Hi all,
As I was poking into the magicks for the batchbuffer, I saw the following
logical bits of code, that make sense by themselves but get me paranoid
together. Firstly in intel_batchbuffer_begin() [ intel_batchbuffer.h, and this
is what BEGIN_BATCH maps to] there is a intel_batchbuffer_require_space() call
that if too much room is needed then calls intel_batchbuffer_begin():
from intel_batchbuffer_require_space():
115 if (intel_batchbuffer_space(brw) < sz)
116 intel_batchbuffer_flush(brw);
and from intel_batchbuffer_space():
80 intel_batchbuffer_space(struct brw_context *brw)
81 {
82 return (brw->batch.state_batch_offset - brw->batch.reserved_space)
83 - brw->batch.used*4;
84 }
Now, for allocating space for state, there is brw_state_batch():
128 offset = ROUND_DOWN_TO(batch->state_batch_offset - size, alignment);
129
130 /* If allocating from the top would wrap below the batchbuffer, or
131 * if the batch's used space (plus the reserved pad) collides with our
132 * space, then flush and try again.
133 */
134 if (batch->state_batch_offset < size ||
135 offset < 4*batch->used + batch->reserved_space) {
136 intel_batchbuffer_flush(brw);
137 offset = ROUND_DOWN_TO(batch->state_batch_offset - size, alignment);
138 }
These taken together, I interpret as meaning that state and commands try to be
separated by atleast batch->reserved_space bytes. I guess state could take up
more than (batch->bo->size - batch->reserved_space) from that second
ROUND_DOWN_TO, but that would only happen right after a flush and any state or
command afterwards would flush too.
Now my questions:
1) it looks like the reserved space is not to be used for either state or
commands. Is that correct? What is it used for?
2) if a function first calls brw_state_batch() to place state and it barely
fits and then calls BEGIN_BATCH that does not fit, then the command will refer
to an offset on the previous batch buffer, that cannot be good. Or does this
never happen for other reasons? If so what are those reasons?
Cheers,
-Kevin
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