On Wed, 2021-02-10 at 09:55 +0100, Thomas Zimmermann wrote:
> Hi
>
> Am 10.02.21 um 09:33 schrieb Khaled Almahallawy:
> > The number of AUX retries specified in the DP specs is 7.
> > Currently, to make Dell 4k monitors happier, the number of retries
> > are 32.
> > i915 also retries 5 times (intel_dp_aux_xfer) which means in the
> > case of AUX timeout we actually retries 32 * 5 = 160 times.
> >
> > So making the number of aux retires a variable to allow for fine
> > tuning and optimization of aux timing.
> >
> > Signed-off-by: Khaled Almahallawy <khaled.almahallawy@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/drm_dp_helper.c | 10 +++-------
> > include/drm/drm_dp_helper.h | 1 +
> > 2 files changed, 4 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_dp_helper.c
> > b/drivers/gpu/drm/drm_dp_helper.c
> > index eedbb48815b7..8fdf57b4a06c 100644
> > --- a/drivers/gpu/drm/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/drm_dp_helper.c
> > @@ -249,13 +249,7 @@ static int drm_dp_dpcd_access(struct
> > drm_dp_aux *aux, u8 request,
> >
> > mutex_lock(&aux->hw_mutex);
> >
> > - /*
> > - * The specification doesn't give any recommendation on how
> > often to
> > - * retry native transactions. We used to retry 7 times like for
> > - * aux i2c transactions but real world devices this wasn't
> > - * sufficient, bump to 32 which makes Dell 4k monitors happier.
> > - */
> > - for (retry = 0; retry < 32; retry++) {
> > + for (retry = 0; retry < aux->num_retries; retry++) {
> > if (ret != 0 && ret != -ETIMEDOUT) {
> > usleep_range(AUX_RETRY_INTERVAL,
> > AUX_RETRY_INTERVAL + 100);
> > @@ -1744,6 +1738,8 @@ void drm_dp_aux_init(struct drm_dp_aux *aux)
> > aux->ddc.retries = 3;
> >
> > aux->ddc.lock_ops = &drm_dp_i2c_lock_ops;
> > + /*Still making the Dell 4k monitors happier*/
>
> The original comment was helpful; this one isn't.
Noted and I apologize for the comment. Was just copy/past original
comment.
>
> Besides that, what problem does this patchset address? Too much
> probing?
The problem mainly with disconnect. When disconnecting, AUX read will
fail because of timing out. The 32 retries cause the disconnect flow
especially for Type-C/TBT Docks with MST hubs taking a long time.
Just trying to reduce this disconnect time. In addition as I noted,
i915 does retry in addition to retries by this function.
Currently this function retry for 4 AUX situations:
AUX_NAK, AUX_DEFER, I/O error reported by driver and AUX timeout.
> If I connect a Dell monitor to an Intel card, how often does it have
> to
> probe?
I guess it depends on how you connect (direct, behind MST hub, behind DOCK with MST). But I believe the most time consuming is the disconnect flow.
Thanks
Khaled
>
> Best regards
> Thomas
>
> > + aux->num_retries = 32;
> > }
> > EXPORT_SYMBOL(drm_dp_aux_init);
> >
> > diff --git a/include/drm/drm_dp_helper.h
> > b/include/drm/drm_dp_helper.h
> > index edffd1dcca3e..16cbfc8f5e66 100644
> > --- a/include/drm/drm_dp_helper.h
> > +++ b/include/drm/drm_dp_helper.h
> > @@ -1876,6 +1876,7 @@ struct drm_dp_aux {
> > struct mutex hw_mutex;
> > struct work_struct crc_work;
> > u8 crc_count;
> > + int num_retries;
> > ssize_t (*transfer)(struct drm_dp_aux *aux,
> > struct drm_dp_aux_msg *msg);
> > /**
> >
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