Re: [PATCH 08/19] drm/i915: Add a relay backed debugfs interface for capturing GuC logs

["Goel, Akash" <akash.goel@intel.com>]

On 8/19/2016 11:33 PM, Chris Wilson wrote:
> On Fri, Aug 19, 2016 at 02:13:07PM +0530, akash.goel@intel.com wrote:
>>  static void *guc_get_write_buffer(struct intel_guc *guc)
>>  {
>> -	return NULL;
>> +	/* FIXME: Cover the check under a lock ? */
>> +	if (!guc->log.relay_chan)
>> +		return NULL;
>> +
>> +	/* Just get the base address of a new sub buffer and copy data into it
>> +	 * ourselves. NULL will be returned in no-overwrite mode, if all sub
>> +	 * buffers are full. Could have used the relay_write() to indirectly
>> +	 * copy the data, but that would have been bit convoluted, as we need to
>> +	 * write to only certain locations inside a sub buffer which cannot be
>> +	 * done without using relay_reserve() along with relay_write(). So its
>> +	 * better to use relay_reserve() alone.
>> +	 */
>> +	return relay_reserve(guc->log.relay_chan, 0);
>>  }
>
> You have to chase through the code a long way to check whether or not
> the allocation is correct.
>
> Please do consider adding a check such as
>
> 	GEM_BUG_ON(guc->log.relay_chan->size < guc->log.vma->size);
> (near the allocation)

Fine, will add a check after the allocation, but not sure how useful it
will be, as we shall trust relay to do the memory allocation for the
sub-buffers as per the requested 'subbuf_size'.

subbuf_size = guc->log.vma->obj->base.size;
n_subbufs = 8;
guc_log_relay_chan = relay_open(NULL, NULL, subbuf_size,
				n_subbufs, &relay_callbacks, dev_priv);


GEM_BUG_ON(guc->log.relay_chan->subbuf_size < guc->log.vma->obj->base.size);


>
> 	GEM_BUG_ON(write_offset + buffer_size > guc->log.relay_chan->size);
> (before the memcpy, or whatever is appropriate).
>
There is a check already for read_offset/write_offset before the memcpy.

I think it would be better to add this check
GEM_BUG_ON(guc->log.relay_chan->subbuf_size < guc->log.vma->obj->base.size);

just before
return relay_reserve(guc->log.relay_chan, 0);

Best regards
Akash

> Just to leave some clues to the reader as to what is going on.
> -Chris
>
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