Re: [PATCH v12 05/12] dmaengine: qcom: bam_dma: add support for BAM locking

[Bartosz Golaszewski]

On Wed, Mar 11, 2026 at 11:00 AM Stephan Gerhold
<stephan.gerhold@xxxxxxxxxx> wrote:
>
> I'm not entirely sure if this actually guarantees waiting with the
> unlock until the transaction is "done", for two reasons:
>
>  1. &bchan->vc.desc_issued looks like a "TODO" list for descriptors we
>     haven't fully managed to squeeze into the BAM FIFO yet. It doesn't
>     tell you which descriptors have been consumed and finished
>     processing inside the FIFO.
>
>     Consider e.g. the following case: The client has issued a number of
>     descriptors, they all fit into the FIFO. The first descriptor has a
>     callback assigned, so we ask the BAM to send us an interrupt when it
>     has been consumed. We get the interrupt for the first descriptor and
>     process_channel_irqs() marks it as completed, the rest of the
>     descriptors are still pending. &bchan->vc.desc_issued is empty, so
>     you queue the unlock command before the rest of the descriptors have
>     finished.
>

Thanks for looking into it. Good catch, I think you're right.

>  2. From reading the BAM chapter in the APQ8016E TRM I get the
>     impression that by default an interrupt for a descriptor just tells
>     you that the descriptor was consumed by the BAM (and forwarded to
>     the peripheral). If you want to guarantee that the transaction is
>     actually done on the peripheral side before allowing writes into
>     config registers, you would need to set the NWD (Notify When Done)
>     bit (aka DMA_PREP_FENCE) on the last descriptor before the unlock
>     command.
>
>     NWD seems to stall descriptor processing until the peripheral
>     signals completion, so this might allow you to immediately queue the
>     unlock command like in v11. The downside is that you would need to
>     make assumptions about the set of commands submitted by the client
>     again... The downstream driver seems to set NWD on the data
>     descriptor immediately before the UNLOCK command [1].
>

If what we have in the queue is:

  [DATA] [DATA] [DATA] [CMD]

And we want to extend it with LOCK/UNLOCK like so:

  [LOCK] [DATA] [DATA] [DATA] [CMD] [UNLOCK]

Should the NWD go with the last DATA descriptor or the last descriptor period
whether data or command?

It's, again, not very clear from reading tha part.

Bart

>     The chapter in the APQ8016E TRM kind of contradicts itself
>     sometimes, but there is this sentence for example: "On the data
>     descriptor preceding command descriptor, NWD bit must be asserted in
>     order to assure that all the data has been transferred and the
>     peripheral is ready to be re-configured."
>
> Thanks,
> Stephan
>
> [1]: https://git.codelinaro.org/clo/la/platform/vendor/qcom/opensource/securemsm-kernel/-/blob/fa55a96773d3fbfcd96beb2965efcaaae5697816/crypto-qti/qce50.c#L5361-5362