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[ Upstream commit873a95e0d5] Currently we only poll for an ACT up to 30 times, with a busy-wait delay of 100µs between each attempt - giving us a timeout of 2900µs. While this might seem sensible, it would appear that in certain scenarios it can take dramatically longer then that for us to receive an ACT. On one of the EVGA MST hubs that I have available, I observed said hub sometimes taking longer then a second before signalling the ACT. These delays mostly seem to occur when previous sideband messages we've sent are NAKd by the hub, however it wouldn't be particularly surprising if it's possible to reproduce times like this simply by introducing branch devices with large LCTs since payload allocations have to take effect on every downstream device up to the payload's target. So, instead of just retrying 30 times we poll for the ACT for up to 3ms, and additionally use usleep_range() to avoid a very long and rude busy-wait. Note that the previous retry count of 30 appears to have been arbitrarily chosen, as I can't find any mention of a recommended timeout or retry count for ACTs in the DisplayPort 2.0 specification. This also goes for the range we were previously using for udelay(), although I suspect that was just copied from the recommended delay for link training on SST devices. Changes since v1: * Use readx_poll_timeout() instead of open-coding timeout loop - Sean Paul Changes since v2: * Increase poll interval to 200us - Sean Paul * Print status in hex when we timeout waiting for ACT - Sean Paul Signed-off-by: Lyude Paul <lyude@redhat.com> Fixes:ad7f8a1f9c("drm/helper: add Displayport multi-stream helper (v0.6)") Cc: Sean Paul <sean@poorly.run> Cc: <stable@vger.kernel.org> # v3.17+ Reviewed-by: Sean Paul <sean@poorly.run> Link: https://patchwork.freedesktop.org/patch/msgid/20200406221253.1307209-4-lyude@redhat.com Signed-off-by: Sasha Levin <sashal@kernel.org>
Linux kernel
============
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Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
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https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.