Re: [PATCH v6 1/6] drm/i915/gen8: Add infrastructure to initialize WA batch buffers

Siluvery, Arun
2015-06-22T15:37:12Z

On 22/06/2015 16:36, Daniel Vetter wrote:
> On Fri, Jun 19, 2015 at 06:50:36PM +0100, Chris Wilson wrote:
>> On Fri, Jun 19, 2015 at 06:37:10PM +0100, Arun Siluvery wrote:
>>> Some of the WA are to be applied during context save but before restore and
>>> some at the end of context save/restore but before executing the instructions
>>> in the ring, WA batch buffers are created for this purpose and these WA cannot
>>> be applied using normal means. Each context has two registers to load the
>>> offsets of these batch buffers. If they are non-zero, HW understands that it
>>> need to execute these batches.
>>>
>>> v1: In this version two separate ring_buffer objects were used to load WA
>>> instructions for indirect and per context batch buffers and they were part
>>> of every context.
>>>
>>> v2: Chris suggested to include additional page in context and use it to load
>>> these WA instead of creating separate objects. This will simplify lot of things
>>> as we need not explicity pin/unpin them. Thomas Daniel further pointed that GuC
>>> is planning to use a similar setup to share data between GuC and driver and
>>> WA batch buffers can probably share that page. However after discussions with
>>> Dave who is implementing GuC changes, he suggested to use an independent page
>>> for the reasons - GuC area might grow and these WA are initialized only once and
>>> are not changed afterwards so we can share them share across all contexts.
>>>
>>> The page is updated with WA during render ring init. This has an advantage of
>>> not adding more special cases to default_context.
>>>
>>> We don't know upfront the number of WA we will applying using these batch buffers.
>>> For this reason the size was fixed earlier but it is not a good idea. To fix this,
>>> the functions that load instructions are modified to report the no of commands
>>> inserted and the size is now calculated after the batch is updated. A macro is
>>> introduced to add commands to these batch buffers which also checks for overflow
>>> and returns error.
>>> We have a full page dedicated for these WA so that should be sufficient for
>>> good number of WA, anything more means we have major issues.
>>> The list for Gen8 is small, same for Gen9 also, maybe few more gets added
>>> going forward but not close to filling entire page. Chris suggested a two-pass
>>> approach but we agreed to go with single page setup as it is a one-off routine
>>> and simpler code wins.
>>>
>>> One additional option is offset field which is helpful if we would like to
>>> have multiple batches at different offsets within the page and select them
>>> based on some criteria. This is not a requirement at this point but could
>>> help in future (Dave).
>>>
>>> Chris provided some helpful macros and suggestions which further simplified
>>> the code, they will also help in reducing code duplication when WA for
>>> other Gen are added. Add detailed comments explaining restrictions.
>>>
>>> (Many thanks to Chris, Dave and Thomas for their reviews and inputs)
>>>
>>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>>> Cc: Dave Gordon <david.s.gordon@intel.com>
>>> Signed-off-by: Rafael Barbalho <rafael.barbalho@intel.com>
>>> Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
>>
>> Sigh, after all that, I found one minor thing, but nevertheless
>> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
>>
>>> +#define wa_ctx_emit(batch, cmd) { \
>>> + if (WARN_ON(index >= (PAGE_SIZE / sizeof(uint32_t)))) { \
>>> + return -ENOSPC; \
>>> + } \
>>> + batch[index++] = (cmd); \
>>> + }
>>
>> We should have wrapped this in do { } while(0) - think of all those
>> trialing semicolons we have in the code! Fortunately we haven't used
>> this in a if (foo) wa_ctx_emit(bar); else wa_ctx_emit(baz); yet.
>
> Uh yes, this is a critical one. Arun, can you please do a follow-up patch
> to wrap your macro in a do {} while(0) like Chris suggested? I'll apply
> the paches meanwhile.

Hi Daniel,

Already sent the updated patch.
I think I got the message-id wrong, the updated patch that I sent is
showing up as the last message in this series.

regards
Arun

>
> Thanks, Daniel