drm/i915: Update workaround documentation

There were several updates in the driver on how the workarounds are
handled since its documentation was written. Update the documentation to
reflect the current reality.

v2:
  - Remove footnote that was wrongly referenced, adding back the
    reference in the correct paragraph.
  - Remove "Display workarounds" and just mention "display IP" under
    "Other" category since all of them are peppered around the driver.

Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com> # v1
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221115192611.179981-1-lucas.demarchi@intel.com

drivers/gpu/drm/i915/gt/intel_workarounds.c

@@ -17,42 +17,62 @@
 /**
  * DOC: Hardware workarounds
  *
- * This file is intended as a central place to implement most [1]_ of the
- * required workarounds for hardware to work as originally intended. They fall
- * in five basic categories depending on how/when they are applied:
+ * Hardware workarounds are register programming documented to be executed in
+ * the driver that fall outside of the normal programming sequences for a
+ * platform. There are some basic categories of workarounds, depending on
+ * how/when they are applied:
  *
- * - Workarounds that touch registers that are saved/restored to/from the HW
- *   context image. The list is emitted (via Load Register Immediate commands)
- *   everytime a new context is created.
- * - GT workarounds. The list of these WAs is applied whenever these registers
- *   revert to default values (on GPU reset, suspend/resume [2]_, etc..).
- * - Display workarounds. The list is applied during display clock-gating
- *   initialization.
- * - Workarounds that whitelist a privileged register, so that UMDs can manage
- *   them directly. This is just a special case of a MMMIO workaround (as we
- *   write the list of these to/be-whitelisted registers to some special HW
- *   registers).
- * - Workaround batchbuffers, that get executed automatically by the hardware
- *   on every HW context restore.
+ * - Context workarounds: workarounds that touch registers that are
+ *   saved/restored to/from the HW context image. The list is emitted (via Load
+ *   Register Immediate commands) once when initializing the device and saved in
+ *   the default context. That default context is then used on every context
+ *   creation to have a "primed golden context", i.e. a context image that
+ *   already contains the changes needed to all the registers.
  *
- * .. [1] Please notice that there are other WAs that, due to their nature,
- *    cannot be applied from a central place. Those are peppered around the rest
- *    of the code, as needed.
+ * - Engine workarounds: the list of these WAs is applied whenever the specific
+ *   engine is reset. It's also possible that a set of engine classes share a
+ *   common power domain and they are reset together. This happens on some
+ *   platforms with render and compute engines. In this case (at least) one of
+ *   them need to keeep the workaround programming: the approach taken in the
+ *   driver is to tie those workarounds to the first compute/render engine that
+ *   is registered.  When executing with GuC submission, engine resets are
+ *   outside of kernel driver control, hence the list of registers involved in
+ *   written once, on engine initialization, and then passed to GuC, that
+ *   saves/restores their values before/after the reset takes place. See
+ *   ``drivers/gpu/drm/i915/gt/uc/intel_guc_ads.c`` for reference.
  *
- * .. [2] Technically, some registers are powercontext saved & restored, so they
- *    survive a suspend/resume. In practice, writing them again is not too
- *    costly and simplifies things. We can revisit this in the future.
+ * - GT workarounds: the list of these WAs is applied whenever these registers
+ *   revert to their default values: on GPU reset, suspend/resume [1]_, etc.
+ *
+ * - Register whitelist: some workarounds need to be implemented in userspace,
+ *   but need to touch privileged registers. The whitelist in the kernel
+ *   instructs the hardware to allow the access to happen. From the kernel side,
+ *   this is just a special case of a MMIO workaround (as we write the list of
+ *   these to/be-whitelisted registers to some special HW registers).
+ *
+ * - Workaround batchbuffers: buffers that get executed automatically by the
+ *   hardware on every HW context restore. These buffers are created and
+ *   programmed in the default context so the hardware always go through those
+ *   programming sequences when switching contexts. The support for workaround
+ *   batchbuffers is enabled these hardware mechanisms:
  *
- * Layout
- * ~~~~~~
+ *   #. INDIRECT_CTX: A batchbuffer and an offset are provided in the default
+ *      context, pointing the hardware to jump to that location when that offset
+ *      is reached in the context restore. Workaround batchbuffer in the driver
+ *      currently uses this mechanism for all platforms.
  *
- * Keep things in this file ordered by WA type, as per the above (context, GT,
- * display, register whitelist, batchbuffer). Then, inside each type, keep the
- * following order:
+ *   #. BB_PER_CTX_PTR: A batchbuffer is provided in the default context,
+ *      pointing the hardware to a buffer to continue executing after the
+ *      engine registers are restored in a context restore sequence. This is
+ *      currently not used in the driver.
  *
- * - Infrastructure functions and macros
- * - WAs per platform in standard gen/chrono order
- * - Public functions to init or apply the given workaround type.
+ * - Other:  There are WAs that, due to their nature, cannot be applied from a
+ *   central place. Those are peppered around the rest of the code, as needed.
+ *   Workarounds related to the display IP are the main example.
+ *
+ * .. [1] Technically, some registers are powercontext saved & restored, so they
+ *    survive a suspend/resume. In practice, writing them again is not too
+ *    costly and simplifies things, so it's the approach taken in the driver.
  */
 
 static void wa_init_start(struct i915_wa_list *wal, struct intel_gt *gt,