To fulfil the Dell 4K monitor, the dpcd max retries has been bumped
from 7 to 32, which may hurt the boot/init time for some platforms,
as the 32 retries may take hundreds of ms.
This patch makes no functional change, but make the max retries
adjustable, so that concerned users/platforms can have an option
to short the wait time.
On a Intel Atom Processor A3960 based NUC, the i915_init() time could
be reduced from 450ms to 200ms.
retries = 3:
[ 0.457806] calling i915_init+0x0/0x51 @ 1
[ 0.662307] initcall i915_init+0x0/0x51 returned 0 after 199694 usecs
retries = 32:
[ 0.465818] calling i915_init+0x0/0x51 @ 1
[ 0.925831] initcall i915_init+0x0/0x51 returned 0 after 449219 usecs
Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
---
drivers/gpu/drm/drm_dp_helper.c | 22 +++++++++++++++-------
1 file changed, 15 insertions(+), 7 deletions(-)
diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index ffe14ec..6054067 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -171,6 +171,20 @@ EXPORT_SYMBOL(drm_dp_bw_code_to_link_rate);
#define AUX_RETRY_INTERVAL 500 /* us */
+/*
+ * 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.
+ *
+ * Since the retry may take quite some boot time, we make it a
+ * adjustable parameter for possible shorter retry time.
+ */
+static int dp_dpcd_max_retries __read_mostly = 32;
+module_param_unsafe(dp_dpcd_max_retries, int, 0644);
+MODULE_PARM_DESC(dp_dpcd_max_retries,
+ "Max retry number for DPCD access (default 32)");
+
/**
* DOC: dp helpers
*
@@ -198,13 +212,7 @@ 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 < dp_dpcd_max_retries; retry++) {
if (ret != 0 && ret != -ETIMEDOUT) {
usleep_range(AUX_RETRY_INTERVAL,
AUX_RETRY_INTERVAL + 100);
--
2.7.4
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