RFC clk: implement clock locking mechanism

Hi J?r?me,

Quoting Jerome Brunet (2017-03-21 11:33:23)

Quoted 7 lines

This RFC patchset is related to the discussion around CLK_SET_RATE_GATE and clk_lock which available here [0]

The goal of this patchset is to provide a way for consumers to inform the system that they depend on the rate of the clock source and can't tolerate other consumers changing the rate or causing glitches.

Quoted 8 lines

Patches 1 to 3 are just rework to ease the integration of the locking mechanism. They should not introduce any functional differences.

Patch 4 is the important bit. It introduce 2 new functions to the CCF API:
clk_protect and clk_unprotect (The "lock" word is already used lot in clk.c. Using clk_lock and clk_unlock would have been very messy. If you found "protect" to be a poor choice, I'm happy to sed the whole thing in future version)

I prefer clk_lock_rate and clk_unlock_rate.

Quoted 4 lines

These calls can happen at anytime during the life of the clk. As for prepare and enable, the calls must be balanced.

Quoted 8 lines

Inside the clock framework, 2 new count values are introduced to keep track of the protection:
* core "protect_count": this reference count value works in the same way as ? prepare and enable count, and track whether the clock is protected or ? not. This is the value that will checked to allow operation which may ? cause glitches or change the rate of clock source.
* clk consumer "protect_ucount": this allows to track if the consumer is ? protecting the clock.

Similarly, s/protect/lock_rate/g

Quoted 10 lines

Requiring the consumer to unprotect its clock before changing it would have been very annoying for the consumer. It would also be unsafe, as it would still be possible for another consumer to change the rate between the call to set_rate and protect. This needs to happen in a critical section.??A solution would be to add "set_rate_protect", but we would need to do the same for the set_parent and set_phase (and the possible future functions). It does not scale well.??The solution proposed in this patch is to use the consumer protect count.

Quoted 9 lines

In function altering the clock, if the consumer is protecting the clock, the protection is temporarily release under the prepare_lock. This ensure that:
* the clock is still protect from another consumer because of the ? prepare_lock
* the rate set on a protected clock cannot change between set_rate and ? later enable
* only a consumer protecting a clock can do this temporary protection ? removal (less chance of people messing with the refcount)

It should be impossible for consumer_A to remove the lock refcount set by consumer_B using something like these patches I wrote for prepare_count and enable_count:

https://lkml.kernel.org/r/1438974570-20812-1-git-send-email-mturquette at baylibr e.com

Quoted 3 lines

* if more than one consumer protect the clock, it remains protected. ? Because the protection is removed for the calling consumer, it gives ? it a chance to jump to another provider, if available.

Hmm, that's a bit confusing. I guess you mean, "if the rate a parent clock is locked, and the child clock has multiple parents to choose from, then the child may select a different parent with an unlocked rate"?

Quoted 5 lines

This protection mechanism assume that "protecting" consumer knows what it is doing when it comes to setting the rate, and does not expect to be protected against itself.

Is this always true? Are there drivers where one code path requires a stable rate (due to glitches) while another code path might try to change the rate?

I can't think of any, but it's a big assumption to make. Anyone else have thoughts on it?

Quoted 28 lines

This was tested with the audio use case mentioned in [0]

One caveat is the following case:
N+1 consumers protect their clocks. These clocks come from N possible providers. We should able to satisfy at least N consumers before exhausting the resources.??In the particular case where all the consumers call "protect" before having a chance to call "set_rate", the last 2 consumers will remains stuck on the last provider w/o being able to set the rate on it. This means that in situation where there is 2 consumers on 1 providers, they will compete for the clock, and may end up in situation where both protect the clock and none can set the rate they need.??This can be solved if, when the rate fails to be set, the consumer release the protection. Then the second consumer will be able to satisfy its request.

Patch 5 is a small change on set_rate_range to restore the old range on failure. It don't use set_rate_range in any driver so I could not test this change.

Patch 6 is just a cosmetic change to clk_summary debugfs entry to make it less wide after adding protect count in it.

Patch 7 fix a warning reported by checkpatch.pl. Apparently, ENOSYS is used incorrectly.

Bonus:

Quoted 4 lines

With this patchset, we should probably change the use the flags "CLK_SET_RATE_GATE" and "CLK_SET_PARENT_GATE" We discussed removing them. Another solution would be to have them automatically gate the clock before performing the related operation.??What is your view on this ?

We'll need to ask the users of these flags. It would be nice to convert those folks over to use clk_lock_rate() instead of those flags.

Regards,
Mike

Quoted 22 lines

[0]: http://lkml.kernel.org/r/148942423440.82235.17188153691656009029 at resona nce

Jerome Brunet (7):
? clk: take the prepare lock out of clk_core_set_parent ? clk: add set_phase core function
? clk: rework calls to round and determine rate callbacks ? clk: add support for clock protection
? clk: rollback set_rate_range changes on failure ? clk: cosmetic changes to clk_summary debugfs entry ? clk: fix incorrect usage of ENOSYS

?drivers/clk/clk.c????????????| 313 ++++++++++++++++++++++++++++++++++----
-----
?include/linux/clk-provider.h |???1 + ?include/linux/clk.h??????????|??29 ++++
?3 files changed, 279 insertions(+), 64 deletions(-)

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2.9.3