[Mesa-dev] [PATCH] i965: fix MakeCurrent when switching a context between multiple drawables.

[Kristian Høgsberg]

On Mon, Apr 7, 2014 at 9:24 AM, Eric Anholt <eric at anholt.net> wrote:
> Iago Toral Quiroga <itoral at igalia.com> writes:
>
>> Commit 11baad35088dfd4bdabc1710df650dbfb413e7a3 produces a regression when
>> switching a single context between multiple drawables.
>>
>> The problem is that we check whether we have a viewport set to decide if we
>> need to generate buffers for the drawble, but the viewport is initialized
>> with the first call to MakeCurrent for that context, so calling MakeCurrent on
>> the same context with a different drawable will have the viewport already
>> initialized and will not generate buffers for the new drawable.
>>
>> This patch fixes the problem by reverting to the previous solution implemented
>> in commit 05da4a7a5e7d5bd988cb31f94ed8e1f053d9ee39 with a small fix to suppport
>> single buffer drawables too. This solution checks if we have a renderbuffer for
>> the drawable to decide if we need to generate a buffer or not. The original
>> implementation, however, did this by checking the BACK_LEFT buffer, which is
>> not a valid solution for single buffer drawables. This patch modifies this
>> to check the FRONT_LEFT buffer instead, which should work in both scenarios.
>>
>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74005
>> ---
>>  src/mesa/drivers/dri/i965/brw_context.c | 9 +++++----
>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_context.c b/src/mesa/drivers/dri/i965/brw_context.c
>> index c9719f5..c593286 100644
>> --- a/src/mesa/drivers/dri/i965/brw_context.c
>> +++ b/src/mesa/drivers/dri/i965/brw_context.c
>> @@ -926,6 +926,7 @@ intelMakeCurrent(__DRIcontext * driContextPriv,
>>  {
>>     struct brw_context *brw;
>>     GET_CURRENT_CONTEXT(curCtx);
>> +   struct intel_renderbuffer *rb = NULL;
>>
>>     if (driContextPriv)
>>        brw = (struct brw_context *) driContextPriv->driverPrivate;
>> @@ -950,6 +951,7 @@ intelMakeCurrent(__DRIcontext * driContextPriv,
>>        } else {
>>           fb = driDrawPriv->driverPrivate;
>>           readFb = driReadPriv->driverPrivate;
>> +         rb = intel_get_renderbuffer(fb, BUFFER_FRONT_LEFT);
>>           driContextPriv->dri2.draw_stamp = driDrawPriv->dri2.stamp - 1;
>>           driContextPriv->dri2.read_stamp = driReadPriv->dri2.stamp - 1;
>>        }
>> @@ -961,10 +963,9 @@ intelMakeCurrent(__DRIcontext * driContextPriv,
>>        intel_gles3_srgb_workaround(brw, fb);
>>        intel_gles3_srgb_workaround(brw, readFb);
>>
>> -      /* If the context viewport hasn't been initialized, force a call out to
>> -       * the loader to get buffers so we have a drawable size for the initial
>> -       * viewport. */
>> -      if (!brw->ctx.ViewportInitialized)
>> +      /* If we don't have buffers for the drawable yet, force a call to
>> +       * getbuffers here so we can have a default drawable size. */
>> +      if (rb && !rb->mt)
>>           intel_prepare_render(brw);
>
> We won't have an rb->mt for the front unless you're doing front buffer
> rendering, so I think you're basically just backing out krh's change.
> Which I think is good -- it looks like he was papering over a bug
> elsewhere, and I think we *should* just prepare_render in makecurrent.
> But if we're going to revert, let's just actually revert.

Here's what I wrote in https://bugs.freedesktop.org/show_bug.cgi?id=74005:
We don't want to revert the behaviour.  The initial patch removed a
call to intel_prepare_render() in intelMakeCurrent().  We're supposed
to call intel_prepare_render() any time we're about to touch the
buffers, but the up-front call to intel_prepare_render() in
intelMakeCurrent covered up a few places where we forgot.  The fix now
isn't to put back the up-front intel_prepare_render() call but to add
it in the rendering paths that are missing it.

Also, for reference, we need the buffer size for the initial value of
the context viewport.  So the first time a context is made current, we
call intel_prepare_render() to get the buffers so we can see what size
they are.  When the same context is later made current with a
different drawable, we have a value for the viewport and we're not
supposed to change it, so there's no point in getting buffers.

Kristian

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