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> > > > 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. > > 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(-) > > > > > > > >
