Re: [PATCH v4 1/6] drm/i915/gen8: Add infrastructure to initialize WA batch buffers

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On 17/06/2015 19:48, Siluvery, Arun wrote:
On 16/06/2015 21:25, Chris Wilson wrote:
On Tue, Jun 16, 2015 at 08:25:20PM +0100, Arun Siluvery wrote:
+static int gen8_init_indirectctx_bb(struct intel_engine_cs *ring,
+				    uint32_t offset,
+				    uint32_t *num_dwords)
+{
+	uint32_t index;
+	struct page *page;
+	uint32_t *cmd;
+
+	page = i915_gem_object_get_page(ring->wa_ctx.obj, 0);
+	cmd = kmap_atomic(page);
+
+	index = offset;
+
+	/* FIXME: fill one cacheline with NOOPs.
+	 * Replace these instructions with WA
+	 */
+	while (index < (offset + 16))
+		cmd[index++] = MI_NOOP;
+
+	/*
+	 * MI_BATCH_BUFFER_END is not required in Indirect ctx BB because
+	 * execution depends on the length specified in terms of cache lines
+	 * in the register CTX_RCS_INDIRECT_CTX
+	 */
+
+	kunmap_atomic(cmd);
+
+	if (index > (PAGE_SIZE / sizeof(uint32_t)))
+		return -EINVAL;

Check before you GPF!

You just overran the buffer and corrupted memory, if you didn't succeed
in trapping a segfault.

To be generic, align to the cacheline then check you have enough room
for your own data.
-Chris

Hi Chris,

The placement of condition is not correct. I don't completely follow
your suggestion, could you please elaborate; here we don't know upfront
how much more data to be written.
I have made below changes to check after writing every command and
return error as soon as we reach the end.

#define wa_ctx_emit(batch, cmd) {       \
                 if (WARN_ON(index >= (PAGE_SIZE / sizeof(uint32_t)))) { \
                          kunmap_atomic(batch);                          \
                          return -ENOSPC;                                \
                  }                                                      \
                  batch[index++] = (cmd);                                \
          }
is this acceptable?
I think this is the only one issue, all other comments are addressed.

one other improvement is possible - mapping/unmapping page can be kept in common path, will update the patch accordingly.

regards
Arun

regards
Arun

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