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

["Siluvery, Arun" <arun.siluvery@linux.intel.com>]

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.

regards
Arun

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