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![]() Modify the wait delay utilize the high resolution timer API to allow for
more precisely scheduled callbacks.
A previous commit added a 1ms retry delay after multiple consecutive
NAKed transactions using jiffies. On systems with a low timer interrupt
frequency, this delay may be significantly longer than specified,
resulting in misbehavior with some USB devices.
This scenario was reached on a Raspberry Pi 3B with a Macally FDD-USB
floppy drive (identified as 0424:0fdc Standard Microsystems Corp.
Floppy, based on the USB97CFDC USB FDC). With the relay delay, the drive
would be unable to mount a disk, replying with NAKs until the device was
reset.
Using ktime, the delta between starting the timer (in dwc2_hcd_qh_add)
and the callback function can be determined. With the original delay
implementation, this value was consistently approximately 12ms. (output
in us).
<idle>-0 [000] ..s. 1600.559974: dwc2_wait_timer_fn: wait_timer delta: 11976
<idle>-0 [000] ..s. 1600.571974: dwc2_wait_timer_fn: wait_timer delta: 11977
<idle>-0 [000] ..s. 1600.583974: dwc2_wait_timer_fn: wait_timer delta: 11976
<idle>-0 [000] ..s. 1600.595974: dwc2_wait_timer_fn: wait_timer delta: 11977
After converting the relay delay to using a higher resolution timer, the
delay was much closer to 1ms.
<idle>-0 [000] d.h. 1956.553017: dwc2_wait_timer_fn: wait_timer delta: 1002
<idle>-0 [000] d.h. 1956.554114: dwc2_wait_timer_fn: wait_timer delta: 1002
<idle>-0 [000] d.h. 1957.542660: dwc2_wait_timer_fn: wait_timer delta: 1004
<idle>-0 [000] d.h. 1957.543701: dwc2_wait_timer_fn: wait_timer delta: 1002
The floppy drive operates properly with delays up to approximately 5ms,
and sends NAKs for any delays that are longer.
Fixes:
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arch | ||
block | ||
certs | ||
crypto | ||
Documentation | ||
drivers | ||
firmware | ||
fs | ||
include | ||
init | ||
ipc | ||
kernel | ||
lib | ||
LICENSES | ||
mm | ||
net | ||
samples | ||
scripts | ||
security | ||
sound | ||
tools | ||
usr | ||
virt | ||
.clang-format | ||
.cocciconfig | ||
.get_maintainer.ignore | ||
.gitattributes | ||
.gitignore | ||
.mailmap | ||
COPYING | ||
CREDITS | ||
Kbuild | ||
Kconfig | ||
MAINTAINERS | ||
Makefile | ||
README |
Linux kernel ============ There are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. Please read Documentation/admin-guide/README.rst first. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. The formatted documentation can also be read online at: 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. 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.