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+.. SPDX-License-Identifier: GPL-2.0
+
+:orphan:
+
+==============
+MSM Preemption
+==============
+
+Preemption allows Adreno GPUs to switch to an higher priority ring when work is
+pushed to it, reducing latency for high priority submissions.
+
+When preemption is enabled 4 rings are initialized, corresponding to different
+priority levels. Having multiple rings is purely a software concept as the GPU
+only has registers to keep track of one graphics ring.
+The kernel is able to switch which ring is currently being processed by
+requesting preemption. When certain conditions are met, depending on the
+priority level, the GPU will save its current state in a series of buffers,
+then restores state from a similar set of buffers specified by the kernel. It
+then resumes execution and fires an IRQ to let the kernel know the context
+switch has completed.
+
+This mechanism can be used by the kernel to switch between rings. Whenever a
+submission occurs the kernel finds the highest priority ring which isn't empty
+and preempts to it if said ring is not the one being currently executed. This is
+also done whenever a submission completes to make sure execution resumes on a
+lower priority ring when a higher priority ring is done.
+
+Preemption levels
+-----------------
+
+Preemption can only occur at certain boundaries. The exact conditions can be
+configured by changing the preemption level, this allows to compromise between
+latency (ie. the time that passes between when the kernel requests preemption
+and when the SQE begins saving state) and overhead (the amount of state that
+needs to be saved).
+
+The GPU offers 3 levels:
+
+Level 0
+ Preemption only occurs at the submission level. This requires the least amount
+ of state to be saved as the execution of userspace submitted IBs is never
+ interrupted, however it offers very little benefit compared to not enabling
+ preemption of any kind.
+
+Level 1
+ Preemption occurs at either bin level, if using GMEM rendering, or draw level
+ in the sysmem rendering case.
+
+Level 2
+ Preemption occurs at draw level.
+
+Level 1 is the mode that is used by the msm driver.
+
+Additionally the GPU allows to specify a `skip_save_restore` option. This
+disables the saving and restoring of all registers except those relating to the
+operation of the SQE itself, reducing overhead. Saving and restoring is only
+skipped when using GMEM with Level 1 preemption. When enabling this userspace is
+expected to set the state that isn't preserved whenever preemption occurs which
+is done by specifying preamble and postambles. Those are IBs that are executed
+before and after preemption.
+
+Preemption buffers
+------------------
+
+A series of buffers are necessary to store the state of rings while they are not
+being executed. There are different kinds of preemption records and most of
+those require one buffer per ring. This is because preemption never occurs
+between submissions on the same ring, which always run in sequence when the ring
+is active. This means that only one context per ring is effectively active.
+
+SMMU_INFO
+ This buffer contains info about the current SMMU configuration such as the
+ ttbr0 register. The SQE firmware isn't actually able to save this record.
+ As a result SMMU info must be saved manually from the CP to a buffer and the
+ SMMU record updated with info from said buffer before triggering
+ preemption.
+
+NON_SECURE
+ This is the main preemption record where most state is saved. It is mostly
+ opaque to the kernel except for the first few words that must be initialized
+ by the kernel.
+
+SECURE
+ This saves state related to the GPU's secure mode.
+
+NON_PRIV
+ The intended purpose of this record is unknown. The SQE firmware actually
+ ignores it and therefore msm doesn't handle it.
+
+COUNTER
+ This record is used to save and restore performance counters.
+
+Handling the permissions of those buffers is critical for security. All but the
+NON_PRIV records need to be inaccessible from userspace, so they must be mapped
+in the kernel address space with the MSM_BO_MAP_PRIV flag.
+For example, making the NON_SECURE record accessible from userspace would allow
+any process to manipulate a saved ring's RPTR which can be used to skip the
+execution of some packets in a ring and execute user commands with higher
+privileges.
Add documentation about the preemption feature supported by the msm driver. Signed-off-by: Antonino Maniscalco <antomani103@gmail.com> --- Documentation/gpu/msm-preemption.rst | 99 ++++++++++++++++++++++++++++++++++++ 1 file changed, 99 insertions(+)