On Mon, May 26, 2025 at 02:58:51PM +0200, Remi Pommarel wrote:
On Mon, May 26, 2025 at 01:35:02PM +0200, Johan Hovold wrote:
On Thu, May 22, 2025 at 05:11:21PM +0200, Remi Pommarel wrote:
Why not move the dma_rmb() in ath12k_hal_srng_access_begin() as below, that would look to me as a good place to do it.
We only need the read barrier for dest rings so the barrier would go in the else branch, but I prefer keeping it in the caller so that it is more obvious when it is needed and so that we can skip the barrier when the ring is empty (e.g. as done above).
Thanks for taking time to clarify this.
Yes I messed up doing the patch by hand sorry, internally I test with the dma_rmb() in the else part. I tend to prefer having it in ath12k_hal_srng_access_begin() as caller does not have to take care of the barrier itself. Which for me seems a little bit risky if further refactoring (or adding other ring processing) is done in the future; the barrier could easily be forgotten don't you think ?
Yeah, that would be the argument for putting in the helper. Big hammer vs adding it where needed after reviewing the code.
There actually is a new ring being added for 6.16-rc1 I noticed after I posted the latest series. That would require a follow-up fix with the barrier-in-caller approach.
dma_rmb() acting also as a compiler barrier why the need for both READ_ONCE() ?
Yeah, I was being overly cautious here and it should be fine with plain accesses when reading the descriptor after the barrier, but the memory model seems to require READ_ONCE() when fetching the head pointer. Currently, hp_addr is marked as volatile so READ_ONCE() could be dropped for that reason, but I'd rather keep it here explicitly (e.g. in case someone decides to drop the volatile).
Yes actually after more thinking, the READ_ONCE for fetching hp does make sense and is also in the patch I am currently testing.
Also for source rings don't we need a dma_wmb()/WRITE_ONCE before modifying the tail pointer (see ath12k_hal_srng_access_end()) for quite the same reason (updates of the descriptor have to be visible before write to tail pointer) ?
Yep, the source rings need explicit barriers for the LMAC case, but there are further issues here too.
And that may also suggest adding the barriers in the start/end helpers for consistency (i.e. use the big hammer).
I'll try to find some more time to fix the remaining bits next week.
Johan