Re: [PATCH] drm/xe/oa: Allow reading after disabling OA stream

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Wed, 11 Mar 2026 21:24:14 -0700, Umesh Nerlige Ramappa wrote:
>

Hi Umesh,

> On Wed, Mar 11, 2026 at 02:01:07PM -0700, Ashutosh Dixit wrote:
> > Some OA data might be present in the OA buffer when OA stream is
> > disabled. Allow UMD's to retrieve this data, so that all data till the
> > point when OA stream is disabled can be retrieved.
> >
> > Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
> > ---
> > drivers/gpu/drm/xe/xe_oa.c | 3 +--
> > 1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/xe/xe_oa.c b/drivers/gpu/drm/xe/xe_oa.c
> > index 72fc4424017bf..f840827960eab 100644
> > --- a/drivers/gpu/drm/xe/xe_oa.c
> > +++ b/drivers/gpu/drm/xe/xe_oa.c
> > @@ -543,8 +543,7 @@ static ssize_t xe_oa_read(struct file *file, char __user *buf,
> >	size_t offset = 0;
> >	int ret;
> >
> > -	/* Can't read from disabled streams */
> > -	if (!stream->enabled || !stream->sample)
> > +	if (!stream->sample)
> >		return -EINVAL;
>
> What if there is no data to be read when the stream was disabled? In polled
> mode, we are probably fine, but in blocking mode, we may just wait forever
> here.

Yes, correct. There is no problem in non-blocking read() (it will return
-EAGAIN if no data). But poll() or blocking read() can block.

However, as far as I see, the responsibility of unblocking the blocked
thread (e.g. by means of a signal) rests with UMD.

Note: blocking read() cannot return 0 (it can, only on EOF, see 'man 2
read' but I don't want to do it because "what is EOF"). Similarly poll()
cannot return if there's no data, UMD will need to specify a timeout in
poll().

Note, KMD release() method will not get called if UMD thread is blocked in
read(). So stream cannot be closed till the blocked thread is unblocked.

I would imagine UMD will use non-blocking read()'s to retrieve data after
stream is disabled, till non-blocking read() returns -EAGAIN. If they don't
use non-blocking read(), they will have to handle the blocked thread using
one of the methods above. There's nothing KMD can do.

Unless you or Robert (Cc'd) can prove otherwise :-)

> >
> >	if (!(file->f_flags & O_NONBLOCK)) {
> > --
> > 2.48.1
> >