[PATCH v5 12/19] drm: writeback: Cleanup job ownership handling when queuing job

[Brian Starkey]

On Thu, Feb 21, 2019 at 11:56:09PM +0200, Laurent Pinchart wrote:
> Hi Brian,
> 
> On Thu, Feb 21, 2019 at 04:02:37PM +0000, Brian Starkey wrote:
> > On Thu, Feb 21, 2019 at 12:42:25PM +0200, Laurent Pinchart wrote:
> > > On Thu, Feb 21, 2019 at 12:32:05PM +0200, Laurent Pinchart wrote:
> > >> The drm_writeback_queue_job() function takes ownership of the passed job
> > >> and requires the caller to manually set the connector state
> > >> writeback_job pointer to NULL. To simplify drivers and avoid errors
> > >> (such as the missing NULL set in the vc4 driver), pass the connector
> > >> state pointer to the function instead of the job pointer, and set the
> > >> writeback_job pointer to NULL internally.
> > 
> > LGTM, it was a mistake not doing it like this from the start.
> > 
> > I do have one suggestion below, but either way:
> > 
> > Reviewed-by: Brian Starkey <brian.starkey at arm.com>
> > 
> > >> Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas at ideasonboard.com>
> > >> ---
> > >>  drivers/gpu/drm/arm/malidp_mw.c |  3 +--
> > >>  drivers/gpu/drm/drm_writeback.c | 15 ++++++++++-----
> > >>  drivers/gpu/drm/vc4/vc4_txp.c   |  2 +-
> > >>  include/drm/drm_writeback.h     |  2 +-
> > >>  4 files changed, 13 insertions(+), 9 deletions(-)
> > >> 
> > >> diff --git a/drivers/gpu/drm/arm/malidp_mw.c b/drivers/gpu/drm/arm/malidp_mw.c
> > >> index 041a64dc7167..87627219ce3b 100644
> > >> --- a/drivers/gpu/drm/arm/malidp_mw.c
> > >> +++ b/drivers/gpu/drm/arm/malidp_mw.c
> > >> @@ -252,8 +252,7 @@ void malidp_mw_atomic_commit(struct drm_device *drm,
> > >>  				     &mw_state->addrs[0],
> > >>  				     mw_state->format);
> > >>  
> > >> -		drm_writeback_queue_job(mw_conn, conn_state->writeback_job);
> > >> -		conn_state->writeback_job = NULL;
> > >> +		drm_writeback_queue_job(mw_conn, conn_state);
> > >>  		hwdev->hw->enable_memwrite(hwdev, mw_state->addrs,
> > >>  					   mw_state->pitches, mw_state->n_planes,
> > >>  					   fb->width, fb->height, mw_state->format,
> > >> diff --git a/drivers/gpu/drm/drm_writeback.c b/drivers/gpu/drm/drm_writeback.c
> > >> index c20e6fe00cb3..338b993d7c9f 100644
> > >> --- a/drivers/gpu/drm/drm_writeback.c
> > >> +++ b/drivers/gpu/drm/drm_writeback.c
> > >> @@ -242,11 +242,12 @@ EXPORT_SYMBOL(drm_writeback_connector_init);
> > >>  /**
> > >>   * drm_writeback_queue_job - Queue a writeback job for later signalling
> > >>   * @wb_connector: The writeback connector to queue a job on
> > >> - * @job: The job to queue
> > >> + * @conn_state: The connector state containing the job to queue
> > >>   *
> > >> - * This function adds a job to the job_queue for a writeback connector. It
> > >> - * should be considered to take ownership of the writeback job, and so any other
> > >> - * references to the job must be cleared after calling this function.
> > >> + * This function adds the job contained in @conn_state to the job_queue for a
> > >> + * writeback connector. It takes ownership of the writeback job and sets the
> > >> + * @conn_state->writeback_job to NULL, and so no access to the job may be
> > >> + * performed by the caller after this function returns.
> > >>   *
> > >>   * Drivers must ensure that for a given writeback connector, jobs are queued in
> > >>   * exactly the same order as they will be completed by the hardware (and
> > >> @@ -258,10 +259,14 @@ EXPORT_SYMBOL(drm_writeback_connector_init);
> > >>   * See also: drm_writeback_signal_completion()
> > >>   */
> > >>  void drm_writeback_queue_job(struct drm_writeback_connector *wb_connector,
> > >> -			     struct drm_writeback_job *job)
> > >> +			     struct drm_connector_state *conn_state)
> > >>  {
> > >> +	struct drm_writeback_job *job;
> > >>  	unsigned long flags;
> > >>  
> > >> +	job = conn_state->writeback_job;
> > 
> > What do you think about adding a defensive WARN_ON(!job)?
> 
> I expect this function to be called from an atomic commit handler for
> the writeback job, which will need to access the job's contents (in
> particular the framebuffer). I thus think we'll never be called with a
> NULL job here, so a WARN_ON would only consume a bit of CPU time and
> memory without adding any safeguard. If your analysis differs and you
> still think there's a risk, I'll add it.

My thinking was that it's up to drivers to make the check for a job,
and that a driver might get that wrong (and bring down the kernel).
With the old signature, it was entirely obvious that you needed a job
as it was one of the arguments to the function. Now it _should_ be
obvious from the naming, but it's not explicit anymore.

Anyway, I don't feel strongly about it. You'd hope a buggy driver
would hit the panic almost immediately during development and never do
any harm.

-Brian

> 
> > >> +	conn_state->writeback_job = NULL;
> > >> +
> > >>  	spin_lock_irqsave(&wb_connector->job_lock, flags);
> > >>  	list_add_tail(&job->list_entry, &wb_connector->job_queue);
> > >>  	spin_unlock_irqrestore(&wb_connector->job_lock, flags);
> > >> diff --git a/drivers/gpu/drm/vc4/vc4_txp.c b/drivers/gpu/drm/vc4/vc4_txp.c
> > >> index aa279b5b0de7..5dabd91f2d7e 100644
> > >> --- a/drivers/gpu/drm/vc4/vc4_txp.c
> > >> +++ b/drivers/gpu/drm/vc4/vc4_txp.c
> > >> @@ -327,7 +327,7 @@ static void vc4_txp_connector_atomic_commit(struct drm_connector *conn,
> > >>  
> > >>  	TXP_WRITE(TXP_DST_CTRL, ctrl);
> > >>  
> > >> -	drm_writeback_queue_job(&txp->connector, conn_state->writeback_job);
> > >> +	drm_writeback_queue_job(&txp->connector, conn_state);
> > >>  }
> > >>  
> > >>  static const struct drm_connector_helper_funcs vc4_txp_connector_helper_funcs = {
> > >> diff --git a/include/drm/drm_writeback.h b/include/drm/drm_writeback.h
> > >> index 23df9d463003..47662c362743 100644
> > >> --- a/include/drm/drm_writeback.h
> > >> +++ b/include/drm/drm_writeback.h
> > >> @@ -123,7 +123,7 @@ int drm_writeback_connector_init(struct drm_device *dev,
> > >>  				 const u32 *formats, int n_formats);
> > >>  
> > >>  void drm_writeback_queue_job(struct drm_writeback_connector *wb_connector,
> > >> -			     struct drm_writeback_job *job);
> > >> +			     struct drm_connector_state *conn_state);
> > >>  
> > >>  void drm_writeback_cleanup_job(struct drm_writeback_job *job);
> > >>  
> 
> -- 
> Regards,
> 
> Laurent Pinchart