allow passing a buffer > > > > > that is not naturally aligned through to a consumer. > > > > > Hence I'm proposing to fix up all existing drivers that might pass > > > > > a buffer with insufficient alignment guarantees. > > > > > Sometimes the timestamp is guaranteed to be in a particular location, > > > > > in which case we can use C structure alignment guarantees to fix this > > > > > in a nice readable fashion. In other cases, the timestamp location > > > > > depends on which channels are enabled, and in those case we can > > > > > use explicit alignment __aligned(8) to ensure the whole array is > > > > > appropriately aligned. > > > > > > > > > > Lars-Peter also noted that, in many of these cases, there are holes > > > > > in the stack array that we never write. Those provide a potential > > > > > leak of kernel data to userspace. For drivers where this applies > > > > > we either need to zero those holes each time, or allocate the buffer > > > > > on the heap (only once), ensuring it is zeroed at that time. > > > > > We may leak previous values from the sensor but currently that seems > > > > > unlikely to present any form of security risk. > > > > > > > > > > As such, this first set contains a mixture of fixes. Where there > > > > > are no possible holes, the buffer is kept on the stack but a > > > > > c structure is used to guarantee appropriate alignment. Where > > > > > there are holes, the buffer is moved into the iio_priv() accessed > > > > > data private structure. A c structure or __aligned(8) is used > > > > > as appropriate. > > > > > > > > > > I've stopped at this point rather than doing all the drivers Lars > > > > > found in order to both throttle the review burden and also to > > > > > see find any general problems with the fixes before doign futher > > > > > similar series. A few of the remaining ones will be rather more > > > > > complex to deal with. > > > > > > > > > > These have been there a long time, so whilst they are fixes we > > > > > will want in stable I'm not that bothered if it takes us a little > > > > > while to get them there! > > > > > > > > > > [1] https://www.spinics.net/lists/devicetree/msg350590.html > > > > > [2] https://patchwork.kernel.org/cover/11554215/ > > > > > > > > > > Jonathan Cameron (8): > > > > > iio:light:rpr0521: Fix timestamp alignment and prevent data leak. > > > > > iio:light:st_uvis25: Fix timestamp alignment and prevent data leak. > > > > > iio:magnetometer:mag3110: Fix alignment and data leak issues. > > > > > iio:imu:bmi160: Fix too large a buffer. > > > > > iio:imu:bmi160: Fix alignment and data leak issues > > > > > iio:pressure:mpl3115: Force alignment of buffer > > > > > iio:adc:ti-ads124s08: Fix buffer being too long. > > > > > iio:adc:ti-ads124s08: Fix alignment and data leak issues. > > > > > > > > > > drivers/iio/adc/ti-ads124s08.c | 13 ++++++++++--- > > > > > drivers/iio/imu/bmi160/bmi160.h | 7 +++++++ > > > > > drivers/iio/imu/bmi160/bmi160_core.c | 6 ++---- > > > > > drivers/iio/light/rpr0521.c | 17 +++++++++++++---- > > > > > drivers/iio/light/st_uvis25.h | 5 +++++ > > > > > drivers/iio/light/st_uvis25_core.c | 8 +++++--- > > > > > drivers/iio/magnetometer/mag3110.c | 13 +++++++++---- > > > > > drivers/iio/pressure/mpl3115.c | 9 ++++++++- > > > > > 8 files changed, 59 insertions(+), 19 deletions(-) > > > > > > > > > > >
