Re: [PATCH 07/12] lib/igt_kms: Add support for the OUT_FENCE_PTR property

[Brian Starkey <brian.starkey@arm.com>]

On Tue, Nov 22, 2016 at 12:10:52PM +0000, Chris Wilson wrote:
>On Tue, Nov 22, 2016 at 11:54:57AM +0000, Brian Starkey wrote:
>> On Tue, Nov 22, 2016 at 11:06:00AM +0000, Chris Wilson wrote:
>> >On Tue, Nov 22, 2016 at 10:53:51AM +0000, Brian Starkey wrote:
>> >>Hi Gustavo,
>> >>
>> >>A little late to the party here, but I was blocked by our internal
>> >>contributions process...
>> >>
>> >>I didn't see these end up in my checkout yet though, so I guess they
>> >>aren't picked up yet.
>> >>
>> >>On Mon, Nov 14, 2016 at 06:59:21PM +0900, Gustavo Padovan wrote:
>> >>>From: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
>> >>>
>> >>>Add support for the OUT_FENCE_PTR property to enable setting out fences for
>> >>>atomic commits.
>> >>>
>> >>>Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
>> >>>---
>> >>>lib/igt_kms.c | 20 +++++++++++++++++++-
>> >>>lib/igt_kms.h |  3 +++
>> >>>2 files changed, 22 insertions(+), 1 deletion(-)
>> >>>
>> >>>diff --git a/lib/igt_kms.c b/lib/igt_kms.c
>> >>>index 4748c0a..f25e1eb 100644
>> >>>--- a/lib/igt_kms.c
>> >>>+++ b/lib/igt_kms.c
>> >>>@@ -175,7 +175,8 @@ const char *igt_crtc_prop_names[IGT_NUM_CRTC_PROPS] = {
>> >>>	"DEGAMMA_LUT",
>> >>>	"GAMMA_LUT",
>> >>>	"MODE_ID",
>> >>>-	"ACTIVE"
>> >>>+	"ACTIVE",
>> >>>+	"OUT_FENCE_PTR"
>> >>>};
>> >>>
>> >>>const char *igt_connector_prop_names[IGT_NUM_CONNECTOR_PROPS] = {
>> >>>@@ -2103,6 +2104,9 @@ static void igt_atomic_prepare_crtc_commit(igt_pipe_t *pipe_obj, drmModeAtomicRe
>> >>>		igt_atomic_populate_crtc_req(req, pipe_obj, IGT_CRTC_ACTIVE, !!output);
>> >>>	}
>> >>>
>> >>>+	if (pipe_obj->out_fence_ptr)
>> >>>+		igt_atomic_populate_crtc_req(req, pipe_obj, IGT_CRTC_OUT_FENCE_PTR, pipe_obj->out_fence_ptr);
>> >>>+
>> >>>	/*
>> >>>	 *	TODO: Add all crtc level properties here
>> >>>	 */
>> >>>@@ -2683,6 +2687,20 @@ igt_pipe_set_gamma_lut(igt_pipe_t *pipe, void *ptr, size_t length)
>> >>>}
>> >>>
>> >>>/**
>> >>>+ * igt_pipe_set_out_fence_ptr:
>> >>>+ * @pipe: pipe pointer to which background color to be set
>> >>>+ * @fence_ptr: out fence pointer
>> >>
>> >>I don't think fence_ptr can be int *. It needs to be a pointer to a
>> >>64-bit type.
>> >>
>> >>>+ *
>> >>>+ * Sets the out fence pointer that will be passed to the kernel in
>> >>>+ * the atomic ioctl. When the kernel returns the out fence pointer
>> >>>+ * will contain the fd number of the out fence created by KMS.
>> >>>+ */
>> >>>+void igt_pipe_set_out_fence_ptr(igt_pipe_t *pipe, int *fence_ptr)
>> >>>+{
>> >>>+	pipe->out_fence_ptr = (uint64_t) fence_ptr;
>> >>>+}
>> >>>+
>> >>>+/**
>> >>> * igt_crtc_set_background:
>> >>> * @pipe: pipe pointer to which background color to be set
>> >>> * @background: background color value in BGR 16bpc
>> >>>diff --git a/lib/igt_kms.h b/lib/igt_kms.h
>> >>>index 344f931..02d7bd1 100644
>> >>>--- a/lib/igt_kms.h
>> >>>+++ b/lib/igt_kms.h
>> >>>@@ -110,6 +110,7 @@ enum igt_atomic_crtc_properties {
>> >>>       IGT_CRTC_GAMMA_LUT,
>> >>>       IGT_CRTC_MODE_ID,
>> >>>       IGT_CRTC_ACTIVE,
>> >>>+       IGT_CRTC_OUT_FENCE_PTR,
>> >>>       IGT_NUM_CRTC_PROPS
>> >>>};
>> >>>
>> >>>@@ -298,6 +299,7 @@ struct igt_pipe {
>> >>>
>> >>>	uint64_t mode_blob;
>> >>>	bool mode_changed;
>> >>>+	uint64_t out_fence_ptr;
>> >>
>> >>IMO this should be:
>> >>
>> >>	int64_t *out_fence_ptr;
>> >
>> >In userspace, fences are *fd*, a plain int. It is only the uabi that we
>> >pass pointers as u64 to the kernel, and indeed that should be limited to
>> >the uabi wrapper.
>> >-Chris
>>
>> Where's the uabi wrapper in this case?
>>
>> Wherever it is, afaik someone needs to have 64-bit type for the kernel
>> to stash its fd in - on the kernel side out_fence_ptr is
>> (s64 __user *), so if there's not a 64-bit variable on the other end
>> of it then someone's going to have a bad day.
>
>We do not have pointers in the uabi because they are different sizes on
>different platforms. The uabi must be a u64 representation of a user
>address to store the result - that is what we pass to the crtc set
>property ioctl.

Sure, but igt_pipe is not a uabi structure. By storing a uint64_t here
we're making it needlessly opaque what the value is actually meant to
be - which is the address of a 64-bit signed integer.

Regardless, tests cannot set out_fence_ptr to the address of an int, I
hope we can agree on that. Where that detail gets taken care of I
don't much mind - but this code as-is is incorrect.

By making igt_pipe.out_fence_ptr an (int64_t *) I thought we'd be
letting the compiler warn anyone else away from incorrect code.

>That it has been futher managled not to pass around fd
>is an interesting twist, but ideally that sillyness should not make
>itself into our API.

Allowing the kernel and userspace to have different ideas about how
big an int is doesn't sound so silly to me. It may not be a
theoretical problem forever.

-Brian

>-Chris
>
>-- 
>Chris Wilson, Intel Open Source Technology Centre
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