| Message ID | 24a577e6e157e1199817ab36631cec51675ef3ca.1695380366.git.mazziesaccount@gmail.com (mailing list archive) |
|---|---|
| State | Changes Requested |
| Headers | show |
| Series | Support ROHM BM1390 pressure sensor | expand |
On Fri, 22 Sep 2023 14:17:49 +0300 Matti Vaittinen <mazziesaccount@gmail.com> wrote: > When IIO goes through the available scan masks in order to select the > best suiting one, it will just accept the first listed subset of channels > which meets the user's requirements. This works great for most of the > drivers as they can sort the list of channels in the order where > the 'least costy' channel selections come first. > > It may be that in some cases the ordering of the list of available scan > masks is not thoroughly considered. We can't really try outsmarting the > drivers by selecting the smallest supported subset - as this might not > be the 'least costy one' - but we can at least try searching through the > list to see if we have an exactly matching mask. It should be sane > assumption that if the device can support reading only the exact > channels user is interested in, then this should be also the least costy > selection - and if it is not and optimization is important, then the > driver could consider omitting setting the 'available_scan_mask' and > doing demuxing - or just omitting the 'costy exact match' and providing > only the more efficient broader selection of channels. > > Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com> Whilst I fully agree with the reasoning behind this, I'd rather we did an audit of drivers to find any that have a non logical order (one came up today in review) and fix them up. A quick and dirty grep didn't find it to be a common problem, at least partly as most users of this feature only provide one valid mask. The few complex corners I found appear to be fine with the expected shortest sequences first. Defending against driver bugs is losing game if it makes the core code more complex to follow by changing stuff in non debug paths. One option might be to add a trivial check at iio_device_register() that we don't have scan modes that are subsets of modes earlier in the list. These lists are fairly short so should be cheap to run. That would incorporate ensuring exact matches come earlier by default. Jonathan > --- > drivers/iio/industrialio-buffer.c | 25 +++++++++++++++++++------ > 1 file changed, 19 insertions(+), 6 deletions(-) > > diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c > index 176d31d9f9d8..e97396623373 100644 > --- a/drivers/iio/industrialio-buffer.c > +++ b/drivers/iio/industrialio-buffer.c > @@ -411,19 +411,32 @@ static const unsigned long *iio_scan_mask_match(const unsigned long *av_masks, > const unsigned long *mask, > bool strict) > { > + const unsigned long *first_subset = NULL; > + > if (bitmap_empty(mask, masklength)) > return NULL; > - while (*av_masks) { > - if (strict) { > + > + if (strict) { > + while (*av_masks) { > if (bitmap_equal(mask, av_masks, masklength)) > return av_masks; > - } else { > - if (bitmap_subset(mask, av_masks, masklength)) > - return av_masks; > + > + av_masks += BITS_TO_LONGS(masklength); > } > + > + return NULL; > + } > + while (*av_masks) { > + if (bitmap_equal(mask, av_masks, masklength)) > + return av_masks; > + > + if (!first_subset && bitmap_subset(mask, av_masks, masklength)) > + first_subset = av_masks; > + > av_masks += BITS_TO_LONGS(masklength); > } > - return NULL; > + > + return first_subset; > } > > static bool iio_validate_scan_mask(struct iio_dev *indio_dev,
On Sun, 24 Sep 2023 17:07:26 +0100 Jonathan Cameron <jic23@kernel.org> wrote: > On Fri, 22 Sep 2023 14:17:49 +0300 > Matti Vaittinen <mazziesaccount@gmail.com> wrote: > > > When IIO goes through the available scan masks in order to select the > > best suiting one, it will just accept the first listed subset of channels > > which meets the user's requirements. This works great for most of the > > drivers as they can sort the list of channels in the order where > > the 'least costy' channel selections come first. > > > > It may be that in some cases the ordering of the list of available scan > > masks is not thoroughly considered. We can't really try outsmarting the > > drivers by selecting the smallest supported subset - as this might not > > be the 'least costy one' - but we can at least try searching through the > > list to see if we have an exactly matching mask. It should be sane > > assumption that if the device can support reading only the exact > > channels user is interested in, then this should be also the least costy > > selection - and if it is not and optimization is important, then the > > driver could consider omitting setting the 'available_scan_mask' and > > doing demuxing - or just omitting the 'costy exact match' and providing > > only the more efficient broader selection of channels. > > > > Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com> > > Whilst I fully agree with the reasoning behind this, I'd rather we > did an audit of drivers to find any that have a non logical order > (one came up today in review) and fix them up. > > A quick and dirty grep didn't find it to be a common problem, at least > partly as most users of this feature only provide one valid mask. > The few complex corners I found appear to be fine with the expected > shortest sequences first. > > Defending against driver bugs is losing game if it makes the core > code more complex to follow by changing stuff in non debug paths. > One option might be to add a trivial check at iio_device_register() > that we don't have scan modes that are subsets of modes earlier in the list. > These lists are fairly short so should be cheap to run. > > That would incorporate ensuring exact matches come earlier by default. BTW I'd have sent these as a separate series as there is potential that this will distract from or slow down the driver + not all the CC list will care about this core cleanup. Jonathan > > Jonathan > > > > --- > > drivers/iio/industrialio-buffer.c | 25 +++++++++++++++++++------ > > 1 file changed, 19 insertions(+), 6 deletions(-) > > > > diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c > > index 176d31d9f9d8..e97396623373 100644 > > --- a/drivers/iio/industrialio-buffer.c > > +++ b/drivers/iio/industrialio-buffer.c > > @@ -411,19 +411,32 @@ static const unsigned long *iio_scan_mask_match(const unsigned long *av_masks, > > const unsigned long *mask, > > bool strict) > > { > > + const unsigned long *first_subset = NULL; > > + > > if (bitmap_empty(mask, masklength)) > > return NULL; > > - while (*av_masks) { > > - if (strict) { > > + > > + if (strict) { > > + while (*av_masks) { > > if (bitmap_equal(mask, av_masks, masklength)) > > return av_masks; > > - } else { > > - if (bitmap_subset(mask, av_masks, masklength)) > > - return av_masks; > > + > > + av_masks += BITS_TO_LONGS(masklength); > > } > > + > > + return NULL; > > + } > > + while (*av_masks) { > > + if (bitmap_equal(mask, av_masks, masklength)) > > + return av_masks; > > + > > + if (!first_subset && bitmap_subset(mask, av_masks, masklength)) > > + first_subset = av_masks; > > + > > av_masks += BITS_TO_LONGS(masklength); > > } > > - return NULL; > > + > > + return first_subset; > > } > > > > static bool iio_validate_scan_mask(struct iio_dev *indio_dev, >
On 9/24/23 19:07, Jonathan Cameron wrote: > On Fri, 22 Sep 2023 14:17:49 +0300 > Matti Vaittinen <mazziesaccount@gmail.com> wrote: > >> When IIO goes through the available scan masks in order to select the >> best suiting one, it will just accept the first listed subset of channels >> which meets the user's requirements. This works great for most of the >> drivers as they can sort the list of channels in the order where >> the 'least costy' channel selections come first. >> >> It may be that in some cases the ordering of the list of available scan >> masks is not thoroughly considered. We can't really try outsmarting the >> drivers by selecting the smallest supported subset - as this might not >> be the 'least costy one' - but we can at least try searching through the >> list to see if we have an exactly matching mask. It should be sane >> assumption that if the device can support reading only the exact >> channels user is interested in, then this should be also the least costy >> selection - and if it is not and optimization is important, then the >> driver could consider omitting setting the 'available_scan_mask' and >> doing demuxing - or just omitting the 'costy exact match' and providing >> only the more efficient broader selection of channels. >> >> Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com> > > Whilst I fully agree with the reasoning behind this, I'd rather we > did an audit of drivers to find any that have a non logical order > (one came up today in review) and fix them up. > > A quick and dirty grep didn't find it to be a common problem, at least > partly as most users of this feature only provide one valid mask. It's always good to hear there is not many problems found :) This patch was not inspired by auditing the existing code - it was inspired by the fact that I would have wrongly ordered the available_scan_masks for bm1390 myself. I just happened to notice the oddity in active_scan_masks while I was trying to figure out if it was the driver, IIO or user-space code which messed my buffer when I disabled timestamps. > The few complex corners I found appear to be fine with the expected > shortest sequences first. > > Defending against driver bugs is losing game if it makes the core > code more complex to follow by changing stuff in non debug paths. I think I agree, although I could argue that it depends on the amount of added complexity. Still ... > One option might be to add a trivial check at iio_device_register() ... this suggestion is superior to the check added in this patch. > that we don't have scan modes that are subsets of modes earlier in the list. > These lists are fairly short so should be cheap to run. Yes. And running the check at the registration phase should not be a big problem. And, if it appears to be a problem, then we can add a registration variant which omits the checks for those rare drivers which would _really_ be hurt by the few extra cycles spent on registration. > That would incorporate ensuring exact matches come earlier by default. Yes. I like the idea, wish I had invented it myself ;) > > Jonathan > > >> --- >> drivers/iio/industrialio-buffer.c | 25 +++++++++++++++++++------ >> 1 file changed, 19 insertions(+), 6 deletions(-) >> >> diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c >> index 176d31d9f9d8..e97396623373 100644 >> --- a/drivers/iio/industrialio-buffer.c >> +++ b/drivers/iio/industrialio-buffer.c >> @@ -411,19 +411,32 @@ static const unsigned long *iio_scan_mask_match(const unsigned long *av_masks, >> const unsigned long *mask, >> bool strict) >> { >> + const unsigned long *first_subset = NULL; >> + >> if (bitmap_empty(mask, masklength)) >> return NULL; >> - while (*av_masks) { >> - if (strict) { >> + >> + if (strict) { >> + while (*av_masks) { >> if (bitmap_equal(mask, av_masks, masklength)) >> return av_masks; >> - } else { >> - if (bitmap_subset(mask, av_masks, masklength)) >> - return av_masks; >> + >> + av_masks += BITS_TO_LONGS(masklength); >> } >> + >> + return NULL; >> + } >> + while (*av_masks) { >> + if (bitmap_equal(mask, av_masks, masklength)) >> + return av_masks; >> + >> + if (!first_subset && bitmap_subset(mask, av_masks, masklength)) >> + first_subset = av_masks; >> + >> av_masks += BITS_TO_LONGS(masklength); >> } >> - return NULL; >> + >> + return first_subset; >> } >> >> static bool iio_validate_scan_mask(struct iio_dev *indio_dev, >
On 9/24/23 19:10, Jonathan Cameron wrote: > On Sun, 24 Sep 2023 17:07:26 +0100 > Jonathan Cameron <jic23@kernel.org> wrote: > >> On Fri, 22 Sep 2023 14:17:49 +0300 >> Matti Vaittinen <mazziesaccount@gmail.com> wrote: >> >>> When IIO goes through the available scan masks in order to select the >>> best suiting one, it will just accept the first listed subset of channels >>> which meets the user's requirements. This works great for most of the >>> drivers as they can sort the list of channels in the order where >>> the 'least costy' channel selections come first. >>> >>> It may be that in some cases the ordering of the list of available scan >>> masks is not thoroughly considered. We can't really try outsmarting the >>> drivers by selecting the smallest supported subset - as this might not >>> be the 'least costy one' - but we can at least try searching through the >>> list to see if we have an exactly matching mask. It should be sane >>> assumption that if the device can support reading only the exact >>> channels user is interested in, then this should be also the least costy >>> selection - and if it is not and optimization is important, then the >>> driver could consider omitting setting the 'available_scan_mask' and >>> doing demuxing - or just omitting the 'costy exact match' and providing >>> only the more efficient broader selection of channels. >>> >>> Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com> >> >> Whilst I fully agree with the reasoning behind this, I'd rather we >> did an audit of drivers to find any that have a non logical order >> (one came up today in review) and fix them up. >> >> A quick and dirty grep didn't find it to be a common problem, at least >> partly as most users of this feature only provide one valid mask. >> The few complex corners I found appear to be fine with the expected >> shortest sequences first. >> >> Defending against driver bugs is losing game if it makes the core >> code more complex to follow by changing stuff in non debug paths. >> One option might be to add a trivial check at iio_device_register() >> that we don't have scan modes that are subsets of modes earlier in the list. >> These lists are fairly short so should be cheap to run. >> >> That would incorporate ensuring exact matches come earlier by default. > > BTW I'd have sent these as a separate series as there is potential that > this will distract from or slow down the driver + not all the CC list > will care about this core cleanup. I was not so worried about the driver being postponed. I was prepared to suggest to merging a subset of the patches if need be - while I can continue work with the rest of the series ;) What comes to people being interested in the core-changes Vs. people being interested in the driver changes - I'd expect the core changes to concern much wider audience than the driver changes. But yes, knowing the amount of mails people go through, limiting the recipient to most relevant ones never hurts. Besides, I think there is no conflicts/dependencies as driver changes don't change core/tools, and core/tool changes don't touch the driver so splitting this to two series should be trivial. Will do that for next version. Yours, -- Matti
diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c index 176d31d9f9d8..e97396623373 100644 --- a/drivers/iio/industrialio-buffer.c +++ b/drivers/iio/industrialio-buffer.c @@ -411,19 +411,32 @@ static const unsigned long *iio_scan_mask_match(const unsigned long *av_masks, const unsigned long *mask, bool strict) { + const unsigned long *first_subset = NULL; + if (bitmap_empty(mask, masklength)) return NULL; - while (*av_masks) { - if (strict) { + + if (strict) { + while (*av_masks) { if (bitmap_equal(mask, av_masks, masklength)) return av_masks; - } else { - if (bitmap_subset(mask, av_masks, masklength)) - return av_masks; + + av_masks += BITS_TO_LONGS(masklength); } + + return NULL; + } + while (*av_masks) { + if (bitmap_equal(mask, av_masks, masklength)) + return av_masks; + + if (!first_subset && bitmap_subset(mask, av_masks, masklength)) + first_subset = av_masks; + av_masks += BITS_TO_LONGS(masklength); } - return NULL; + + return first_subset; } static bool iio_validate_scan_mask(struct iio_dev *indio_dev,
When IIO goes through the available scan masks in order to select the best suiting one, it will just accept the first listed subset of channels which meets the user's requirements. This works great for most of the drivers as they can sort the list of channels in the order where the 'least costy' channel selections come first. It may be that in some cases the ordering of the list of available scan masks is not thoroughly considered. We can't really try outsmarting the drivers by selecting the smallest supported subset - as this might not be the 'least costy one' - but we can at least try searching through the list to see if we have an exactly matching mask. It should be sane assumption that if the device can support reading only the exact channels user is interested in, then this should be also the least costy selection - and if it is not and optimization is important, then the driver could consider omitting setting the 'available_scan_mask' and doing demuxing - or just omitting the 'costy exact match' and providing only the more efficient broader selection of channels. Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com> --- drivers/iio/industrialio-buffer.c | 25 +++++++++++++++++++------ 1 file changed, 19 insertions(+), 6 deletions(-)