For any remote call to DSP, after sending an invocation message,
the fastRPC driver waits for a glink response, during which the CPU
can enter low power modes. This adds latency to the fastRPC call
due to CPU wakeup and scheduling overhead. Add polling mode support
where the fastRPC driver polls a shared memory location for
completion after sending the invocation, avoiding CPU wakeup and
scheduling latency and reducing fastRPC overhead. If the poll times
out, the call falls back to the normal interrupt/glink-based
completion path.
Poll mode is only applied to dynamic modules running in a user PD
(handle > FASTRPC_MAX_STATIC_HANDLE), since static/root-PD handles
are not expected to benefit from, or require, this optimization.
Support is advertised per SoC via fastrpc_soc_data, with a closed
exception list for older platforms whose DSP firmware is known to
support polling but which otherwise use the default soc_data.
Poll mode can be enabled by userspace via the FASTRPC_IOCTL_SET_OPTION
ioctl with the FASTRPC_POLL_MODE request id.
Since context IDs (ctxid) are allocated from a fixed-size, per-channel
cyclic IDR shared by all processes on a DSP, a context ID can be
recycled for a new request soon after it is freed. In poll mode the
context can be considered complete (and released) as soon as the poll
memory is updated, while the corresponding glink COMPLETE response
from the DSP may still be in flight. If that response arrives after
the ctxid has been reused, it would otherwise match the new context
and incorrectly signal completion for it while the DSP may still be
operating on the new context's buffers. To prevent this, embed a
monotonically increasing per-channel sequence number in the unused
upper bits of the ctxid/message context and validate it in the
rpmsg callback, dropping any response whose sequence number does not
match the current owner of that ctxid slot.
Signed-off-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
Signed-off-by: Srinivas Kandagatla <srini@kernel.org>
Link: https://patch.msgid.link/20260729094352.111065-8-srini@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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